head gaskets

Is it a good idea or bad idea and why, to reuse gaskets from one engine to the other of the same make. If the gasket is in good shape, why shouldn't one reuse it?

because one should only do the job once... seriously though, with different expansion and contraction rates of metals in the head, block, and gasket why would you want to take the risk with a gasket thats already been through hundreds of heat cycles and has already been compressed once? its just too cheap and easy to put new head gaskets on it. if you dont have time to fix it right the first time, how are you gonna have time to do it again...

Head gaskets shouldn't be re used. They are designed to crush a certain amount during installation. They crush to fit each individual head and block surface, so putting a used one on a different engine will cause massive leakage. Even putting the same gasket back on the same engine after its been untorqued will cause massive leaks.

The one possible exception is a non-embossed soft copper gasket. They're not typically used on the street for several reasons, but if you're careful they can be reused. They typically require a sealant which makes a mess, but in race applications where you just run them once without sealant and disassemble they can often times be reused.

depends on the gasket. Spring steel headgaskets can be reused, but its not advised by most (although I've been doing it for years without issue)

Use that advise at your own risk of course