look through the exhaust port and put the piston at top dead center. Then remove the backplate. Take a small wooden dowl or toothbrush handle and place it to where it locks the rod in place and wont let the engine turn over. You should now be able to remove the flywheel nut without the engine spinning. However they do make a piston stop tool and is fairly affordable but the other method always worked for me. Hope that helps. also I would replace the collet that the flywheel sits on because it is closing prematurely or you could just hold the flywheel and remove the nut.