Re: weird "slipping" noise while starting, starting getting harder...
There are very few times a shim(s) are needed but it is immediately obvious when they are. You won't find it in most GM service manuals as the problem tends to occur only with rebuilt or after market starters. There is a pinion tooth to flywheel tooth spec which could be checked with the engine out of the boat and the starter installed. I'll dig that spec out and post it. I want to say it's around .030" but don't hold me to that. The good thing is that the starter need only be loosened to slip the shim in, provided the one bolt hole is notched. As far as I know, there is only one thickness and I've never had to use more than one shim to solve the problem. To complicate matters, there is no way on most boats to view the pinion/flywheel engagement so its a "try it and see" situation. The telltale sign is a "grinding" sound during rotation, not the "teeth being chewed up screech" noise a pinion makes when its not fully engaging the flywheel. The reason some starters require shimming is due to "build-up of tolerances" which is engineering speak for you have a bunch of parts that are all at the upper or lower end of their manufacturing tolerance (or out of tolerance) and the combination creates interference. Flywheel OD at max, starter pinion at max OD, starter housing at min OD, and block mounting surface at minimum all combine to place the pinion closer to the crankshaft centerline. In practice, if those specs are met, the starter should work without shimming. However, inspection of every piece-part no longer occurs either at the manufacturer or upon receipt at the end user. That, and the fact one never knows where individual parts come from these days, it's quite possible for a rebuilt starter to fit too well so to speak. Generally a single shim between the starter and the block is all that's needed to quiet things down. In my opinion, the main culprit is the starter mounting housing. Those starters get rebuilt over and over and some shrinkage is inevitable.
