failure rate is pretty low on the flywheel bolts, but they do fail. less than the valves but they don't fail that often either, especially if the ski is maintained properly and not run in saltwater. I normally don't recommend people change the flywheel bolts unless it has high hours or you are in the pto housing anyway, the kit is only $13. sc fail constantly, even before the 100 hours especially if they have some years on them.