3.4 header bolt confusion

[malc_p]

So, I got a set of pacesetters for my '95 3.4 bird (I may write up the issues I had to resolve in fitting the damn things later!:rant:). Problem I have is with the header bolts - none came with the headers. They're a 5/16, and I would have expected 18tpi (thread per inch), as this is the standard for American Unified Coarse (UNC). Nope. These measure out at 20 tpi. Which is weird - Unified Fine (UNF) should be 24tpi so they're not UNC or UNF. I've tried Metric 8mm but that is about 21tpi, and an M8 bolt goes in 3 turns and locks, so they're not M8 either.
Anyone have any clues? At present I've got them loosely fitted in place with the old bolts and spacers, which I will NOT take the car on the road with!
Cheers
Malc (UK)

[blackandgold]

Re: 3.4 header bolt confusion

Stage 8 makes some really nice bolts for them, whether you use the locking fasteners or not, and they do fit perfectly, thats what I had on my 3.4l

[malc_p]

Re: 3.4 header bolt confusion

Well, slap me silly! A bit of research tells me the ARE an M8!:eek:
I need an M8 tap to clean the old holes out, which is why I couldn't run a new M8 all the way in (the old bolt I took out is a bit rusty so I misread the thread pitch on it).
Duh.

[viper98885]

Re: 3.4 header bolt confusion

LS1 exhaust manifold bolts make things very easy because their head is a 10mm which is much easier to fit into the tight places.

[malc_p]

Re: 3.4 header bolt confusion

I've sourced some allen-head (socket head) bolts in high-tensile that should do the job. One thing that looks weird though is the thread profile - the old header bolt (on the right in the attached pic) seems to have a flat area between the threads and a narrower thread profile whereas a new M8 (left) has the classical zigzag profile. Might have to find out if there's a special profile for header bolt threads?

[viper98885]

Re: 3.4 header bolt confusion

There is not a "special profile." That is what you will see from different bolt manufacturers.

[Bandit127]

Re: 3.4 header bolt confusion

Nothing really special about these bolts, just M8.

I have a hodge-podge of bolts holding my headers on, from Stage 8 to bolts bin to even a couple lower intake mani bolt from a 3100. LOL oops.