Moby
GOLD Star
Just so I have seconded this, we tried recrimping the tops on my old 80 exhaust stud nuts also, they're not reusable and the threads are actually a little tight to start with when new, sort of a built-in locking/intentional crossthread feature.
If I'm remembering correctly they have 2 (I think) little knubs near the top of the nuts that provide part of the interference fit. When you re-use the nuts the top portion of the nuts don't provide enough tension to drive the nubs with enough force into the threads of the studs because they've been expanded once. Sometimes there is enough tension left to work OK. If you tap them you remove these nubs and probably completely lose the interference fit.