...For a rebuild,which bolts would you definitely not re-use?
...head bolts...rod bolts..main cap bolts???

If it's a stock motor you should be fine with the factory hardware. If it's something that is going to be driven with enthusiasm then you'll need to swap head, rod, and main bolts.

It's cheap insurance on any build.

Heads and mains should be fine. I like the idea of replacing rod bolts BUT that has to be done before and only if you're resizing the big end.

Correct. Again, if you plan on driving with enthusiasm it is a necessary evil.

Rod bolts! They are the most stressed part in the engine. Big end of rods are usually re-sized after replacing the rod bolts.

All the rest of the bolts can be reused if in good condition. I have had issues with the stock 1/4 bolts breaking (valve covers, and other places they might be used?) That's likely from me over tightening them.

Factory bolts...

Heat-cycled, stretched, and stressed who knows how many times...


Bolts...

New bolts...


Better to have them, than wish you did...

...and because of the weird 7-degree angle big-block valve covers screws are installed at!

Rick

If your going to do a stock type build with rings, bearings, valve job and maybe a little bigger cam I would take the rods with the stock bolts torque to 45 ft lbs with oil to someone who has a good dial bore gauge and have them check the roundness of the rods(all 8) big end bore. If they are all round I would, and have many times, reuse those rods and put the money for the new rod bolts and resizing the big end of the rods to a better use If you don't know the history of the motor and you have ANY doubts about reusing the rods or bolts swap them out

.....grade 8 bolts are very hard.I'm thinking they would break if they were to fail?
who have seen a broken or stretched main cap or head bolt?

I've always reused main cap bolts.

Head bolts usually get replaced, but not always. My brother has assembled many engines for himself and friends and says used head bolts will produce a rubbery feel during the final torque down. Doubt they would ever break, but they may not hold tight enough which could mean the head gaskets could fail. Best to replace if in doubt.

Rod bolts? I always get new ARP ones.