My engine was also very tight, had to use a screw driver on the flywheel a fair few times to get the piston unstuck, that was with my modified GX5-R Godzilla engine that had a matched piston & sleeve, pre-heating makes a big difference, but for the first few tanks I was having trouble because I could only pre-heat to 160F so decided to keep the engine going & run a few tanks together instead of just one, saved alot of stuffing around & probably saved my starter box from stuffing up completely as well. Slightly loosening the glow plug made things a little easier too.