Homemade Exhaust for Your Old Lawnmower




You can build your own, homemade exhaust system for an old Briggs and Stratton lawnmower engine!

When taking my junk-picked 70s-era lawnmower apart, besides finding out that this motor was made the same year I was, I realized that the exhaust threads are a standard pitch 1/2 inch pipe thread!

The parts you will need are:
A short 1/2 inch nipple (short pipe, threaded on both ends)
A 1/2 inch x 1 inch hex bushing (female 1/2 inch on one end, male 1 inch on the other)
A 1 inch 90 degree elbow
A length of 1 inch pipe
A 1 inch cap
A drill and small drill bit(s)
Cutting oil for drilling (WD-40 or even motor oil will probably work)
(optional) Pieces of fiberglass tape or cloth for sound-deadening material.

PLEASE use black gas piping, NOT galvanized pipe. Galvanized pipe is coated inside and out in zinc. Zinc heated to very high temperatures can offgas and make you feel very sick. There is some debate as to whether zinc exposure leads to long term health effects, but I have gotten some stern warnings against welding zinc, so have used only black gas piping for this project.

You could also probably do this project with aluminum or threaded copper pipe, but for safety and affordability, I went with the steel. You probably wouldn't want to use soldered copper, in case the solder were to melt and cause trouble.


Step 1: Remove Old Muffler

On these old engines, the muffler is easy enough to remove. Simply unscrew it from the engine block. (lefty loosey, and please do this when the muffler is cold!)

Step 2: Assemble the Manifold

The manifold (Part of the muffler that attaches to the engine block) is easy to make from a threaded nipple, a hex bushing and a 90 degree elbow.

Assemble these parts tightly. Depending on the final required orientation of the muffler, it may be necessary to attach some parts with a bit of fiberglass tape to allow more flexible positioning of the joints. Here, I used fiberglass cloth left over from a canoe pontoon project, but you could also use fiberglass drywall tape. You might not require the fiberglass at all. I didn't use plumbers tape because I was concerned about the heat generated by the muffler burning the tape and causing a stink.

Step 3: Fabricate and Assemble the Muffler Body

To make the muffler body, I used a 12 inch length of 1 inch diameter pipe, into which I drilled 5 3/16 inch holes. I discovered that two holes was not enough and 5 seemed to be OK. You can experiment with the number of holes if you like, and a 12 inch pipe is probably overkill.

Before and during drilling, I wet the steel with WD-40 (cutting oil may work better) to cool the drill bit so as not to wear it out. The metal is quite thick and I broke 2 drill bits by trying to rush things. Take your time and use plenty of oil.

After the holes are drilled, clean up the pipe to remove any metal particles and cut a short length of fiberglass tape. Roll it up and stuff it into the end of the tube. The fiberglass will serve to deaden the noise a little,but it may also come flying out of the exhaust holes when the engine is running. It helps to cut the fabric as little as possible so as not to create many loose threads.

Screw the pipe cap on tightly. You can remove the cap or disassemble the muffler entirely to repack or repair it in the future.

Step 4: Attach the Muffler

The original muffler has a stop on it to prevent you from over-tightening it and stripping the threads. Your new muffler will not have this safety feature, so be careful when threading the new unit into place. The engine block is aluminum and very soft in comparison to the steel pipe. Do not use a wrench to thread the pipe into the block - hand tight is plenty.

Once you are satisfied with the tightness and orientation of the manifold, screw the muffler body into it. You have a muffler!

You should now be ready to test the exhaust. Be careful when testing and running the engine!

1) The new muffler is much heavier than the old muffler, and may cause stress damage to the engine block if it is not installed and supported properly.
2) The fiberglass tape may come out of the exhaust holes - be aware of this and on alert in case the engine decides to suck up a strand.
3) The muffler gets VERY hot, VERY fast. Do not EVER touch a muffler after running the engine for more than a few seconds.
4) Please do NOT use galvanized pipe. Only use black gas piping. Galvanized pipe is coated in zinc and when exposed to extreme heat, it could offgas and make you feel very sick. My original attempt was with galvanized pipe, and I got a bit light headed after the muffler heated up for the first time. Also never weld galvanized metal for the same reason.

Please be careful. This project involves extreme temperatures, hot gasses and fast moving parts. Do not rush things and be always aware of where your hands and feet are in relation to the mechanisms.

Step 5: Fit and Finish

The final steps are up to you.
The muffler may need extra support, which could be accomplished with cleverly placed bolts, hose clamps or pipe support straps. Use your head and think of where the forces are going to be transferred.

