Bolts like header bolts stay locked tight because of tension loading. The boat either compresses something elastic or stretches enough to keep preload on the bolt. The preload distortion and friction prevents it from turning.
If a header bolt is coming loose either something in the bolt clamping stack, like a gasket, is yielding and the bolt is going to zero tension as it shakes around. The stack the bolt is compressing gets shorter. Then it can rattle out.
The other cause is something is the bolt itself is stretching or the threads are pulling out.
Either you have soft gaskets compressing and letting the stack get shorter which removes tension, or the bolt or threads are yielding and not staying in the elastic area that maintains tension.
The only times in my life I have had header bolt issues was running thick soft gaskets. I'm sure some bolts are junk, but I've never run into that.
I have had a 40 pound turbo hanging off my header and never once had a bolt loosen, and I never used anything special for hardware or glue. As a matter of fact I coat ALL of my threaded things going into aluminum with anti-seize lubricant (just like I do lug nuts that get repeatedly changed). I never have and never will use a lock washer or thread glue on header bolts and I never every have a problem unless I use a weak gasket.
My headers get red hot in a pass and have 40-50 psi exhaust pressure and the bolts never come loose. I don't care what fastener I use, Cal Custom, Auto Zone, or fancy pants ARP. I do make sure my flanges are flat and I never use gaskets other than a special high temperature RTV.
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89 LX. 363, single turbo, Super Vic EFI, TFS high port heads by TEA, solid roller, glide. Holley HP EFI. (exact combo varies)