Introduction: How to Make a Racing Lawn Mower (Updated!)

 I've recently completely turned this mower into a new build. If you've read this before, proceed to step 12 for the latest updates. Otherwise, start reading below for the original build.Thanks to everyone who has commented before. As always, feel free to ask questions and I'll do my best to answer.Click Here to proceed to step 12.

In this demo, you'll get some ideas of how you can make a real racing riding mower used in national events. Have fun turning what used to be the family lawn mower into a fire-breathing high speed racing machine. Also- I'm constantly making changes and modifications to the final machine so check back to see what I've done. The next plans I have include steering upgrades.

Please read the following paragraph before proceeding.

Before we start, there's a bit of safety to discuss.Yes, racing lawn mowers from an outward perspective is sort of funny ( which it is!)But its important to realize that racing mowers such as these are heavily modified to handle much greater speeds than the original mower was designed for. Many of these mowers go 50MPH or more.Making a race mower isn't as simple as taking a stock tractor and making it go fast without any alterations. So its important that the frame, brakes, steering, engine, and wheels are modified or altered to handle this additional speed.So to make this point doubly clear, it is NOT a good idea to take a bone stock mower and make it go fast. You can, and will get hurt if you do so, and trust me- I've seen enough people wreck due to this very reason. So play it safe. Secondly, if you do plan on racing, make sure and check out the rules for your chapter and wear appropriate safety gear such as a helmet ( motorbike), gloves, boots, and long pants.My mower is built using ARMA ( American Racing Mower Association) rules and regulations. Lastly, your mower must have an approved jet ski/snowmobile style safety tether switch. If you fall off ( which we often do) the mower must automatically shut down or it'll keep right on going! Racing mowers might seem silly, which it sort of is, but you can get hurt if you're not careful. So be safe!

Ready, let's get started! The 'victim' I chose for this build is a late 60's Grants mower. Tiny little mowers like these were produced back when riding mowers were still deemed a luxury. They're little more than a seat sitting on top of a mower deck. Most used smaller engines. The advantage of using such a little mower is that you can reduce the weight dramatically by simply having a 'legit' riding mower complimented with a larger engine, hence a higher power/weight ratio. Don't get attached to it. When its done, there won't be much left of the original.

The first step is to strip the mower down to the frame. Modern mowers usually have a single stamped piece of steel. Older mowers like this one have frames made of square tubing or slabs of steel. This will give you an idea of how much of the mower is actually usable and how you can lay out the drive, steering, and brake components. Besides the hood, what's leftover to use isn't much. The rest are worthless such as the stock wheels, steering wheel, and transmission.

Step 1: Configuring the Build.

The next step is probably one of the more difficult parts of the build: configuration and finding parts. Building one of these is sort of like building a small car with all its various systems. Since all of these racing mowers are one-off type builds, finding the parts that will work can be a pain. I've had a lot of questions about where the tires, clutch, and right angle gear box ( transmission) comes from. the gear box can be found on ebay. The tires are go cart tires and can be found online on any site that sells go cart parts. The same is true for the brakes and rear axle components. The front axle is a custom unit built by a company called Acme Mowersports.

A list of parts needed for this build are:

A: Engine
B: Transmission ( right angle gearbox)
C: Centrifugal clutch
D: sprockets
E: Front axle and spindles
F: Rear axle and axle hangers
G: steering wheel
H: Brake system
I: chain
J: electrical components
K: Wheels, tires, and hubs
M: Gas lever
N: high endurance engine components

Many of the others need to be made by hand. One thing that's helpful is that many of these components such as the rear axle and hangers,wheels, hubs, spindles, and brakes are basically go-cart components. Some golf cart and motorbike components work as well. Sprockets and such can be had from sites such as Mcmaster -carr.

Once you get all the parts, the build actually goes pretty quickly.

Step 2: Frame Reinforcement

The next step is to beef up the frame or make alterations that will work with your components.Its important to realize that these mowers will be racing on what tends to be really rough dirt track. They have no suspension, thus the frame takes a severe beating. Reinforcement is critical to avoid having the frame flex and ultimately crack from fatigue. The rear of the frame was cut about 6" from the rear. Throughout the build, I used 1x1 square tubing which is easy to weld and work with. This is what I used to create the square frames in which the mounting brackets were welded into to hold the rear axle bearings. I chose to use a 1/1/4" rear axle because that size is highly common and thus easier to get parts like wheels and sprockets for.

These square frames were welded into the frame, then the end I cut off was welded to the back. The minimum height requirements for my class is 4" from the frame to the ground. So its important to know what size wheels you plan to use and where to mount the axles in order to meet that requirement. The lower you can go, the better handling the mower will be. Mine site just at 4" off the ground.

Next, I welded two lengths of square tubing along the top of the axle brackets to the front tubular frame. I did this because the transmission will go underneath. A piece of diamond plate will cover it, and above will be the seat. This will give me easy access to servicing the chain and transmission and also protect me from flying debris or potential chain failures.

I am using a right angle gearbox for this build. Why? Because the other choice is to use a 3-5 speed gearbox used as standard equipment on mowers. These work fine, but it also means you'll have to change the grease in them and perhaps invest in hardened gears since the originals will strip out much easier. With a right angle gear box, or RAGB, there's only two moving parts. Plus, they are made for higher speed applications and therefor perfectly suited for this application. More simplicity means more reliability.

