Locking exhaust manifold studs on a L89

[Joseph W.]

I am in the process of finishing up the build on my 1969 L89 coupe.
This car has power steering so the stud is the most front spot on the drivers side exhaust manifold.
the stud part number is 3928905.
You can not run a 3/8-16 nut on it.
My question is, will it strip the soft aluminum stud hole in the 842 heads?
Are these studs a one time use type of thing?

thanks for any help

[Gary B.]

Joseph,

In an old post Joe Lucia said “The end stud for cars with power steering was GM #3928905. This stud was 3/8"-16--5/16"-18X 2-5/32". It was discontinued quite some time ago and replaced by GM # 14103124. I do not know the configuration of the new part number.”

Gary

[Joseph W.]

Thanks Gary

I have the correct stud with self locking threads.
Joe Lucia posted on a 2007 thread about this stud a long time ago, and he asked the OP to take a correct size nut and try and thread it on the stud. You can’t do it because of the self locking threads.
It’s this self locking feature that eliminates the French locks on the exhaust manifolds.
My concern is that it will mess up the threads on the 842 aluminum heads when I thread the stud in place.
It sure feels like it would strip the threads.
I guess I’m also curious about what’s different with the threads on these studs.

[Patrick B.]

I think you are right about being careful with the threads in aluminum. I would run a die on the stud thread and use blue locktite if you think it would loosen up

[Joe L.]

Thanks Gary

I have the correct stud with self locking threads.
Joe Lucia posted on a 2007 thread about this stud a long time ago, and he asked the OP to take a correct size nut and try and thread it on the stud. You can’t do it because of the self locking threads.
It’s this self locking feature that eliminates the French locks on the exhaust manifolds.
My concern is that it will mess up the threads on the 842 aluminum heads when I thread the stud in place.
It sure feels like it would strip the threads.
I guess I’m also curious about what’s different with the threads on these studs.

Joseph-----

I do not know if the locking thread studs (or bolts) were actually used on big blocks with aluminum heads. However, I am very reluctant to use them with aluminum heads. In that regard, I did not use them on the "ZL-1" engine I built.

[David M.]

Get yourself some pitch gauges/identifiers and find out what the heads threads are. Then locate the matching fastener.
Is this double sided fastener with two different thread sizes? Do you have pictures?

Do not run a tap down the hole. Use a thread "chase." Each time you run a cutting tap down a fixed diameter you are widening thread interference making the fastener less effective at clamping the designed load. A chase just cleans out the mountains n valleys without much if any material removal.

There are sooo many different thread types. 1/4-20 is a 1/4-20 but you need to consider the pitch angle, valley peek & depth, round vs flat et al. Do a little home work.
ANSI example https://www.engineersedge.com/screw_threads_chart.htm

The fasteners and gauges are available at several specialty vendors. If you have an old school hardware store they may be of some help. I'd recommend grade 5. Grade 3 is not enough grade 8 is too much.

https://www.mcmaster.com/products/thread-gauges/thread-size-identifiers-2~/

Use a small amount of Henkel 243 primer-less removable loctite on clean fasteners going into aluminum with a steel fastener.
Torque according to the softer material...aluminum in your case.
Never use green or red on this application unless its on as heli-coil.