DIY expansion chamber exhausts

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To learn more about how expansion chamber exhaust systems work, read this great primer: How two-stroke expansion chambers work and why you should care Arbeitsweise Zweitakt.gif

Required Reading

To familiarize yourself with two stroke engine tuning, read the Two Stroke Tuner's Handbook by Gordon Jennings as well as Two-Stroke Performance Tuning by A. Graham Bell.

Click here for a .rar file that includes both of these sources.

A slightly better scanned version of the Jennings Handbook may be found here. Another link to Graham Bell's book

Calculating dimensions

  • If you want to learn something about making your own exhausts or you have a special interest in Puch and Tomos exhausts, you should look at this exhaust nomograph.
  • You may also use this calculator with this picture to calculate exhaust dimensions.


Low Carbon steel, stainless steel, or Titanium are common materials for fabricating expansion chambers. Sheet metal around the thickness of 16 gauge (.060") is most commonly used.

Rolling plate

Flat sheet metal is rolled into cones and round sections, which are then welded together section by section. Although time consuming, it is usually the method chosen for development of a new design due to its flexibility, accuracy and low tooling costs. Use this calculator to make templates for the cones.

Cone1.GIF Cones2.JPG Pipes.jpg


Two flat representations of the required finished pipe are cut out of sheet metal. The edges of the two identical flat cutouts are welded together forming a sandwich. On one end of the pipe a fitting is welded and high-pressure water is pumped into the cavity between the sheets. The pressure inflates the flat sheet into its final rounded shape. This method can be quicker than hand forming and only slightly more costly in tooling, however it requires a number of trials before a finished design as accurate as hand formed or stamped can be produced. All curves must be made in a single plane so cutting apart and re-welding is often required but the final product can be as good as a stamped pipe if enough care is taken to be precise.


Flat sheet metal is pressed between a male and female mold in the shape of the required pipe. Each half of the pipe is stamped this way and the two halves are welded together. Stamping requires expensive tooling and machinery and is used only for mass production.