Dress up your exhaust tube and add a margin of safety with an exhaust tip or cover.

Don't be afraid to experiment with the amount of packing material, number, size and placement of holes, etc. This muffler is completely customizable.

I originally designed a much smaller muffler with larger diameter pipe, but when I moved to the black gas pipe, the larger pipe diameters were not available.



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    82 Discussions


    3 years ago on Step 5

    Bad ass instructable!


    10 years ago on Step 5

    OK, I see some problems here. 1. This idea needs to have the amount of backpressure figured out so you don't burn the valves or cause power loss. 2. I have seen pipe systems like this before and the weight of the pipe usually wears out the threads in the engine because you cannot keep them tight. 3. Spark arresting features of this system probably will not meet US Dept. of Forestry specs. 4. Might be more dangerous if fuel is spilled on the mower. 5. Bigger exhaust system = more possibility of skin burns or setting dried grass on fire. 6. The cost of the pipe and fittings is way more than the $2-3 for an approved muffler.

    On the plus side, it looks cool, especially with the chrome cover.! LOL!

    15 replies

    Reply 10 years ago on Step 5

    First things first. You're a killjoy, and if you're so worried about obscure, irrelevant and stupid regulations and rules, then what are you doing on this website? I'm really quite serious. I know I read three articles on bomb-making and two about lockpicking before breakfast. 1. The brilliant thing with projects like this is the overkill. Drill more than enough holes and it won't cause any back pressure. 2. So far as the stripping or cross-threading is concerned, the man even suggests supporting the weight somehow. Also, it's a thirty-year-old law mower, what does it matter if it does strip? He can take the entire muffler off and have a SUPER loud lawnmower just for the heck of it. 3. Who cares? 4. Oh, wow... My lawnmower is on fire. Hold on while I call a few people to come over and watch and maybe bring some thermite so we can do this right. 5. Oops. I burned myself on an exhaust. Again. I'm tattooed up both arms and legs from exhausts from motorcycles and cars and ovens and god-knows what else. Go somewhere else if you're going to live in fear of life. Some of us do things with the full expectation of getting hurt. It's half the fun to have a scar to associate with something really fun or really stupid your did. Most times both. 6. Did he ever mention this was going to be a money-saving endeavor? The whole point of this is that the stock muffler wasn't quiet enough for this man's liking, so he built a bigger one and put a chrome tip on it. Just because he could. Welcome to America, a country founded upon the principal of experimentation, the Rule of Cool, and getting away from killjoys. I, for one, would try to make an exhaust cover like they put on the AC Cobra's side pipes, or maybe built a mini diesel smoke stack just to make it look cooler. I bet with a little welding and some pre-planning, you could give your lawnmower twice-pipes. I'd absolutely love to try this on a gas-powered rotary mower, just for the look. Maybe even vent the exhaust to the side, and put a spark plug in it hooked to a battery. Nothing says "Good mornin'!" like flaming exhaust when you're mowing the lawn. Maybe you could even vent it straight up and brace it, so everyone can enjoy the flames.


    Reply 3 years ago on Step 5

    I so so so SO SO SO so so SO SO SOOOOOOO agree 100% with what you said here. Thank you! Ideas like this is why I LOVE this site so much!


    Reply 10 years ago on Step 5

    You know something, I really don't care what anybody builds here. All I tried to do is make sure the builder is being careful to make sure he isn't hurt, maimed, or killed. If any of the three happened to the builder, I'm sure it would kill somebody's joy in a hurry. Let's not call people names and criticize why they are here. Every year someone modifies a lawnmower in a way that causes them pain or death. The next thing that happens is that the manufacturer and the dealer get sued. I don't care what anybody builds here but please be careful and when things go wrong, take responsibility for any negligence on your part..


    Reply 10 years ago on Step 5

    Firstly, nobody called you a name. Calling you a killjoy is no more calling names than saying that someone who runs professionally or competitively is a runner.

    Secondly, you are again being incredibly offensive by, effectively, calling the builder a moron incapable of foresight or any knowledge of safety. For all you know, this gentleman is an engineer who designs two-stroke engines for NASA, or some other type of engineering hyper-genius. Both of your posts are insulting and degrading to the gentleman who wrote this article and I am, again, deeply offended by your treatment of someone who took time out of his life to share his experience and ingenuity and bring a smile to people's faces. So, because of your condescending posts, I'll respond in kind.

    "You know something, I really don't care what anybody builds here."
    "I don't care what anybody builds here..."
    - Then why did you bother to comment in the first place?

