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
Step 2: Assemble the Manifold
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
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
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 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.