Thank's
The flywheel diameter is 108 mm. The cold cylinder was in brass because I had not yet made a new one in cast iron. The first version of the engine had both cylinders in brass and both pistons in aluminium with quadring seals, but it didn't work well. When I made the hot cylinder and the hot piston with cast iron, this was much better. And finally I made the cold cylinder and his piston in cast iron and this time it works very well.