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Step 8Engine 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- cheapmowerparts.com 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.
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- cheapmowerparts.com 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.
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If he doesn't have any there, email him and tell him what engine you have.
http://www.arcracing.com/servlet/StoreFront
If you don't see it o the site, email the store. He should have them available.
http://www.g-team.us/