Office of Steam Forum for Model & Toy Steam Gas & Hot Air Engines

The correct size is, OD-22mm, ID-20mm, diameter is 1mm. In looking at your screen shot it looks like it shows a OD of 20mm.  I have bought some replacements and the 22-20-1 are a perfect fit.

While I am not sure, I believe that all of Jins hit and miss engines have a 20mm piston. One of his newer engines uses a 11mm diameter piston, but that is not hit & miss.