So I juuuuust finally finished my tranny swap! Went from a C6 to an Np435 behind a 400. Everything, unbelievably, seems to be working first try!

The sole issue is a common one I know, but I'm not sure how to diagnose it, as I've never experienced it before.

My starter grinds as I start the engine. It powers up, engages instantly, turns strong, and then disengages just as quickly as it always has. The engine then runs fine, and without noise. The issue though is that just aaaas I engage the starter, and while it's turning the flywheel, it grinds pretty bad, erratically.

I know you can shim your starter, but is that the only possible issue??

The teeth of both my starter and the flywheel were fine when installed.

Could the starter be too far out sideways from the wheel, or too close in? And if it iiiis just the usual too far forward or back (depth into the tranny), why does that produce a grinding noise??

AleX