[EricH]

If you turn down a gear .020"...

... Your chance of the gear slipping will go up, albeit slightly.
... Your chance of messing up the tooth profile will go up--you really don't want to mess that up, so make sure you clean up carefully after whatever machining op you do.
... If the gear is heat treated, you're probably going to change the heat treating. This will probably lead to unintended consequences, good and bad (but most likely mostly bad).


But, you should be able to get away with it. For a little while, at any rate.

Now, I have to wonder... Why not just move the gear .020" farther away from the other gear, in some direction? And, what's with asking if the CIM comes in under its stated diameter? If you're building a really tight gearbox, I suggest that you also CAD in the assembly tools for that gearbox, just to make sure they will actually work...