Exhaust Manifold Bolts

[e_digg]

I have a 91 360 with stock headers. I am looking for the exhaust manifold bolts and I understand they are different sizes but the 3/8" inch bolt seems very small in the manifold hole itself. The 5/16" bolts fill the space. What am I missing? See pic to get a better understanding.

[babywag]

not missing anything...
for whatever reason they used 2 different sizes.

[FSJunkie]

Why would you say a 3/8" bolt is too small and a 5/16" bolt fills the space better? That makes no sense. 3/8" is larger than 5/16"! I think you have the two sizes confused.

The space around the bolt allows for thermal expansion of the exhaust manifold. Big chunks of cast iron expand quite a bit when they get as hot as exhaust manifolds do, and the manifolds would crack if they were not allowed to shift and "creep" around the bolts as they expand and contract. The bolt holes in the center of the manifolds are smaller because they are there to hold the center of the manifold in place while the far ends of the manifolds expand and contract. That is why the bolt holes out at the far ends are larger than the bolt holes in the center.

You see this even more on big long inline sixes and inline eights.

[e_digg]

Why would you say a 3/8" bolt is too small and a 5/16" bolt fills the space better? That makes no sense. 3/8" is larger than 5/16"! I think you have the two sizes confused.

The space around the bolt allows for thermal expansion of the exhaust manifold. Big chunks of cast iron expand quite a bit when they get as hot as exhaust manifolds do, and the manifolds would crack if they were not allowed to shift and "creep" around the bolts as they expand and contract. The bolt holes in the center of the manifolds are smaller because they are there to hold the center of the manifold in place while the far ends of the manifolds expand and contract. That is why the bolt holes out at the far ends are larger than the bolt holes in the center.

You see this even more on big long inline sixes and inline eights.

The end or outer ports bolt holes are 5/16" but the hole in the manifold that the bolt goes through is more than 1/2". The inner ports are 3/8" and the holes in the header are about 1/16" bigger in diameter. I am thinking that I need to try and find something like this to make it work (less spring of course).

[babywag]

do whatever you wish but normal bolts work just fine.

[asphaltrockdweller]

do whatever you wish but normal bolts work just fine.

Yeap!

[gopher_6_9]

do whatever you wish but normal bolts work just fine.

This.

They seal fine if your manifolds are true.

[Bob Barry]

That clearance is to accommodate the expansion/contraction of the cast-iron manifold. You need a strong washer under the head of the bolt to put appropriate clamping-force on the manifold.

[nograin]

See if you can find a set of original bolts. Might be the same as used on other AMC engines. The tip of the bolts is hollow. That detail apparently was worth the expense to insure the bolts maintained proper tension in threads.


Exhaust-Manifold-Bolts-3527

[Bob Barry]

See if you can find a set of original bolts. Might be the same as used on other AMC engines. The tip of the bolts is hollow. That detail apparently was worth the expense to insure the bolts maintained proper tension in threads.


Exhaust-Manifold-Bolts-3527

Chrysler did NOT think it critical enough for the 5/16" bolts that they sold over the counter in 1998 to be hollow, though (the 3/8" replacements were still hollow).

I think the main usefulness of the drilled-centers on the bolts is to give you a nice pilot-hole for an EZ-out to remove them when they break off in the heads...

[nograin]

Chrysler did NOT think it critical enough for the 5/16" bolts that they sold over the counter in 1998 to be hollow, though (the 3/8" replacements were still hollow).

I think the main usefulness of the drilled-centers on the bolts is to give you a nice pilot-hole for an EZ-out to remove them when they break off in the heads...

Chrysler probably couldn't care less about the AMC engines in '98.

Seriously, I thought I had a couple web links about the history and reasons AMC went this route for those bolts.
Can't find them right now.

edit: Well here's a few.

http://theamcforum.com/forum/forum_posts.asp?TID=26371&PID=307615&title=exhaust-manifold-bolts#307615

http://theamcforum.com/forum/forum_posts.asp?TID=73710&PID=666875&title=390-exhaust-manifold-bolts#666875

http://theamcforum.com/forum/forum_posts.asp?TID=90721&PID=805075&title=i-need-hollow-exhaust-manifold-bolts#805075

If the information is correct:
Maybe as early as 1969 AMC started using these bolts, and was a service replacement very early in the 70s.
Made by Rockford and at least some have special head markings (I'll have to look at mine again)
Grade 5, Zinc Plated and "waxed", 3/8-16, Class 3 thread, drilled thread end so they wouldn't loosen.
Use with a thick strong washerunder the head .

[e_digg]

That clearance is to accommodate the expansion/contraction of the cast-iron manifold. You need a strong washer under the head of the bolt to put appropriate clamping-force on the manifold.

NO! There is an extra 1/4" gap on the outer ports and that cannot be for expansion. Waaaay too much! You are correct on that with the inner bolts and there is minimal clearance there.

[Bob Barry]

NO! There is an extra 1/4" gap on the outer ports and that cannot be for expansion. Waaaay too much! You are correct on that with the inner bolts and there is minimal clearance there.

Well, the outer ports used a 3/8" bolt up until '78, and the passenger-side manifold design remained the same. Perhaps when they switched to the 5/16" outer bolts in '79, they did not change the exhaust-manifold blueprints for correspondingly-smaller holes on the outer ports, or perhaps your manifolds came off an earlier truck?

[FSJunkie]

It worked fine from the factory. Just put it together the way the factory did and it'll work fine. Problem solved.

[e_digg]

Thanks for all the input. I'm going to go and get some grade 8 bolts and use those.