Additionally, I am using a centrifugal clutch. This is a higher quality, higher HP rated unit that is heavier duty than typical go-cart clutches. The springs can be adjusted for higher or lower engagement.

Step 3: Steering System

The next step is one of the most important of the build. Many people go out on the track with the stock steering setup. That's a big mistake for a number of reasons. For one, the stock components aren't made for going 50MPH, as is none of the other stock components. Secondly, there's more to steering besides making the wheels turn. You also need to have the proper caster, pitch, and turning radius so that the chassis will handle corners better.

Most mowers come with a gear driven steering setup. These are worthless and tend to pop out of place. So you'll need to make a "direct steering" system. In other words, a solid connection between the steering wheel and the front wheels.

For this build, I bought a pre-built front axle from a guy in Texas. He has a small business called " Acme mowersports" and can be found at His front axles are a good deal because even if you were to build your own, the cost would be only slightly less. With the Acme axle, the proper caster and degree of inclination are already built-in, which will save you lots of time. These come with the radius arms as well as connections for the steering axle, which on mine runs down the center of the front of the mower over the top of the engine.

Next up is the installation of the steering shaft running along the front of the frame. This mower has an unusual setup where the steering linkage runs over the top of the engine. An a arm runs from the steering wheel pitman arm to a shaft running down the front of the frame to the radius arms of the front axle spindles. First, I got some 1/1/4" steel pipe and cut some lengths about 2" long. On each end, I placed a bearing in which the steering shaft fits through.

The top of the front steering shaft has a removable lever to attach the piece of linkage coming from the steering wheel. This enables you to remove it if needed. If you look at the pic entitled "pitman arm detail", this is the steering wheel shaft with the pitman arm welded on. As you can see, the arm on the end is rounded and has three holes. There's a reason for this, which is to prevent the heim joints, which are the screw-on ball bearing pieces on the ends of the rods from binding. The reason for the three holes is to give you adjustments to the steering sensitivity. Further out gives you more slack. Further in tighter. It is also important that the arm running across the top of the engine area has threads on either end. This way the heim joints can be screwed in or out to adjust the amount of right and left turn in the wheels.

In The pic entitled: "Steering arm", you can see how this system works together. Lastly, the "turn right" pic shows the underside linkage and radius arms. If you see the "t" shaped piece, that's where the radius arms connect. The "T" is welded to the bottom of the front steering shaft.

Another step is to determine the angle of the wheels. Generally, it is better to have the left wheel turn in more than the right. I usually have the left wheel turn in @ 10:00 and the right at 2:00.

Lastly, you will need to install what are known as "stops", which are basically welded on rods or bolts to prevent the wheels from turning too far. If they turn too far, the steering wheel will turn completely over, thus reversing your steering! Not good! For this build,. all I did was weld two 5/16" pieces of steel rod to the front of the axle, right where the spindles swing in and out. The spindle arms simply hit the stops. I held the wheels in place at the correct position and placed the stops at exactly where the spindle arms hit, then welded them into place.

Once you have the steering done, then you've just completed one of the hardest steps!

Step 4: Installing the Transmission Or- the RAGB

The next step is to install the RAGB ( transmission). This was a tedious task because the RAGB I chose has an unusual triangular shape and a strange bracket. Additionally, the RAGB shaft has to be far enough off the ground, yet not too far up as to protrude above the diamond plate covering it. When all the measurements were done. I had 1/4" between the top of the RAGB and the cover. The RAGB sits on two cross members welded in the frame. If you look at the pic called: "Battery bracket", you can see how it is configured. In the next pic you can see the RAGB bolted into place.

Before we get any further, you'll need to determine what your gearing ratio will be. The RAGB is a 2:1 ratio, meaning that two turns go in, one comes out of the output shaft. I suck at math, but my gear setup is as follows: Centrifugal clutch is 14 tooth. Input on RAGB is a 14 tooth. Output on the RAGB is 12 tooth, and the rear axle sprocket is a 40 tooth. That worked out well, but I have no clue what the final ratio is. The engine speed is around 4,500-5,000 RPM.

I'm using an all chain drive system. Many use belts, but I like the idea of using a chain. I used #35 chain for this build, but many use #40.

The next step is to install chain tensioners. The chain needs to be somewhat taut, but not tight. All chains will loosen and 'stretch' with use. So you need to have tensioners to keep that tension at the right level. The RAGB to rear axle tensioner was a problem because there was very little space to install one. My solution was to install a skateboard wheel that slides up and down in a 1x1 piece of steel with a slot milled along it's length. This enables you to slide the skateboard wheel up and down against the chain. You wouldn't think skateboard wheels would hold up, but they work great. Seeing as how they are designed to handle a person riding on asphalt, chain doesn't cause any damage to them at all. In this setup, I just have enough room to get into the area with a wrench to loosen/tighten the tensioner.

Step 5: The Brake System

Next up is brakes. There are several types of brakes you can use: mechanical or hydraulic. The later is generally better and easier to install because you can route the brake lines to wherever you want them. The brake system I have is an MCP go cart brake kit. They run around $150 including the master cylinder, rotor, rotor hub, caliper, and brake line. You can also use motorbike brakes if you have any laying around.

