Starter not engaging?

[Tomahawk30]

Hey guys... I've been searching for similar threads, but haven't found exactly this issue described elsewhere. In a nutshell, the starter will spin, but doesn't engage the flywheel consistently. It was starting fine when I got it back from the shop that replaced the engine and did my paint and body work. After starting it a bunch of times to troubleshoot a carb / timing issue, it started just making a grinding sound about 9 times out of 10. Every so often it would engage fully and fire right up.

I initially suspected the flywheel, but it looks perfect. I pulled the starter and took it in to NAPA where they said it tested fine. When I was taking it back to reinstall, it rolled off the tailgate and busted the solenoid housing. I replaced the solenoid with a new one from NAPA, reinstalled, and got exactly the same issue again. The starter tries to engage, but it sounds like the starter isn't extending far enough to fully engage the flywheel gears. When I had the starter off, I could see the gears on the starter side were wore at the very leading edge. I tried shimming it, but when I do that it doesn't even touch the flywheel gears at all and sounds like it is just spinning free.

At this point, I suspect that the starter isn't engaging far enough into the flywheel gears. Is it possible that it would look good on a bench test, but not have enough force to engage properly into the flywheel gears? I don't know enough about the indicators of a failing starter to troubleshoot it very well. I might just pick up another starter tomorrow and give it a try, but thought I'd see if you guys have any input first....