So I have a 66 mustang coupe with a 289 block and hooker long tube headers. Everything was fine and fitting fine. But then i started getting a bit of an exhaust leak so i had to change the gasket on one of the headers. Sounds and leak went away for the most part. Buuuutt.... one of the bolts will not go in now. Its either one of the end bolts that just wont go in. I tried and tried and cant figure out how to get the last bolt in. I figure the headers have moved around with the use. Any help will be appreciated.

 

Loosen all the bolts a bit. Then replace the missing bolt.

This is one of the reasons I switched from headers to 289HP manifolds, I got tired of tightening header bolts. The manifolds have never come loose or leaked.

 

+1 to that. loosen all of them so you can move the header around, put all bolts in then tighten.might even be the gasket in your way.
Best thing to do is remove all bolts, then use a gasket that will not leak.
the felpro 1415's are quite good but if you have any bit of warp going on then there's a set fo header gaskets that are compressible. material similar to the exhaust donuts

 

If loosening all the bolts doesn't do it you may have to try enlarging the hole in the header flange or grinding the bolt end to somewhat of a point.

 

That's why so many Ford original bolts had a dog point.

 

I would try removing all the bolts for that header. Then, thread the bolt into it without the header to make sure you don't have another issue. Align the gasket with the header and double check alignment with the block. Next, install the header with that bolt first.
Do not tighten it all the way. On my headers, I had to thread a bolt in with the headers tight to the heads. Otherwise, I couldn't get a wrench on it.

One of the counter guys at my local performance shop suggested that I wrap the bolt threads with teflon tape before installing. It is supposed to act like a binder without locking it up.

 

Oh i did all that, first it was the bolt closest to the firewall that didnt want to go in, took them all off, then put that one in first, and now the one closes to the grill wont go in, im pretty sure the header got warped a bit over time. I tried loosening them all and putting them in like that, no success. Spent around 2 hrs trying to get it to work with no luck.

 

Welcome to the wonderful world of headers. They are always like that, even the expensive ones.

 

One of several reasons I yanked the headers and went back to 289HP manifolds years ago.

 

This is when you insert a die grinder in the hole and make it a slot.