Exhaust manifold fasteners for C3 big blocks

[Mark E.]

I'm researching the proper exhaust manifold fasteners for C3 big blocks.

In this thread, Terry talks about "tri-lobe locking fasteners" used on the outside exhaust manifold cylinders.
https://www.forums.ncrs.org/showthre...light=tri-lobe

In these threads, Joe says this bolt is GM #3909821, and has a "LOCKING THREAD design. In this design, the minor thread dimension (i.e. the bottom of the thread "grooves") is increased by a small amount, thereby making the bolt an interference fit."
https://www.forums.ncrs.org/showthre...manifold-bolts
https://www.forums.ncrs.org/showthre...d-French-Locks

Questions:

- Is GM #3909821 used only on the outside hole for cylinders 1, 2, 7, 8 for a total of four bolts per engine? Or are they used on both holes for these cylinders for a total of eight bolts? (I understand cars with AC or PS use studded bolts which replace one or two of these bolts.)

- How are these bolts designed? Terry's "tri-lobe" description seems different than Joe's description of shallow cut interference threads. I want to understand what I'm looking for when I order these.

[Joe L.]

Re: Exhaust manifold fasteners for C3 big blocks

I'm researching the proper exhaust manifold fasteners for C3 big blocks.

In this thread, Terry talks about "tri-lobe locking fasteners" used on the outside exhaust manifold cylinders.
https://www.forums.ncrs.org/showthre...light=tri-lobe

In these threads, Joe says this bolt is GM #3909821, and has a "LOCKING THREAD design. In this design, the minor thread dimension (i.e. the bottom of the thread "grooves") is increased by a small amount, thereby making the bolt an interference fit."
https://www.forums.ncrs.org/showthre...manifold-bolts
https://www.forums.ncrs.org/showthre...d-French-Locks

Questions:

- Is GM #3909821 used only on the outside hole for cylinders 1, 2, 7, 8 for a total of four bolts per engine? Or are they used on both holes for these cylinders for a total of eight bolts? (I understand cars with AC or PS use studded bolts which replace one or two of these bolts.)

- How are these bolts designed? Terry's "tri-lobe" description seems different than Joe's description of shallow cut interference threads. I want to understand what I'm looking for when I order these.

Mark------


As far as I know, the "locking thread" bolts were only used on the manifold end bolts. However, when a stud was used on one of the end bolts, I believe the stud may have also used the locking thread. I'm not sure of this, though.

Attached are photos of a GM #3909821 bolt, the bolt originally specified by GM. The original manufacturer's ID marking may have been different than that seen on the pictured bolt. However, otherwise I believe the head configuration was the same. Note the "hexagonal" design at the perimeter of the head. I believe that this design denotes a "locking thread" type bolt.

These bolts were used on big blocks from some time in 1967, if not from the outset, and through 1974.

[Mark E.]

Re: Exhaust manifold fasteners for C3 big blocks

Thanks Joe!

I've seen bolts with head markings like this in years past, but didn't appreciate what they were.

To clarify:

- Only four of these per engine, one at each end of each manifold. And in cars with PS and/or AC, one or both of the two front bolts were replaced with bolts which feature studs. So for my car with PS and AC, only two of these special bolts are used, one at the rear most mounting hole for each manifold. Correct?

- Aren't the "tri-lobe locking fasteners" Terry mentions a different design, perhaps for a different application? If these are what I think they are, they have an uneven threaded end, shaped a bit like a 3-leaf clover. Can't remember where I've seen these.

[Joe L.]

Re: Exhaust manifold fasteners for C3 big blocks

Thanks Joe!

I've seen bolts with head markings like this in years past, but didn't appreciate what they were.

To clarify:

- Only four of these per engine, one at each end of each manifold. And in cars with PS and/or AC, one or both of the two front bolts were replaced with bolts which feature studs. So for my car with PS and AC, only two of these special bolts are used, one at the rear most mounting hole for each manifold. Correct?

- Aren't the "tri-lobe locking fasteners" Terry mentions a different design, perhaps for a different application? If these are what I think they are, they have an uneven threaded end, shaped a bit like a 3-leaf clover. Can't remember where I've seen these.

Mark-----

As far as I know, your car with C-60 and N-40 would have used only 2 of the 3909821 bolts. However, I'm pretty sure that the studs used at the other two end locations used the same type of "locking thread".

I am unfamiliar with the type of "tri-lobe" fasteners that Terry described . However, I do know that GM #3909821 bolt, still available today from GM, is the part number of the bolts originally used. So, unless the design was rather drastically revised at some point, without a change in part number, I don't understand how the "tri-lobe" design could have had the 3909821 part number.

[Rick N.]

Re: Exhaust manifold fasteners for C3 big blocks

Big blocks used 8 locking bolts.
2 on each end of the exhaust manifold.
Two for Cylinder #1 and #7
Two for Cylinder #2 and #8
The exception would be if power steering or A/C is used which require the studed bolts.

[Joe L.]

Re: Exhaust manifold fasteners for C3 big blocks

Big blocks used 8 locking bolts.
2 on each end of the exhaust manifold.
Two for Cylinder #1 and #7
Two for Cylinder #2 and #8
The exception would be if power steering or A/C is used which require the studed bolts.

Rick-----


What you say is possible but it's not my understanding of how it was done.

It's also possible that due to confusion at Tonawanda, it was done both ways.

[Mark E.]

Re: Exhaust manifold fasteners for C3 big blocks

Is there an "engine AIM" to answer these questions?

And why didn't Chevy use interference threads for all manifold bolts?

[Terry M.]

Re: Exhaust manifold fasteners for C3 big blocks

Here it is:

http://www.abs-metal.com.tw/tapping-screws-series/tri-lobe-thread-pt-thread-screws

[Mark E.]

Re: Exhaust manifold fasteners for C3 big blocks

Here it is:
http://www.abs-metal.com.tw/tapping-...-thread-screws

That's a tri-lobe. But not the design of the 3909821, as shown in Joe's post. Right?

[Joe L.]

Re: Exhaust manifold fasteners for C3 big blocks

Is there an "engine AIM" to answer these questions?

And why didn't Chevy use interference threads for all manifold bolts?

Mark------


There were engine AIM's. I even have a copy of the Mark IV engine AIM for the 1971 model year. However, unfortunately, it does not show part numbers for any of the engine components. Apparently, this was handled separately with a "Bill of Materials" for each specific Mark IV variant that was being manufactured and I don't have those documents.

[Joe L.]

Re: Exhaust manifold fasteners for C3 big blocks

That's a tri-lobe. But not the design of the 3909821, as shown in Joe's post. Right?

Mark------


Yes, the 3909821, at least the examples I have seen do not use the tri-lobe locking design. They use a "shallow thread" design.

[Mark E.]

Re: Exhaust manifold fasteners for C3 big blocks

Big blocks used 8 locking bolts.
2 on each end of the exhaust manifold.
Two for Cylinder #1 and #7
Two for Cylinder #2 and #8
The exception would be if power steering or A/C is used which require the studed bolts.

Rick,

Is hour information based on your experience with original cars? Or another source?

[Rick N.]

Re: Exhaust manifold fasteners for C3 big blocks

Hi Mark,
My information is based off my experience.
I've worked for a Corvette service/restoration business for 20+ years.

FWIW... The 5th edition 70-72 judging manual has it mentioned using 2 bolts per outer cylinders.

[Mark E.]

Re: Exhaust manifold fasteners for C3 big blocks

Thanks Rick!