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

Another thing to try, in fact is easier than what I posted below, so might try this first.

Unscrew the other end of the connecting rod where it screws into the crank. Then rotate the flywheel to where the crank is furthest away/outboard from the piston (ie the longest part of the stroke). Then with your hand, pull the connecting rod out to the same distance as the crank pin hole.  You might be able to feel with your fingers on the connecting rod whether the piston is hitting the cylinder end.  Repeat for the 180 degrees position of the crank, ie the most inboard travel of the crank.

You can of course do this without disconnecting the connecting rod from the crank, but I find it is easier to feel the piston hitting and so easier to diagnose.

Once again, this may not be your issue, but it is a good place to start your troubleshooting.