First, I drilled two holes through the frame where the bolts holding the caliper would go through. Many people create a caliper mounting bracket. Mine just happened to conveniently work without it. After that, I bolted on the caliper and made sure it aligned properly with the rotor. The rotor and other components on the axle are adjustable and slide back and forth on a keyway milled into the axle shaft. Once adjusted, you lock them down with set screws.

Next, I installed the master cylinder. This is bolted through the frame. The aluminum piece with the three holes is the connection for the brake pedal connecting rod, which is made out of 1/4" steel rod bent @ 90 degrees. The other end of the rod connects to the pedal. I welded a nut to the end of the rod and drilled out the threads. Make sure you do a real hot, molten weld here because you don't want that to fail. The pedal is made out of scrap steel and a short length of 1x1 square tubing with a hole drilled through the bottom in which to run a bolt through the frame. A nylock nut on the other side allows it to stay in place yet swivel back and forth.

Once you've gotten everything to work smoothly, you'll have to bleed to brakes. But hold off on that until closer to the end.

Step 6: Installing the Floor Plates,"mower Deck" Battery, and Electrical System

Next up, I install the "mower deck", or in this case, the simulated mower deck.In some classes ( yes there are different classes of mower racing machines) you can install a replacement for the original mower deck as long as it is in the approximate location and height. Mine is made out of more of the steel square tubing welded into "C"s with wire mesh tacked on top. This not only makes for a nice foot rest, but it is much lighter than the original deck. These are simply welded directly to the bottom of the frame.

Then I installed the battery, which was originally to go under the seat. The seat is sitting on top of the original bracket I welded directly to the top of the deck frame. But the battery was too tall. Its a small lawn tractor battery, and the ONLY place it would fit was right behind the engine. I welded two angle iron pieces pointing upwards to form a bracket in which the battery sits. The space is TIGHT. This actually works really well because it moves the center of gravity to the center of the mower, adding further stability.

Following that was the installation of the floor plates made of thick diamond plate. These were cut to size and screwed on with self-drilling screws so that both panels can be removed for servicing the RAGB, transmission. The fit was very tight and I glued on a piece of rubber to the battery to avoid abrasion from the diamond plate. The fit was perfect and snug, which is what you want with batteries.Diamond plate is costly stuff, so try and find scrap if you can. The same was done for the dash, which is where the electrical stuff goes.

That brings us to the next step, which is the creation of the electric panel. In order to keep everything neat and serviceable, all of the electrical components were screwed and bolted to the dash panel. This includes the starter button, starter solenoid, and tether switch.

Step 7: "Puke Tank", Seat, and Engine Mounting.

Our next step is to make what is known in the racing world as the "Puke tank". No- it isn't for seasickness, but rather for the engine. Since the engine will be running at sometimes 50% faster than it was originally designed for, the crank case will sometimes spit oil from the crank case breather. Since you don't want that stuff all over the track or you for that matter, you make sometimes called a puke tank. Mine is made out of ABS plastic pipe, brass hose fittings, and PVC for the breather on the right side (in white). This is mounted to the back of the mower under the seat, connected with a hose to the engine valve cover.

Next, I decided to machine slots into the engine mounting pan. I made keyhole shaped slots that enable the engine to be slid forwards and back so that the chain can be fitted and tensioned properly. This is the longest chain on the build, thus a spring-loaded tensioner was fabricated in addition. Since the chain here is hanging horizontally, you want to have constant, gentle pressure applies to keep it from falling off. The tensioner is simple, using a channeled nylon idle pulley typically used on mower decks. Like the skateboard wheel, nylon holds up just fin on chain. The pulley is mounted to a arm that is bolted to the underside of the simulated mower deck via a nylock nut and bolt. A spring is attached to it as well as a bolt welded to the underside of the engine pan. As you can see, the chain fits nicely from the centrifugal clutch to the RAGB.

Now the seat. This step is actually kind of important. You'll find that staying on these mowers on a bumpy dirt track with sharp corners is difficult.The inertia will threaten to throw you off. But at the same time, if you do fly off, you want to clear the "scene of the accident" and not get run over by your own mower. So it needs to be low. The solution is to have a low seat with low sides that keeps your rear attached to the seat. It'll actually give you more control. One last word- it helps to have padding. A steel seat with no padding can be... painful. My Wife covered mine with upholstery from an old chair.

Step 8: Engine Modifications and Installation.

Now that the chassis and other vitals are now complete, now its time to work on the engine. The engine I'm using is a 12.5 HP Briggs and Stratton with a cast iron sleeve. These are one of the most common engines on riding mowers. Mine in particular is what's known as a "flathead" because the other variant has overhead valves. These engines are extremely simple and inexpensive. The flathead takes up less space too.

