Some causes.
- Overheating
- Age and coolant rot.
- Faulty materials/design. Rare.
- One that I may veer off of from the rest of the world and that is the lack of cylinder head bolt retorquing in regards to all aluminum or aluminum head engines.
I understand the principle of Torque to Yield bolts but I’m just not completely sold on it.
Back in the 80s Nissan had a policy of head bolt retorquing (on their dime) and did away with it in a cost-cutting move. This was soon followed by numerous head gasket leakage problems from an oil pressure port. Just one example.
If the head gasket is leaking externally some goop 'em may stop it. If the gasket is leaking into the combustion chamber then it needs to be replaced.