To make a long story short, I was changing the midway gasket between the 4-2 pipes in my header, and found out one nut has come off and is gone while driving, and one of the other 3 was loose almost all the way down the thread. Should I use lock washers?

 

I made a replacement gasket with expensive material. I think the reason is either because of the anti-seize I used too much, or the gasket shrunk so much that there was significant space between the nuts and the bolts. Cause I tightened the crap out of them.

 

I'd have to agree with the gasket shrunk that happen between my haeder and cat

 

Check www.fastenal.com for locknuts that can withstand heat. You DO NOT want to use nylocks or starnuts, but Fastenal has various other locknuts that should work well.

 

this happened to my header too. I decided to use lock washers and I haven't had that problem again.

 

Like Kommando said, use lock nuts. But of course any type involving plastic is just going to melt. Get the type that has 3 of the 6 sides dented in so they aren't quite round.