Broken Rocker Stud

[jimbrandberg]

Probably just a fluke if they're stock length unless someone bought new ones. I'm a fan of good original over new Chinese on just about everything.
I've used poly-lock nuts with longer studs on roller rocker and stud girdle applications but can't remember using them for stock. I've also used them for head nuts on race engine.
Jim Brandberg
Isanti, MN
CorvairRepair.com

[Nashfan]

I "believe" I got my ARP rocker arm studs from Clarks 20 some odd years ago. They were bought to use Harlan Sharpe roller rockers for a 140 that I had then and changed my mind and went to shaft mounted roller rockers that became available then, and ended up using the rocker arm studs on my turbo. It has gone roughly 50000 miles with those on, zero issues. One thing that is a known cause of broken rocker arm studs has been the use of polylocks+stud girdles, as the stud girdle clamps to the polyloc and ends up placing a bending load at the last thread of the stud. I wouldnt think this same dynamic would apply if not using stud girdles, but since the stud did break on the thread, maybe somehow someway the use of the poly loc did contribute to the breakage. Be very suspicious if it happens again!

[erco]

I have seen some rocker studs with a groove worn on one side from cockeyed rockers rubbing on them. Those would certainly increase stresses at that point and could lead to a failure. Any telltale marks on your broken rocker?

It's almost certainly a fatigue fracture. Do you by any chance have access to a scanning electron microscope to examine the fracture surface for classic fatigue striation pattern marks?

I did! My 140 broke a piston when I was in college at Virginia Tech for mechanical engineering. The whole top of the piston broke off at the oil slots, exposing the inner steel anti-expansion struts. I used that as a project in my very timely "Mechanics of Materials" class. Got to use the university's electron microscope to make microphotographs of the fracture surface. I factored in as many variables as I could, stress raisers, alloy, temperature, forces, and ballparked the number of cycles based on mileage. The numbers fit neatly into the fatigue failure envelope and the fracture photos showed classic fatigue marks. I got an A on my report.

[jimbrandberg]

I see what you mean about the place where the pushrod contacts the rocker arm doesn't look good.
Jim Brandberg
Isanti, MN
CorvairRepair.com

[vairmech]

Hey Everyone, I had a weekend of racing so I was sort of incommunicado.

Anyway, I did get the email and I answered but here there were pictures here! In the first picture I have to say that the stud looks like it had a fracture already started, see the dark area starting at the base of the thread? That has been there a while, the rest is pretty much a fresh break.
Then looking at the rocker, what is up with the lower left corner? And as has been mentioned the wear on the tip looks more galled than just sliding. Do wipe off the oil before you take the picture, I don't know what it is but oil really screws with the texture of the picture.
One more thing, the valve springs look massive, which Isky springs did you get? They look to be dual spring.

[Auto Anatomy]

Hey Everyone, I had a weekend of racing so I was sort of incommunicado.
Then looking at the rocker, what is up with the lower left corner?

It's the picture, nothing marred or discolored in the rocker slot.

One more thing, the valve springs look massive, which Isky springs did you get? They look to be dual spring.

They're Isky 4005 with 115# spring pressure.

I'm hopeful this was just a fluke failure with an older part. Like I said, wasn't doing anything crazy with the car just riding home from a long drive.

New parts on order and should be here tomorrow. Hopefully with a new stud it'll be back up and running again without any further issues.

Sean

[vairmech]

LOL, I could see those were larger than stock springs! Yeah, 115# is a pretty good load for a stock rocker stud. But the broken one still looks like it has had a small fracture for a while with that dark area.

What valve spring height are you at to get the 115#?