But to use an engine such as this stock would be a mistake. First of all, the governor will be removed. enabling engine speeds to approach double what the engine was designed for. This means that parts are going to take a severe beating and certain items should be replaced. First and foremost- the flywheel. The original is made out of solid cast iron and weighs in at 12-15 pounds. Cast iron is somewhat susceptible to fracturing from stress. Even microscopic cracks, not seen by the naked eye can cause a flywheel to explode at high speeds. While this rarely occurs, its something to think about since the flywheel will be less than a foot away from you -know-what. There are two fixes, once better than the other. The first is to make a scattershield, which is a 1/4" frame made to fit around the top of the engine shroud. These can be bought at a number of sites, including G-team racing. These will at least contain the explosion if it occurs, but not absolutely.

The safer, albeit more expensive solution is to purchase what's known as a billet aluminum flywheel. These are milled via computer guided mills out of solid billet aluminum- the stuff used to make aircraft components. Not only will the wheel be lighter, but stronger and almost indestructible.Considering the cost of the milling machine, the cost is reasonable. Around $350.

Next, the governor is removed. You want to remove it entirely, including internally. That involves removing the oil pan. You will see a brass sleeve in which the throttle lever slides through. After you remove the throttle, use a punch to remove the brass sleeve. To seal the hole, I simply use a bolt and a nut, washer, and rubber o-ring.

The second specialized part is what's known as a "dogbone" connection rod for the large cast iron counterweights. These large weights help the engine run smoother. The crank runs directly through it. The wimpy aluminum factory original is prone to breaking. If that happens, the counterweight will slam into the sides of the crank case, breaking right through it. Say bye-bye to the engine if that occurs. So again, a billet aluminum replacement is used.

Additionally- as mentioned in the new "updates" section, the stock piston rods in these engines do not deal well with the additional engine speeds. So as a recommendation, invest in a billet aluminum rod. You will also have to buy another piston, most often being a magnesium Briggs unit. I ordered mine from a company called G Team racing out of North Dakota. They are well worth the money not having to worry about an engine blowing out on the track, which trust me- is rather scary and potentially costly.

Next, the linkage for the carburetor needs to be setup. For the gas throttle, A bicycle brake lever and brake line is connected to a spring that pulls against the throttle control of the carb. The pull of the spring is what snaps the brake lever back in place.The lever is attached to the steering wheel. As you can see, I simply bent two scrap pieces of diamondplate aluminum. These face each other. On one side, the spring goes to the throttle lever of the carb. The brake cable comes from the other side, attaching into the same hole as the throttle spring. To hold the brake cable in place, I used a brass nipple fitting that stops the cable sheathing, but allows the actual cable to come through. The spring holds the cable into position.

Lastly, I fabricated the exhaust system. It is a 2.5 foot long pipe. To get the correct curves, I bought a muffler header pipe that is made to fit a Snapper riding mower from a mower parts supply site- This will fit 8-12 HP engines, hence it fits mine. This is cut and a second curved piece was made from the exhaust pipe of a 1980's Honda Civic( which apparently had tiny exhaust systems). These two curves gave me the right geometry to curve out and under the foot rests. The end of the pipe is simply a length of straight muffler pipe. These are held to the bottom of the foot rests via pipe hanging brackets used for electrical conduit. By the way- this exhaust will make your engine extremely LOUD. Just a word of warning in case you have... neighbors. close by.

Step 9: Paint and Finishing It Up.

Now for the funnest part of the build: Painting. Now all of that hard work gets to get shown off. But before painting, you need to do some prep. The chassis is likely oily and dusty from welding. All of the welds are also likely not "clean" meaning they're splattered. You can quickly clean up the frame with sandpaper flap discs attached to a grinder. Clean all of the rust, splatter, and rough edges to a smooth surface.

I used a oxide primer primer, which is a good foundation for the paint. I used plain ole' black spray paint. I chose spray paint because the frame is very likely going to get pitted with rocks and future mechanical modifications. So it can easily be touched up. Plus its cheap. The fiberglass hood was painted with orange engine paint, which I find dried quick and smooth.

Now the entire mower is put back together. Wheels, engine, brakes, electrical, etc. The mower went back together surprisingly quick. Less than two hours was all it took.

With the mower put back together, all that's left is to bleed the brakes, install the battery, fill the engine with oil, and then give her a test run.

Wanna' see it run? Watch the video.

Anyhow, it was lots of fun making this mower and so far I've been in 3 races this season. Feel free to ask questions if you want to make your own. if you want to see me in action, visit my racing club's web site,

Step 10: Updates.

Its been a year since this mower was completed. After a year of racing I've learned what the weak spots were on the machine. First, stock pistons in Briggs flathead engines do not hold up well under stress. I wound up blowing two engines early in the season when the rods snapped. So the first improvement was in purchasing a billet aluminum rod and a lightweight magnesium piston. So far this rod has held up well under the stress.

Secondly, any nuts and bolts that can come loose will come loose unless they are secures using nylock nylon lock nuts and loctite ( the blue bottle). Otherwise the severe track vibration will cause things to fall off, which they did frequently. So spend the few extra dollars and secure everything down and save the headaches.

Lastly, the cheap stamped bearings I went with in the steering system have way too much slop in them. I replaced all 8 of the bearings with sealed ball bearing units with steel snap rings. This removed all of the slop in the steering and made the handling much more accurate and responsive, which in turn is much safer. So make sure and invest in better high quality bearings for everything that moves.

Oh yeah- one more thing- Orange was not a good color. So now it is all-black.

