Exhaust collars

Has anyone had luck removing exhaust collars, and reusing the copper gaskets without having an exhaust leak?

You will want to buy new exhaust gaskets. Once crushed, they will not reseal if reused. If you really need to try it, you could try annealing them before reuse... that is, heat them red hot with a torch then drop them into water. This will restore the copper someone and they may recrush some but I wouldn't bother.

I've removed and reinstalled many exhaust pipes without having to replace copper rings. That said I always had a set available in case I had a leak. More got lucky than anything else. A leak is noisy and can damage exhaust valves by allowing a stream of oxygen to enter exhaust port. You might get lucky but best to have a set on hand and do not drive with exhaust leak.

Jeff Saunders at z1enterprises sells them for around $5.00. I keep several sets around. I have a 70% hearing loss and i can hear a exhaust leak. Roy