GM-GMC: bolt sizes, thread sealant, bolt sets

Question
This doesn't have to do with diesels but is general: I have a missing bolt on my transmission on a 1988 GMC 3/4 ton Serria 4X4 with a 350 engine, that connects the bell housing to the engine block and I need to know what the size is without removing another bolt? Also if you know what the size of the blots are that connect the flywheel cover to the transmission? What would cause my bolts to loosen like that?

Answer
GM is famous for using a mix of metric and SAE fasteners.

I cannot guess at what might be present on your vehicle.

Your best bet, on the bell housing to engine block bolt, is to remove one of the other bolts to get an exact match.

The inspection cover bolts are not as critical and you may be able to use nut and bolt sets to secure it.

Vibration generally causes these fasteners to loosen.  Use lockwashers or thread sealant to help keep them in place.