The video is of our first event of the year. Enjoy!

Step 11: Updates: New Rear Axle Chain Tensioner.

Here's another update, and this time its again for the rear chain tensioner. I had a lot of issues with the chain jumping off the sprocket for the rear axle. I tried a few things, namely the experiment I mentioned in the previous step involving two roller skate wheels, one on top of the chain, one underneath. That system didn't work.

So I came up with a new tensioner that involves using a chain sprocket idler which is spring loaded. The old tensioner idler was mounted to a bracket with a slot cut in the side to enable the idler wheel to be moved up and down. This is shown in step 4. So I used the same bracket but instead of having the idle sprocket be stationary, it "floats" up and down in the slot with the help of a sturdy spring that ensures that it keeps constant pressure on the chain. I accomplished this by using 2 large fender washers on each side of the mounting bolt going through the bracket, then having nylock stop nuts on either side, backed up just enough to allow the mounting bolt to move freely up and down. I welded a bolt to the frame above the idler sprocket so that the tension spring could be attached. The idler sprocket itself is mounted to a thick piece of 1/8" steel.

This has worked out great all season. The tensioner can not only move up and down, but also slightly forward and back thus not matter what the chain does, the tensioner keeps constant pressure on it. With both chains staying put, the mower is now pretty reliable.

The rebuilt engine has held up great this year. Its now broken in so the synthetic oil stays almost crystal clear for a good 2-3 races.

We also just redesigned our web site and you can check us out at

Step 12: Updates for 2011: Total Overhaul

  After over 3 years of racing the mower with the same configuration it was time for some serious upgrades. There were some major shortcomings with the old design. That and as the years went by, others on my team "upped the anty" and had upgraded their machines. A lot of things have changed in mower racing since I built this machine. A lot of custom, high performance engine parts are now available making more durable, highly reliable engines that can withstand higher RPMs over longer periods.

 Before we begin read this :
I'll reiterate a bit about safety. As I mentioned at the very beginning of this insctructable,  racing mowers are actually pretty dangerous and if you're not careful you can very easily get hurt. There are several racing organizations out there that all have very specific rules and regulations as you'd find in any motorsport: Rules meant to keep the driver and those watching safe. Think of these as go-carts except they look like mowers. We have no safety cages or restraint systems. Thus you must wear correct safety gear like a DOT approved helmet- preferably a full face helmet. You must also wear gloves, boots, long pants, a neck collar- or even better- a neck brace. Your mower must also have various safety features like real brakes ( not the crappy ones the mower came with), a safety tether that cuts power if you fall off, and others as well. In order to understand these in detail visit and check out the rules. If you show up at an event and you or your mower are not equipped safely- you won't be racing. Lastly... DO NOT simply take the ole' family mower and make it go fast without anything else other than swapping pulleys. Stock mowers are meant to go 2-5MPH. Not 50MPH. If you do something like this, you will have a greater chance of getting injured.

 Lastly, if you're welding, drilling, grinding, or working with any power tools, please use proper safety gear- as in gloves, boots, goggles, respirators, and so on. Use common sense and you'll avoid a trip to the emergency room ( trust me- I've been there enough to know)
Whew! Now that that's over- let's get goin'!

 The changes I made to the mower were as follows:
 1: Extend the length of the frame. Why? The original frame was around 38" long. The width from wheel to wheel? 36". The problem was that when you're basically driving a square, you have very poor handling. I decided to lengthen the frame to the maximum length permitted for Mod-X machines: 42". Thus I would add 6" to the frame.

2: New engine with a plethora of high performance parts. This includes a billet rod, billet ARC flywheel, High-torque starter, billet crank with built-in lightweight counterweights, a high performance cam, Higher strength valve springs and better valve keepers. That and the rings would be filed to an absolute close tolerance to increase compression. Other considerations would be porting and polishing. The old engine was a very mild build. This new engine was a total build-out. As this was my first major mod job of an engine, there was a lot of learning.

3: New sheetmetal. Luckily I found a "donor mower" pretty quick.

4:A new fully adjustable steering system. The old setup wasn't adjustable at all. The new one is fully adjustable and the caster and camber can be carefully tweaked. This is important because no matter what engine you might have, a mower that steers poorly and plows into corners will be impossible to handle.

 The first thing was to strip the old mower down. After 3 years of abusive tracks and racing, it was actually in pretty good shape. Most of the parts I'd used like the brakes, wheels, axle, transmission, and clutch could be re-used, which mean a lot less money. The old 12.5 Briggs Flatty served me well but will be replaced with a 14.5 Briggs Overhead Valve engine.

 I had previously gone to a scrap metal yard and gotten a piece of channel iron to extend the frame 6". The frame on mine is a simple box frame with a tubular frame up top. Thus I was able to extend my mower versus something like a stamped steel machine, which would be more problematic. The channel iron was cut and the frame was sliced near the front. The idea was to preserve the more complicated rear of the machine which has the engine, transmission, and brake system mounts so it could be used as-is without modification.  These new pieces of angle iron was welded into place.

 Since the old fiberglass hood was now way too short for the now-lengthened mower, I needed to either cut and extend the old hood or find a new hood. Luckily for me, the group leader or our group has what I'd call lawn mower heaven in his back yard- a whole fleet of old riding mowers. One was an old Montgomery Wards and the hood and fenders from it fit perfectly! Easy.

