Cammer Rocker Problems

[jayb]

I have a customer who is running a race SOHC engine with T&D rockers.  He has been having problems with these rockers for quite some time, with one particulary vexing issue.  His #2 intake rocker has been regularly breaking. At his most recent race the rocker broke again, and part of it actually exited the engine through the valve cover!  Obviously a violent failure.

I have had my share of problems with T&D rockers but they have been confined mostly to needle bearing failures.  A long time ago I had a piston to valve clearance issue that caused some bending of the rocker body around the roller wheel axle, basically oblonging the hole, but outside of that I've found the T&D rocker bodies themselves to be quite strong.  To hear of one actually breaking and blowing a hole in the valve cover is really surprising, especially since it happens repetitively.

My customer has put a borescope into the cylinders and hasn't found evidence of any piston to valve clearance problems.  He also says his engine has been gone through recently by a knowledgeable SOHC guy and doesn't have coil bind issues.  He is inclined to blame the quality of the T&D rockers for these failures, but I am not so sure that this is the problem.

I have a few thoughts of what could possibly be going on with this engine, but certainly can't say I know for sure.  I thought I would throw this situation out for comment.  Any ideas?

[ToddK]

Is there any evidence of lack of oil going to that rocker? What lift is the cam, has the lobe been measured to confirm?

[My427stang]

Jay, do you know spring pressures and cam lobe profile?

I hate guessing without numbers or seeing the failure, but maybe that one's on the edge end going out of control?  I doubt it'd break from too much pressure.

[jayb]

I don't know about the cam and spring pressures; I will ask the customer and post back.  He didn't mention oil starvation issues and it seems like it would be unlikely for that one rocker to have the problem, since all the intakes on that side are oiled from the same shaft.  I am leaning towards chain whip being an issue, although that would result in piston to valve contact, and my customer doesn't see anything with the bore scope.  But it can sometimes be hard to see that...

[WConley]

Like Ross, I'm guessing the spring pressure and installed height on that cylinder are just right for a resonance.  You can't rule out timing error due to the chain, but wouldn't you see that more on the exhaust valve?  I guess violent chain whip could cause the intake valve to advance enough at certain moments, especially if the starboard cam is installed really advanced...

[hbstang]

i wonder if these have the screw adjusters on them.at least they didnt break a valve .

[blykins]

Aluminum or steel?  What keeps them located axially?  Spacers?

[jayb]

Aluminum or steel?  What keeps them located axially?  Spacers?

I assume steel, because T&D hasn't made aluminum SOHC rockers for a long time (as far as I know).  The spring clips on the shafts locate the rockers axially.

[blykins]

I'd be looking real hard at coil bind on that particular spring.  Just because the rest of the springs have sufficient coil bind clearance doesn't mean that this one does.  I have seen some springs vary by .030". 

SOHC valve spring loads should be no issue whatsoever for a steel rocker.

I would also see if that rocker has any axial clearance.

[shady]

I doubt it's a T&D problem or failures would be random cylinders. IDK, maybe a valve guide or stem issue.

[Urgefor]

I second Shady's assumption.  Not that all aftermarket parts are junk, but I've seen a number of things from the aftermarket lately that are almost, but not quite right dimension wise.  To clarify, by lately I mean over the past 3 to 5 years.

[pumpbldr]

I am going to take a crack at this from a different angle. Are his shafts regular thickness or HD ones that are thicker? We all saw Bill Connely spintron video years ago. As a MFG  of shafts I had access to a spintron  long ago but with a wedge head at a different angle than his. Viewed the shaft long ways. Stock shaft vs, spacers vs Hd shafts vs spacer. What a difference in the wave. Get a surf board LOL. But an idea. This solved alot and yes frequency changed/ resonance. Maybe what you guys think.

 Doug aka pumpbldr
Precision Oil Pumps

[WConley]

I am going to take a crack at this from a different angle. Are his shafts regular thickness or HD ones that are thicker? We all saw Bill Connely spintron video years ago. As a MFG  of shafts I had access to a spintron  long ago but with a wedge head at a different angle than his. Viewed the shaft long ways. Stock shaft vs, spacers vs Hd shafts vs spacer. What a difference in the wave. Get a surf board LOL. But an idea. This solved alot and yes frequency changed/ resonance. Maybe what you guys think.

 Doug aka pumpbldr
Precision Oil Pumps

Doug - Shaft bending is certainly a possible contributor.  There could be just the right combination of soldiers marching in step on this bridge.  I have a feeling this is going to be a really weird combination of factors.  As Brent says, a steel rocker should have no issues at typical cammer spring loads.

[jayb]

Here are some more details on this failure.  The engine is a 461" SOHC in a drag race car.  The valves are the Ferrea SOHC valves, which if I recall correctly are hollow stem and weigh about 130 grams on the intake and 118 grams on the exhaust.  The engine is run hard, it is shifted at 8500 RPM and goes through the traps at 8800 RPM.  Details on the cam specs and spring specs are below, as well as a picture of one of the broken rockers (not the one that exited through the valve cover).  There is no sign of overheating on the rocker, and after it breaks the bushing is in good condition and still spins freely on the shaft. 

At those engine speeds I think he is running out of valve spring to control those valves.  Chain whip probably also plays a role.  Further comments welcome - Jay

[blykins]

Not enough spring load, IMO.

Valves are probably bouncing off the seat, springs are in surge.  Harmonics everywhere.