I just replaced my starter with an F body LT1 starter. Sometimes it works just fine, sometimes all I hear is the clank of what I assume to be the gear actuating and just hitting the flywheel gear and it doesn't crank. I've tried playing with the alignment and it still does it sometimes and I can't start the car. I tried it with no shims, and I tried it with both included shims. The one I took off was also an LT1 starter and it worked perfectly (before it went) without shims. Might i need more shims? What else can I check? It never grinds, just a clink and nothing. The battery is fine. When it does work, it works great.

 

Did you check to make sure all of the teeth are still there? Sounds like you either have a section of the flexplate that's missing teeth or you have the mesh set too tight.

 

Presumably, when it goes 'clank' it's not spinning?

Sounds like the contacts in the solenoid itself are toast (it'll still pull in, just won't spin).

 

Had the exact same problem with my LT1 (parts store rebuild) starter a few years back. You're right- the clank is the solenoid trying to extend the gear but it just hits tooth-to-tooth against the flexplate gear and doesn't drop in. I actually crawled under the car and watched the engagement, or rather the lack of it. My case was rather extreme- sometimes I could hit the starter a dozen times or more in a row before it would engage. Once it did engage it would crank the engine over just fine. Like you, I tried shimming a zillion different ways with no effect.

Long story short, the spur gear on the starter was simply machined incorrectly. I took it back for an identical replacement and it worked fine forever after and with no shims.

While I had the new one and the old one laying next to eachother I compared the gears on them. The "problem child" had noticably fatter teeth on it than the good starter.