Step 13: Update the Old Steering System to a Fully Adjustable One.


Perhaps the single most important thing you can do for a racing mower is get your steering right. Before I go into the build, let's talk a little about steering geometry because if you understand how it works, you'll have a much easier time building one. I have to admit it took me a long time to figure it out. The guys I race with in many cases raced stock cars thus they have it down to a science.

 Basically the problem that we have on the track is that the rear axle doesn't have a differential. As such both rear wheels are traveling at exactly the same rate. With a differential the outer wheel naturally travels more than the inner, thus making turning into a corner easier. In order to correct this problem, you'll want to remove weight from the front right and place that weight onto the front left so that weight is removed from the right rear. In doing so you'll offset the effects of having no differential. This is done so by adjusting three things: Caster ( the angle that the spindle sits- either positive or negative, the Camber- the angle at which the wheels lean in or away from the mower, and toe out- as in how much the wheels point away from the mower.  We'll go into this later when final adjustments are made.

 The old spindles were cut off of the front axle. I re-used the old axle instead of installing a new one. The new spindles are fully removable from the axle and are mounted with large heim joints. The mounting plates for the spindles are welded to the ends of the axle and have machined slots so that the spindles can be turned forward or back ( which gives you your caster adjustment). These plates need to be welded to the ends of the axle at a 10 degree angle with the tops of the plates pointing in towards the mower. This is critical because this in turn gives you your camber- or  the degree in which your wheels will lean in towards the mower. Having this adjustment will allow you to carefully adjust the  camber in such a way as to help push more weight onto the left front wheel meaning you will be able to "hook up" more easily in the turns. 

 With the mounting plates welded in place, the spindles could be attached. These spindles come in a kit and you can get them from various go-cart suppliers. Some people make their own spindles using bolts. If you do that make sure to use Grade 8 bolts as they are less prone to bend than conventional bolts.

 The last major thing you'll have to do is to attach the spindle arms for attaching the radius rods.  There's a pretty simple method for doing this: With both spindles absolutely straight on both sides, attach a piece of string from the ends of the spindle mounting bolts and attach the string to the center of the rear axle. Weld the spindle arms as if you are lining them up with the path of the string. Easy enough.

 Once the steering system is installed, you'll have to do some adjustments to the setup to ensure the mower is handling properly. But that will come later once the machine is completed.

Step 14: Porting and Polishing the Engine.

Porting and Polishing

 The next step was to start working on the engine. The engine I got was a early 90's Briggs 14.5 OHV. There are a lot of similarities between it and the old 12.5 flathead I had. The crank, piston, rod, and carb in the 12.5 are basically the same. Thus I was able to scavenge the old magnesium piston and billet rod I'd been running. The rod is an ARC rod and has replaceable bearings. Its always important to replace these if you're sticking them in a new engine. I also ordered new rod bolts.

 I'd also decided to try out a new product from ARC, which is a billet chrome molly billet crank. Just as I got started on the engine they came out with it. For years we've had a dilemma where the old heavy cast iron counterweights that came with these engines not only presented an issue of having to deal with more mass, but on occasion these have come loose, smashing through the crank case. The solution was to get an old-style 80's or early 90's crank with removable eccentrics and install lightweight brass counterweights. ARC developed a billet crank with the counterweights machined into the piece- thus its all one piece. I opted for this part.

 Also ordered was a new set of valves and more "modern" valve keepers. The old style of keepers uses a sort of cap with a slot cut in the top. These have been known to come loose out on the track- causing damage to the head. The new keepers are similar to ones you'd find in a car and are highly unlikely to come loose.

 Lastly, while I thought I would be able to re-use my old billet aluminum flywheel, the one I had was solid aluminum-including the hub. The hub was badly damaged from having slipped numerous times on the crank. I ordered an ARC billet flywheel with a removable steel hub- thus making it also adjustable. In doing so I also had to order a high-torque starter. The starter looks like something you'd find in a car also. Its made by Denso- the folks that make parts for Toyota.

 Anyway, one way to increase engine performance is to do something called "porting and polishing". What this entails is to basically provide an easier way for gasses to enter and exit the engine. From the factory  the intake and exhaust ports have rather sharp edges. Run your finger inside the area where the port enters the valve area and its almost sharp enough to cut you. These edges will need to be shaved down and smoothed over. I used a stone mounted to my Dremel tool to slowly cut and blend these areas so that the airflow would be smoother. I then followed up with some polishing compound and gave it a mirror polish. This took hours but its worth it in the end.

Step 15: Building the New Engine.

