Years ago I was discussing small diesel engines on a phone call with George Aldrich and mentioned I had picked up a VA Mini-Elfin engine and the contra-piston was very tight. George said he had the same engine and used a drill press and a wooden dowel to work the fit with jewlers' rouge. Like Tony said, only fit it at the very top of the cylinder, don't go too low.

George Alfrich loved small diesels and I picked up several AH minis for him.

I still have the engine and wanted to put it on an .020 size Playboy or similar so I may dig it out and get to work.

Here's a picture of the engine. This is the .41cc version, I think they made a .020ci version too.