    "All I tried to do is make sure the builder is being careful to make sure he isn't hurt, maimed, or killed."
    - Via a text post, after the fact, online, anonymously, with alleged "facts" of things that are so improbable it's insulting? Also, this contradicts your previous statement that you "care what anybody builds here". Has the Nobel committee been made aware of you?

    "but please be careful and when things go wrong, take responsibility for any negligence on your part.."
    - LOL. What?

    "Spark arresting features of this system probably will not meet US Dept. of Forestry specs."
    - Please submit a detailed diagram of exact blue-prints of the builder's design and highlight the specific faults you have found. Also, please post the specific and applicable "US Dept. of Forestry specs" along with exactly how they affect this build. Please cite your sources.

    "Every year someone modifies a lawnmower in a way that causes them pain or death."
    - Please list specific examples of both death and injury each year starting from the invention of the lawn mower by Edwin Beard Budding in 1827. Please cite your sources.

    "I have seen pipe systems like this before and the weight of the pipe usually wears out the threads in the engine because you cannot keep them tight."
    - Please cite each and every specific instance you have personally had direct experience with. Please note dates, locations, and potential witnesses. Also, please post the exact weight of the exhaust system the builder implemented, along with equations to prove your hypothesis. Please show the exact torque at the fulcrum and detail the exact tolerances of the materials and threads on both the engine block and the exhaust system. Please cite your sources.

    "5. Bigger exhaust system = more possibility of skin burns or setting dried grass on fire."
    - Please explain how. The chrome tip on the exhaust will actually act as a heat shield as there is minimal contact with the muffler, and the exhaust is not venting openly, but inside the tube of the exhaust tip which would act as a crude flame arrestor along with the fibreglass inside. Plus, the muffler is pointing forward, away from the operator position, which would severely diminish the likelihood of ever coming in direct physical contact with it. Also, note the builder's own writing, "Dress up your exhaust tube and add a margin of safety with an exhaust tip or cover".

    "6. The cost of the pipe and fittings is way more than the $2-3 for an approved muffler."
    - Please cite examples. Lawnmower mufflers can cost up-wards of $11 or more (http://www.amazon.com/b?ie=UTF8&node=155375011). Please list for us the exact model number of the specific "approved muffler" for this engine, along with a website showing an average price a consumer could expect to pay in USD. Please cite your sources.

    "1. This idea needs to have the amount of back-pressure figured out so you don't burn the valves or cause power loss."
    - How do you know the builder did not? He, in fact, states quite clearly that, "I drilled 5 3/16 inch holes. I discovered that two holes was not enough and 5 seemed to be OK". Please list the exact exhaust pressure for this specific model of engine, taking into account it's exact current age and operating condition. Please also note what the optimal operating exhaust pressure range should be, taking into account weather and other operating conditions, and an equation showing exactly how to calibrate a custom muffler to operate within these specs. Please also note the exact amount of back-pressure created by the current exhaust system and note whether or not it is operating within acceptable tolerances and, if it is not, how to modify it to bring within the optimal operating specs. Please cite your sources.

    Please also list any degrees, certifications, qualifications, and/or training you have that make you qualified to provide criticism on this project, including your practical experience with federal regulations. Please cite your sources, examples, and proof of above.

    In the future, you may want to try acting like less of a buzz-kill and just enjoy things. You don't need to be condescending, rude, confrontational, or even just plain mean to get attention or start a conversation. Perhaps a better way of approaching the matter would have been to ask, "Hey, have you had any problems with the back-pressure or it flaring up on you". That would have brought his attention to your concerns, over what you perceive to be a death-trap, in a neutral, non-confrontational, and almost friendly way. Then it leaves the door open for him to say if he has had the problems and, if so, how he resolved them, or to note that he hasn't and won't have the problems because he took things into account.

    Something to keep in mind is that most of us who tinker around with these, and more potentially dangerous projects, are most usually pretty darn aware of not only the physics and chemistry of what we're playing with, but the safety precautions as well. This project is about as far from dangerous as one can get without using Play-Do, yarn and blunt-tipped scissors. The engine is so under-powered that even if he had completely plugged the exhaust, nothing more dangerous would likely have happened except the engine dying.

    Of exhaustion. *rimshot*

    You could have handled this all much better and in a far nicer way. You could have at least approached this in a manner that didn't involve insulting people and coming off as such a condescending, arrogant, and mean person. I'm not saying that you are any of these things, just that you are coming off as such. You could have just been social and human.