Building the Engine

 With the parts all ordered and received I spent about a week building the engine. One thing I did was ordered a set of over-20 rings. This means that the gap between where the ring when it is fully compressed in the cylinder is .020 over the stock gap. This is done for a few reasons. Mainly its if you had an engine with a worn cylinder and needed more available ring to fill that void. The ends of the ring are then filed down until a desired ring gap is met. In my case I did this to intentionally make the ring gap far tighter than stock. Usually a stock engine will come with a 0.10-0.20 ring gap. In my case I wanted to have a 0.004-0.006 gap. This would give me more compression overall. Doing this is tricky if you're like me and don't have a ring grinder tool handy. But it can be done. Simply mount a mill file in a vise straight up and down and very slowly pull up on the ring with the end of the ring filed as level as possible. Only file one side of the ring too because doing so means a greater chance of having an uneven gap. Every few strokes put the ring in the cylinder and tamp it level with the top of the piston. Measure the gap with a feeler gauge. Eventually you'll arrive at the correct gap. I accidentally filed too much and wound up having it down to a .008 gap. Ooops. Not that big of a deal so that's what I installed it as.

 The crankcase was thoroughly cleaned and the hole which the governor lever came out of was blocked using a bolt and nut. The next step was to install the new valves. In addition to ordering new valves and keepers, I also ordered a stronger set of valve springs. These are double layer springs with a smaller inner spring that is removable.  What I wasn't aware of was that you don't want to use the center spring. I spent hours sweating bullets trying to install the new valve and keepers because the dual springs were so stiff. The first time I did this I was pressing the spring down with a socket when it slipped. The tiny keepers flew off into the wild blue yonder, never to be seen again. What a pain! I never found them as they were hopelessly lost somewhere in the shop. Thus I ordered 3 extra sets. With the center springs removed it was much easier to push them down and drop in the keepers. Slowly let the pressure off the springs and the keepers will usually fall into place around the groove of the valve stems.

 With that done it was time to install the piston and rod. The rod needs to be installed using exacting torque settings. The final is @ 28 foot pounds. First, apply a bit of oil to the bolts. This will enable you to do what's called " Wet torquing". Using a torque wrench, set it for 16 foot pounds. go back and forth from one bolt to another until you arrive at 16 foot pounds. Then go in 4 pound increments until both are snugged to 28 foot pounds. This is vitally important. Incorrect torquing can mean a snapped rod.

Step 16: Building the New Engine, Part 2.

More Engine Building...

 Next up was the installation of the ARC billet flywheel and high torque starter. In regards to the flywheel its important to understand that the flywheel is mated to the crank via a friction fit. There is a key but all that key does is provide the means to properly align the flywheel magnets to the magneto. In order to get a tight fit, especially with 2 new parts, you need to do something called "lapping". Simply put, it consists of using a very fine abrasive to mate the two pieces together. You can actually buy lapping compound to do this. Simply smear a bit of this onto the crank, place the flywheel on the crank and twist it back and forth. You don't have to do this all that much. Just enough to make sure that the two pieces will be perfectly matched. Clean off the compound and then torque the flywheel down with about 100 foot pounds of torque. You do so via an air impact wrench.

 Once you install the flywheel you'll need to install the starter. This is a bit tricky with the high torque starter. You don't want the teeth of the starter to be too tightly meshed with the ring gear of the flywheel. If you do this the flywheel will bind. The starter comes with a number of thin washers that you use as shims. Unlike typical small engine starters, this is more of an automotive starter and you can't just "walk" the starter gear up. Instead the starter gear is tucked down out of sight. Why should you care? Because in order to get the proper gap between the ring gear and starter gear that starter gear needs to be pushed up all the way.  Here's how you can do it ( or at least how I did it). Get a small hex key, stick the short end of the hex key behind the starter gear and pull UP. With the starter gear fully pulled up, stick another hex key under the starter gear so that when you release it it won't snap back down.

 Now you'll want to hold the starter up to the flywheel. Stick the 2 mounting bolts through the starter and into the threads in the side of the engine. Here's where it gets "fun". Get a paper clip. Not a weanie one either- but a large paper clip. bend the end of it down and measure the gap between the valley of the ring gear and the tip of the starter gear. The proper gap is the width of the paper clip.  In order to make this work you'll need to insert enough washers to get the correct gap. Doing this was a pain in the ass because the washers kept falling from the ends of the bolts as I tried to hold it up to the engine. It took around 3 washers on each bolt for mine to get the correct gap.

  Now that we've finished doing that its time to install the blower housing. You'll see right away that the ARC wheel is not as tall as the stock unit. As such the fins fall short of the blower housing by over an inch. This is not acceptable because air won't be getting sucked into the engine cooling fins and you'll very quickly fry the engine- ruining all those lovely high performance parts. So, as un-fun as it is, you'll have to cut enough of the shroud to get the flywheel fins within 1/8" of the blower housing. I measured from the top of the fins to the blower housing. I marked the amount with a permanent marker around the bottom of the blower housing and used a grinder wheel to cut it off. Again, you'll want to make sure that you have about 1/8" of clearance from the blower housing to the top of the flywheel fins.

 With most of the engine together its time to install the "tins" and the breather plate. I re-used the plate I'd made for the old 12.5 flathead. The stock breather plate will not work. You need something that will not leak and one that will allow more air to pass through. Thus the steel plate I made has a threaded piece of pipe coming out of it with a brass barb screwed to this. I usually use a gasket making material- usually the blue stuff- and barely tighten the bolts. You'll want the gasket material to harden before you snug down the bolts. As far as the tins, there have been some people who for some reason don't re-install these. Without them the engine won't cool properly. So make sure and install them and make sure they're not bent. With that the engine is done- minus the carburetor.

