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

Edward - Benny makes good points.

I would add that I have a nice D20 I acquired from our honored Member classixs.  A great engine out of the box, but I noticed a knock developing over time. You might check that your piston rod is not backing out of the piston.  Try removing the screw holding the connecting rod to the piston rod and then twist (turn) the piston rod clockwise to see if it feels like your are tightening it into the piston.  If the piston is traveling too much one way or the other, it may cause a knock.  May be not your issue, but it was mine.

You may have to remove the cylinder cover (shroud, painted blue or gray) to really get a good look.

Scott