DIY 2-stroke Expansion Chambers

For two-stroke riders looking for a cheap, power-producing alternative to aftermarket expansion chambers, try the following: find the point on your existing pipe(s) where the diameter becomes largest (usually a foot or so from the downstream end of the headpipe) and cut it (them) there. Then braze the end (converging) cone and stinger from a junk dirtbike expansion chamber to it. You're usually dealing with a large enough and cool enough pipe that braze will be plenty strong, if you do it right. Try to find dirt bike pipes off a model with a similar displacement to each cylinder of your bike. I had some success doing this to an old GT380 triple and a friend's GT185 twin years ago. I used two RM125 pipes and one that was probably off a YZ125 for the 380. I used all of the YZ pipe except the headpipe, in fact, because the middle cylinder of a 380 splits into two exhausts...all I could use of it was the headpipe. The YZ pipe tucked right around the rear wheel with a little cutting and brazing, and that thing would do easy second-gear wheelies afterwards, even with no rejetting. The 185 gained more top-end but lost a lot of low-end, and ended up holing a piston. Multicylinder two-strokes sound incredible with unmuffled expansion chambers. This approach saves a lot of involved mathematics and should work on most restrictive old two-stroke exhausts, at least with some rejetting, but be advised that you'll trash your original pipes doing it. Might want to try it on a spare set of pipes first, and if you run it lean and hole a piston or two, blame yourself for not jetting it right and not doing enough math--like I did. Good luck!
Ben - USA
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Advice given in this section may or may not be accurate, safe or wise. Always use common sense and when in doubt assume that any advice given herein is incorrect and liable to end in terminal engine or part failure and possible injury or death. www.ratbike.org and its contributors can accept no responsibility for anything, anywhere, regardless of cause.


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