When Flywheel problems showed up on the 1960 Evinrude Bigtwin/Lark 40 horse, they changed the crankshaft and flywheel. The crankshaft and flywheel is heaver and the flywheel key is smaller. The key is only there to locate the flywheel so it is synchronized with the breaker points. The flywheel has a slightly different taper than the crankshaft and when the Flywheel is properly torqued to the required 105 foot pounds, The flywheel hub will expand to tightly grip the crankshaft. For this reason, this is one of the most difficult flywheels to remove. Standard quarter twenty bolts will break. Schedule 8 bolts must be used.
If less torque is used, moisture can creep in and as iron expands as it oxidizes, the rust will weld the two together. Less torque will shear the key, which was never designed to hold the flywheel. Many have lapped the damaged flywheel to the crank to avoid the cost of new parts. It will be very difficult to get the tight grip of the original coupling. It will involve using more compound at the top of the crank to get the proper taper. Yes, heat is often needed.
I recall one motor I had worked on for an hour with a strap wrench, heat and penetrating oil. I decided to take a lunch break and when I slammed the shop door there was a loud report. When I checked to see what had happened, I found the flywheel had popped loose with such force that it was now balanced on TOP of the crankshaft. So, had I just lifted the flywheel and tapped the center bolt one more time, it would have come loose. However it may have ended up on the floor . . . . 😆
