Stock bolts here too. WD-40 on them a few days before the header install and they came right out. Had them out again when I did my engine swap and didn't even hit them with WD, no issues at all.

 

Just got my 05 Denali. Has the rear most two bolts missing on both sides. Just loosened the 4 on the passenger side that were still intact, and luckily didn't break them. I'm going to stud and nut them all with high quality studs and nuts ASAP.

 

The best way I've found getting the bolts out that are broken flush is to use an arc welder and welding a nut to it. Mig welder doesn't work that well but arc welder does. We don't even attempt to drill them anymore.

 

Two of the bolts, one in each end of my driver’s side manifold were snapped off. Just the heads were popped off, so they came out easily. It’s been six years since I installed the headers using factory bolts, and every one is still intact.

 

Just noticed the exhaust leak pattern on my '99, bolt heads still look attached.
Where should I get good replacement bolts?

Is there a gasket I need to replace as well?

 

I got some grade 10.9 allen head bolts from fastenal, seem to work great, just be sure to use some antiseeze