Are you running 6 or 12 volts? It sounds like you've got a mismatch between the starter pinion gear and the flywheel teeth. A 6V flywheel has 139 teeth and a 12V (55-up) flywheel has 168. Both starters have 9 teeth but the gear pitch and diameter is different. A 12V starter has about a 1/2" diameter shaft, and a 6V starter has a shaft about 5/8" in diameter. There is a fiber thrust washer under the rear cover of most starters that is supposed to control the end play. Yours might be worn out.
Jerry


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