Step 17: Installing New Chain Tensioner.

Chain Tensioner.

One of the biggest problems I had with the old mower was the rear axle chain setup. The chain repeatedly either came loose or snapped. This might have been due to the old chain being a #35 size, which is a bit small. I elected to upgrade the chain to a #40 size which meant new sprockets. At the same time I also installed a new RAGB transmission. The old one had always had a lot of play and slop in the gears. This never caused a problem. But I decided to install a duplicate I found on Ebay a year ago. The old one will serve as a spare.

 The key to having the rear chain not fall off is the correct combination of proper alignment and tension. All chains will stretch as they wear. Thus the best setup involves a spring-loaded idler. In this case I used a idler sprocket. The fatter chain meant that it would be hitting the top of the transmission mount. The old #35 barely cleared it. Thus I had to cut out one side of the mount and weld a new piece behind where that piece had been. Thus the mount was staggered back enough to allow the chain to clear.

 A great deal of time was spent ensuring that all 3 sprockets were absolutely aligned. This is done with a flat steel ruler. Since the tranny is more or less not adjustable the idler and rear axle sprocket both needed to be aligned with the tranny sprocket. Lay the ruler flat over all 3 sprocket surfaces in such a way as that all of the gear's sides are totally flat- as in the ruler lays flat across all 3. Gently tap the sprockets until this happens. This needs to be absolutely perfect so spend some time.

 The tensioner I use is one I made 2 years ago. I modified it to use the larger idler sprocket. I also relocated the tension spring in such a way as so it pulls directly up. Previously the spring was pulling slightly forward, which I think might have caused the idler to jump off the chain. The tensioner itself rides in a slotted piece of square tubing. A bolt runs through the slot and thus allows the entire tensioner to float up and down. The spring keeps the tensioner against the chain.  As such even if the chain stretches, the tensioner will always apply the same amount of pressure.

Step 18: Re-doing Exhaust, Paint, Brakes, and Other Stuff.

Finishing up

Something I realized right away was that the engine was reverse of what the old 12.5 engine had been: The starter, carb, and exhaust were all on the other side. At first I didn't think this would be that big of a deal. But the problem was that the new high torque starter stuck out so far on the right that it interfered with the brake pedal- which was on the left where the starter would now be. This meant moving the brake pedal and master cylinder to the left. This wasn't that difficult: The MC was moved to the left and new holes were drilled for its mounting bolts. The problem arose when I realized that with the carb on the left the intake was extremely close to the brake pedal. I didn't want to accidentally kick the carb when racing. After doing some configuring I simply welded an extension onto the end of the pedal making it wider. A piece of angle iron was welded to the top of the pedal. This ensures that my foot will not slip and wind up sliding close to the carb. After 2 races its worked out well.

 The exhaust was a bit tricky. Since the exhaust was on the other side it had to be cut so the bend could be reversed to face the other way. On top of that the exhaust port was higher up than the one on the engine. In order to accomplish the additional length I welded in a piece of pipe. A word of warning: Do not weld galvanized steel. The pipe I used was galvanized but I spent a great deal of time grinding this off. If you do weld Galvanized you can get very sick.

 The engine was then installed and the old clutch bolted on. This has been a fantastic clutch with over 3 years of use and hardly any wear. I did disassemble it and applied a very light coat of grease to the bearings. I also cleaned the shoes a bit.

 The new hood and fenders were sanded down and painted. I used Duplicolor high heat ceramic engine paint. For some reason I've found its one of the best rattle can paints for getting a nice smooth finish. The color I got is called " Cast Iron". Pretty cool actually, and different from what most people use on their racing mowers. Seems like crazy fluorescent colors are very popular.

Step 19: Finishing Up and Final Adjustments.

Final Adjustments

With all of the sheetmetal painted the mower was fully assembled. Pretty nice looking if I don't mind saying so. Now there was only one thing left to do- which was to race it and see what it would do. The first race was actually what we call our "Test-n-tune" day where all the new "mowchines" are brought out and tested... and tuned up.

 Mine had some serious handling problems. The mower was pushing into the corners. It was so bad that I could barely stay on the track. As it turned out my steering system was wayyyyyy out of whack. To fix this I did some serious adjusting. This involved doing a few things:

1: adjusting the front right wheel so that it had more camber. As it turned out I had somehow welded the right spindle mount on at the wrong angle. Luckily all I had to do was simply thread the top heim joint inward until the camber was corrected. The right wheel was also dropped downward a bit. This enabled the right front to push down onto the front left.

2: Adjust the toe-out. As mentioned before, toe-out means how much the wheels splay outwards- as in they turn away from the mower a bit. It doesn't take much. Only 1/8" on each side. Doesn't sound like much but this will greatly help the mower become far more stable.

 The first "real" race we had was a huge difference. The mower did much better. That said, there are still some issues. The biggest is that I chose not to install a high performance cam. This would greatly help with low end torque and faster acceleration out of the corners. This is relatively simple to do and only involves removing the bottom of the engine. I'll decide on what to do about this soon.

 In the meantime if you want to see the mower racing in action, check out our racing group's web site which is chok-full of videos.

 Thanks for reading and stay tuned for even more updates!

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