As stated earlier the piston sleeve is narrower at the top. If it is tough to turn at the top that is a good thing! As you turn the flywheel counterclockwise the piston will get very tight. With the backplate & head off , watch the piston travel as you turn the flywheel. It should be loose when the piston is at the bottom of its stroke and tight at the top (where it gets tighter). How old is your engine? As the engine runs, it gets hot and the metal expands. You want it to be tight to keep compression in the engine. If you want, you could heat the engine will a heat gun, and then try turning the flywheel, (use gloves or a towel) it should be looser.
You might check that the piston is in correctly, usually there is a half-moon cutout that goes towards the exhaust port.
Also check how the shaft turns without the conrod & piston in the block. If your front bearing is not seated completely if could cause the shaft to have a little wobble. If that is the case it might cause the conrod and piston to bind.
Last edited by C&B RC; 04-14-2008 at 06:16 AM.
Reason: more suggestions