exhaust manifold to head bolts

Foe anyone that reads this and is located in NJ there is a supplier called Fazzio's they have everything under the sun aand the sell bolts and nut by the pound. What you pay for three bolts in the big orange box store you can get a pound of them. Trust me if you work on you own trucks cars, etc.... they have everything, metal, aluminum, ss, nuts bolts channel, tubing, flat stock, diamond plate, reg stell , aluminum etc...... look them up on the web, its worth the look.

Dealing with a 2.9, or 4.0 the threads on both of those engines are 8mmX1.25

I recommend drilling then threads out entirely with a clearance drill
which would be a 21/64" drill bit... and re-tap the holes for Heli-Coils.

I've had eight 2.9 engines before I switched t a 4.0 and I long ago gave up using 8mmX1.25 threads to hold the exhaust manifolds on...

THE issue is that getting bolts the correct lengths is difficult, because NOBODY stocks ALL the correct lengths.

at this point I'll tell what I do, I drill the threads out and cut them for 5/16"-18 NC helicoils

EVERYBODY in North America that sells 5/16 coarse thread bolts in a wide variety of lengths, I've been to a few places that had every length from 1/4" through 2" in 1/16" increments....

I helicoiled my 4.0 exhaust manifold flanges for 5/16"-18 and you'd love the bolts that hold my headers on... they have 12-point heads... I tighten them with a 5/16" box wrench.

But with some effort I can get a socket onto most of them, which for a headers installation is amazing

No backing out either as I used special helicoils that act like elastic stop nuts
Technically "Prevailing torque fasteners" so they cannot vibrate loose...

I long ago gave up on using the factory fasteners or even the factory thread standard...

the factory exhaust flange bolts on the heads of the 2.8, 2.9 & 4.0 engine are 8mmX1.25

The problem is that the factory manifolds are of a thickness that requires some odd length bolts...

So What I did was to take both of my exhaust manifolds and but 5/16-18 US standard bolts of appropriate lengths with washers so they stuck through by 12mm or 1/2"

Then I drilled the factory threads out completely with a 21/64" drill bit.

This drill size is the proper tap drill size for the installation of Heli-Coils
for either 5/16-18 OR metric 8mmX1.25.

Switching to 5/16-18 allows me to get bolts in the exact lengths needed and in a variety of materials that suit my internal "Materials Engineer"...


Like when I switched to Headers I could use some 5/16-18 bolts with tiny little 12-point heads that allow me to get a socket or box wrench onto every single bolt.

THE issue with the FACTORY bolts is the method they use to prevent them from vibrating loose... they use a "Truncated root" thread form, that deforms when they are threaded into place.

I employed a sneaky trick of my own... I don't buy my heli-coils from an auto parts store...
I get mine from an industrial supply that caters to machine shops, so I bought special heli-
Coils that have one intentionally distorted thread inside the coil that turn any hole you insert one to into a "prevailing torque" lock-nut...

I personally recommend against using stainless steel bolts with heli-coils
what I recommend is using grade-8 bolts, because there will not "Gall" (Friction weld)
together like a stainless bolt will tend to do to the stainless steel heli-coil...


Over Christmas/New Years last year I had to pull my manifolds to replace them (one had cracked) and a friend who was watching me do it was fascinated that all twelve bolts unscrewed using only a 1/4drive ratchet and a 12-point socket...

My other sneaky trick is that while I use header gaskets from Hedman's Headers I wrap them In 0.1mm (0.004") copper foil.


If there is a job that is more annoying or inconvenient to do it is replacing Header gaskets to fix an exhaust leak...
Well I can't think of it...

Remember, working on your vehicle to make improvements is "Fun"
Working on the same vehicle to do repairs is NOT.


AllanD