    Perhaps this can all end here, as I know that I won't bother responding to any further posts here. This is the end of my fun and it's going to just get dumb after this. I really can't believe that you are so oblivious about your behaviour, but perhaps you will keep it all in mind next time you post a comment to a project someone posts and is obviously proud of. Tearing people down is not the way to get positive attention. In the end, however, there's certainly nothing more for you to say as I've really kind of put you in a corner. So let it drop and walk away or you're going to look silly. This isn't a fight, so just be human and social.

    Good luck in your future endeavours. I really do mean that.

    maize5Waste Of Space

    Reply 9 years ago on Step 5

    I'm actually a little upset that nobody has specifically made a comment on my joke about the engine dying of exhaustion. I really thought that was funny.


    Reply 10 years ago on Introduction

    Woah, woah, hold it. No one needs to defend me here - I'm not offfended, and in fact I'm quite proud that my little project has created so much healthy debate! Segcd's concerns are real and legitimate. Yes, this thing gets HOT. I mean real hot. I mean crazy hot. Don't touch it! I did and I got burned! That's why I put the loony muffler tip on it. It didn't help a whole lot, but it looks outrageous, so I like it. People get burned all the time when they step out of their $50,000 AC Cobra replicars onto the shiny muffler. Protect yourself, put a heat shield on it, or design yours better. This is by no means a copyrighted design. Use it, improve on it. Or use it, burn yourself, and give up on it. Yes, it will probably start fires if you throw dry grass at it. Don't do that! The project as I envisioned it was never intended to be used as a lawnmower. I wanted actually to put a pulley and V-belt on the bottom and to power a recumbent bicycle project with it. The lawnmower actually has no blade on the bottom. No, it's not much of a spark arrestor. It doesn't even quiet the engine down all that much. Maybe your design will work better than mine did. Yeah, it's heavy and it WILL bugger up your threads if you don't support the weight of the muffler. Black pipe is much MUCH heavier than the stock muffler. I literally rescued this lawnmower from the curb. It's somewhat of a miracle I could get it running actually, considering it had no working throttle or governor system, and I had no previous experience with small engines. The aftermarket mufflers probably work much better than my pipes, and are probably cheaper and easier to deal with in the long run, AND they're approved for all your spark-arresting, lawn-mowing needs. I have another (much newer) lawnmower for that requirement. I basically created this project to see if it could be done, and to contribute something eccentric and creative to Instructables - a site which I love dearly. I consider it a success on both fronts - it DOES work, albeit with the caveats above, and I'm thrilled that the project was actually picked as a feature on Instructables! As with ALL projects on Instructables... Builder BEWARE! Most of all though, have fun! Now go thread a pipe into your 30 year old lawnmower, yank the starter cord, annoy the neighbors, invite some friends, stand around your creation and be proud. Then put it back in the garage, and go buy a purpose-built, USDA-approved muffler. Lastly, "be nice", as the little pink box says.


    Reply 10 years ago on Introduction

    point of order, Mr speaker, relevance. come on lads, it looks good, that's all that counts. didn't your mother's ever teach you that if you don't have anything nice to say, then don't bother commenting on other blokes instructables.

    Kaptain krunchsegcd

    Reply 9 years ago on Step 5

    No one cares about safety, seriously, there are many more dangerous things than a funkin lawn mower muffler. Second, with a 4 stroke you dont need backpressure, as long as you have some sort of pipe longer than a few inches you WILL NOT burn out your exhaust valve, and there is no way to lose power by not having back pressure on a 4 stroke. gas evaporates fast enough, that if you spilled a bit on the muffler, by the time it got hot enough to ignite gas, all the gas would be gone.


    Reply 10 years ago on Step 5

    Yes, I know it's ridiculous. ;) You can buy premade quiet mufflers for these things too, but I wanted to have a bit of fun and see if I could do it.


    4 years ago on Step 3

    Simple solution which works! I guess steel wool could be used instead of fiberglass tape in the end.


    4 years ago

    Oh my god, that was some beautiful trolling. Now all I can think about is a smoke stack with flapper valve on a junker.... A one man convoy riding the grassy highways of America. Maybe I can replace the self propelling bar to engage an air horn. Getting misty just thinking about my dream mower, like purple mountain majesty.


    5 years ago on Step 3

    Good article, just what I needed! I wasn't sure how to make the bit with the holes. I've used some scrap brass plumbing fittings which which are BSP threaded also and copper pipe for moving a silencer which was uncomfortably close to a carburetor. Copper pipe could probably be used also for making a silencer because it withstands the heat and is probably easier to drill than steel.