L78 failed Rocker Stud after 50 years!!!

[Gary C.]

One of my Rocker Studs failed with metal fatigue on my 396/L78.
I think I should replace them all.
All as I can seem to find on the net are aftermarket roller rocker types.
Does anyone know where I can find NOS studs?

Thanks,
Gary
Perth, Western Australia.

[Gary C.]

Re: L78 failed Rocker Stud after 50 years!!!

Measuring the old stud I would say the ARP equivalent would be this one;

https://arp-bolts.com/kits/arpkit-detail.php?RecordID=393

[Gary C.]

Re: L78 failed Rocker Stud after 50 years!!!

Here are the dimensions;

[Joe L.]

Re: L78 failed Rocker Stud after 50 years!!!

Measuring the old stud I would say the ARP equivalent would be this one;

https://arp-bolts.com/kits/arpkit-de...p?RecordID=393

Gary------


Yes, these are the ones (even though there is an error in the listing which shows them to have metric lower thread. The 135-7201 actually has 7/16-14 lower thread). They have an even stronger stud available in their Pro Series line with p/n 235-7201.

Here's the problem, though: ARP does not recommend the use of any of their rocker studs with stock-type, self locking nuts. I have used them with self-locking nuts without apparent problem but that does go against the ARP recommendation. In addition, the Federal-Mogul/Speed-Pro stud which replaces the original GM stud comes with the same warning.

By the way, I consider the stock-type self locking nuts to be the absolute best design. If it were possible to use them even with roller rocker arms, I would do so.

The GM studs have been discontinued for quite some time.

[Patrick B.]

[Carl N.]

Re: L78 failed Rocker Stud after 50 years!!!

Change all of them. Check you springs and push rods if they are original they need an upgrade. Inner springs are prone to failure. Lots of good after market options for BBC does not have to be Corvette specific. Hope NCRS never makes you pull valve covers for judging. Always change locknuts !

[Patrick B.]

Re: L78 failed Rocker Stud after 50 years!!!

Valve springs for 65 to at least 69 big blocks did not have an inner spring. They had a damper and they were prone to failure of the spring not the damper.

[Joe L.]

Re: L78 failed Rocker Stud after 50 years!!!

Valve springs for 65 to at least 69 big blocks did not have an inner spring. They had a damper and they were prone to failure of the spring not the damper.

Patrick------


Yes, those were the GM #3859911. They were used for all big blocks except L-88 from 1965 thru late 1969. They were a single spring with damper. GM #3859911 was discontinued from SERVICE in November, 1970 and replaced by GM #3970627. The latter included a double spring with no damper + a special valve cap with attached umbrella seal. This was the best valve spring assembly GM ever devised for a street big block. Absolutely the best. Period. Unfortunately, these were discontinued several years ago.

Attached are photos of a GM #3859911. This one came from my 1968 Chevelle SS 396 with L-78 and failed me just about 51 years ago. The car was only about a year old at the time with about 15,000 miles. I was lucky, though, and the valve did not drop and destroy the engine.

So, how much would an NOS GM #3859911 be worth today. In my opinion, ZERO.

[Gary C.]

[Joe L.]

Re: L78 failed Rocker Stud after 50 years!!!

I replaced the valve springs, seals and nuts about 300 miles ago.
So “lessons learnt”, I should have replaced the studs as well!

So with the change to the double spring springs, as Joe mentioned above a set of new ARP studs (#135-7201)may be a better option than NOS?

Gary-------


There's still the issue of the fact that ARP instructs not to use their studs with stock-type self-locking nuts. I have done this without apparent problems but it still worries me a bit. I'm going to try to contact ARP to see if I can get some more info on this issue.

[Gary C.]

[Ray K.]

Re: L78 failed Rocker Stud after 50 years!!!

I would also inspect the push rod, rocker arm and ball, and guide plate, for a clue as to why a a broken stud. My L78 has original studs as well.

Ray

[Gary C.]

Re: L78 failed Rocker Stud after 50 years!!!

Thanks Ray, I have checked those already to no avail.
I have replaced with the stud in the short term with an old one till I replace them all with a new set.

[Duke W.]

Re: L78 failed Rocker Stud after 50 years!!!

One of my Rocker Studs failed with metal fatigue on my 396/L78.
I think I should replace them all.
All as I can seem to find on the net are aftermarket roller rocker types.
Does anyone know where I can find NOS studs?

Thanks,
Gary
Perth, Western Australia.

It looks like, from the left photo, that this is a classic fatigue failure. Most of the break appears smooth except for the crescent on the right side that appears rougher, but I can't say for sure without closely inspecting the part with a magnifying glass, and if my analysis is correct I'm surprised the crack propagated as far as it did before failure occurred.

What happens is that a crack develops from a microscopic surface flaw and then propagates through the material due to cyclic loading. As the two faces of the crack move ever so slightly relative to each other with each load cycle the faces become "polished", and then when insufficient material is left to carry the load, sudden failure occurs and leaves a slightly rough surface.

Magnaflux inspections will reveal cracks and may even reveal microscopic flaws that can develop into cracks. This is why parts, like connecting rods, that are subject to high cyclic loading should be Maganfluxed, even when new and certainly when used.

I believe the "pink" rods used on SHP 350s were first Magnafluxed in production, and those that passed had the bolt seats shot peened, which "rearranges" the surface by eliminating microscopic flaws that can lead to propagating cracks that ultimately cause failure after anything from a few load cycles to millions of load cycles.

For steel parts, maximum loading should not stress the material beyond it's "fatigue limit", which is something on the order of half to three-quarters of its yield stress, and then if it passes a 10 million cycle test, fatigue life is considers infinite, but then you get one out of a zillion parts with a microscopic surface flaw and the theory goes out the window.

Duke

[Terry M.]

Re: L78 failed Rocker Stud after 50 years!!!

Gary-------


There's still the issue of the fact that ARP instructs not to use their studs with stock-type self-locking nuts. I have done this without apparent problems but it still worries me a bit. I'm going to try to contact ARP to see if I can get some more info on this issue.

Joe

If there is oil residue build up on the stud removing the lock nut could unscrew the stud rather than the locking nut. It doesn't seem to me to be much of a problem in our V8 applications, but a possibility none-the-less. Other applications could be more problematic.