3/8" rocker stud broke right above the truion, just below the polylock nut.
I have a feeling it was the stiff dual springs, but it sure broke in an odd place...I guess the PR rod and the spring pressure would be pushing up on the stud at that point..or at least at a point below the polylock nut. The stronger the spring, the more pressure at that point.

 

When did it break? With the engine running or when you were locking it down(etc)? What are the open and closed pressures for your installed spring height?

 

seriously doubt it was a spring pressure issue.

 

Are there pushrod guides on those studs?

Hal

 

It broke while cruising down the freeway at a constant 1800-2000rpm. Been going that speed for about 15-20 minutes, never got above 2500 rpms the entire day.

Nope, no guide plates, SA rockers. They're Comp studs...so I don't put it past cheap foriegn materials again with them....
The spring pressure is around 420 when at max lift...165 I beleive at install height if I remember correctly.

 

I would guess its possible that upon tightening the thing a slight crack or flaw developed and over time cracked all the way accross.

Did you tighten the thing after the stop screw had ben tightened? Even a tweek afterwards could possibly have done it.

 

I'm hoping someone with experience in this issue will post up.

What I would do is make sure that the piston didn't kiss the valve. If the valve is not bent, the pushrod straight, the lifter okay and none of the other studs are breaking......then install a new one and see what happens.

If it's an ARP stud, I'm sure they would like a chance to examine it.

 

That's the weakest point of the stud that's why it broke there. It was probably flawed from day one but not enough to show any signs, only to allow it to break down over time. I used to bolt door on at GM and I used Impact sockets and I was able to call my team leader BEFORE the socket broke so he could swap them out and not have to chase the line down to torque the bolts down... There was a certain sound that changed very slightly when the socket was about to break. The team leader came over once and I told him my socket was broken and he looked at it and said it wasn't... I took my finger and broke it completely off so it was obvious and he just smiled. As part of preventative maintanence you could replace these things even though they weren't broken yet and never have any down time...but they didn't like that idea...literally every 2,500 cars one would break...give or take a couple hundred... If one broke sooner it was a weaker one...but most fell within the 2,300-2,700 range. I wouldn't worry about it too much. Just replace it and if the same one breaks again you can worry at that point in time.

 

Intake huh? Lucky.

I guess this is a good reason to switch to ARP studs.

Hal

 

They were Comp studs... PR is straight, rockers looks fine. I thightened the polylock set screw down to 30ft/lbs max. Lifter seems ok, at least pushing down on it with the PR, I haven't taken the intake off to look at closer.

 

Did you have any head work done? IE: bigger valves or new guides? I had this happen to a set of regular SBC heads after a valve job. The guide was installed wrong and through off the geometry of the valve and put side strain on the stud. But mine broke down next to the head, and was a BI*CH to get out.

 

Did get bigger valves, the guides were ok so they are the stock originals, LE2 port job.
I installed new studs today, the new ones are ARP studs.

 

I couldn't tell you for sure, but I have done it three or four times now. People have suggested over-reving is/was the cause of this, but I highly doubt that because I have never over-reved any of my motors.

 

I noticed a couple of things. The stud looks like it was hardened all the way through. That would be typical of an alloy steel like 4140 or even a medium carbon steel like 1144. (My metallurgy class is now being useful.)

The failure doesn't show me any sign of flexing or bending before snapping off. That means the part is brittle, but strong. It won't flex under load, it will just resist the load then snap when it reaches the ultimate strength of the material. This happens when a steel part is too hard and undergoes bending loads which rocker studs do.

The failure occurred at the weakest point on the stud, the root of the thread. That is also an area of a sharp v even if the thread was rolled rather than cut. The studs I have looked at have rolled threads. If a failure like this is going to occur, that's where it probably will be. It takes a number of cycles to fail if the crack starts in a groove. The failure can happen quite a while after the highest loads were applied, like during high rpm. (Strength of Materials class is also starting to pay off)

These were 3/8 studs. That makes them only about 74% as strong as 7/16 studs.

I think that the stud that broke may have been harder than it should have been. The problem is that some of the other studs in that set may have been from the same heat treating batch. There is no good way to tell except by hardness testing each one.

If it were my engine I would change the type or brand of studs. I looked at the Summit catalog and found Comp High Energy studs for about $20/set, Magums for about $35/set, ARP High Performance for about $36/set and APR Pros for about $68/set. My choice would be the ARP Pros because with the selfaligning rockers you are restricted to the 3/8 diameter. The chances of getting a bad heat treat on ARP Pros is a heck of alot less than on $20 stuff.

I don't know everything about metallurgy and strength of materials, but this looks like a classic failure from the textbook. Maybe I only know enough to be dangerous.


Joe Urban