Auto Parts: P-30 Starter, fly wheel, cars ford

Question
QUESTION: I have to replace my start about every 6 months, what can be causing this?, This is a rebuilt engine by GM so the starter should from the beginning be the right one, NAPA looked it up because they can't come up with and answer. My first problem was the first starter ate the fly where we replaced both, since within 11/2 years 3 more starters, they seem not to stay engaged long enough to start motor. The starters need two spacers to line up on the fly wheel,when new it starts nice and fast, then will start to disengage to soon to start the engine after 4,5 months to the point I may sit for 5-10 minuets sometimes longer trying to start my Chevy P-30 food truck, any help will help.

ANSWER:      It's almost certainly the shims.  Buy a pack of shims and experiment with which one works best.

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QUESTION: Question so I understand what I'm looking for, by adding or taking away shims (there are two shims in place now)from the existing starter that will not stay engaged long enough to start the engine, should correct the problem and hold the starter gear out long enough to start the engine or experiment with a new started which I know will start right away, I think I just answered my own question. If I am on the right track experimenting with shims on the existing start, how will this help the cycloid to stay in engaged long enough to push that gear out long enough for the engine to start/ Sorry for the questions, but understanding what and why helps me in the long run.

Answer
    Shimming the starter motor controls the distance traveled by the starter drive on engagement.  There has to be some way to do this on all cars.  Ford and Chrysler do it by manufacturing to a closer tolerance; GM does it by shimming, which is controlled by the shims and the installation procedure.