Re: Water Getting in Top Cylinder Yamaha 40hp Twin

[Don]

Well, usually the flywheel nuts don’t require any Loctite. But, a few drops on the threads only surely can’t hurt. I’m thinking you are correct in suspecting the new crank tapers weren’t mating up nicely with the flywheel tapers. I would surely try some fine lapping compound to see just how the surfaces are mating. Again, please protect the upper seal area, it is probably best to apply the compound to the flywheel tapers only so excess compound will be forced up the flywheel tapers instead of down towards the upper seal area. Just like doing a valve job, you will know pretty quickly how the two surfaces are mating. Needless to say, the lapping compound must be carefully removed, don’t spray clean the excess down into the seal area. The tapers must be clean and dry for installation, no Loctite, grease or anything else. The flywheel key is really only needed to properly locate the flywheel initially. Once properly torqued, those tapers should lock in place. Please find the correct torque spec and use it. Tightening flywheel nuts with an air gun/impact gun is a mistake most of us are guilty of from time to time. Not enough torque, the tapers don’t lock, and the flywheel key will shear. Too much torque will distort the taper mating surfaces, resulting in a sheared flywheel key and unusable flywheel.
I can’t comment about whatever washer Yamaha might use/require for this set up. So, please investigate and follow the OEM instructions, it may be that this washer must be replaced every time, don’t know. As a rule, OMC rarely ever used any washers between the flywheel and nut.