690 HP at 1.1 bar runs for 20 hours perfectly then breaks #6 rockers (both). No further damage, no bent valves, no contact to piston...leaks down perfectly. Does this again!...disassembly shows NO adverse wear or clearance issues anywhere. New Rockers all around (again). Four twenty minute tests on track, fine, then fifth time out # 2 cylinder rockers break.
Again, no contact with piston and leaks down perfectly. Any rocket scientists have an opinion....BTW-this combination has proven consistantly reliable for years-until NOW. I've beat myself up on this already-hit me with your best shot. :-) Mark

 

Weakening valve springs? You didn't say how high you rev it, but if you float the valves, even without piston contact, the cam can come around and slap the rockers, and they will break. My car with stock springs is fine to 7K. Take it to 7,200 and it will break rockers. It is not the rocker arms because they will survive much higher rpms with the proper rate valve springs.

 

Mark,

Larry's points are well founded.

Without knowing what kind of revs you use nor what valve springs you have, its difficult, if not impossible to offer much concrete advice.

I would tell you that I hope you have titanium retainers because the OEM sintered metal ones easily fracture when either the valves float or there is even slight piston-to-valve contact.

 

Steve:
the springs were relatively new (aasco) as well as the ty retainers, we also run a unique valve..all of which has worked before. I had orginally installed "polished" rockers which lasted the 20 hours (app). no spring bind either...all three heads on 4-6 bank were tested for correct dimensions after second time of rocker breakage. Note that BOTH intake and exhaust went south. Then....it ran for app 1 hr before #2 rockers broke (at the shaft-split. NO ONE I know has ever seen this repetitive event before....this is a headache...no one to ask anymore. I've decided to go to button rockers as they were originally in the 962 engine....should be really enjoyable doing all twelve in the friggin' engine bay of a mid engined car. After a million of these, this is really making my day(s) bad, as in attitude.:banghead. You know as well as I, at this level, problems are supposed to be easier to solve! Mark
BTW-engine did NOT see more than 7300 RPM

 

I would have guessed either valve float or the springs are coil bound at max lift but you've addressed those - too tight clearance on the valve stem?, excessive valve-rocker clearances if great enough would put additional stress on the rockers?

 

Thanks Mark,...

The quality of the OEM cast rockers has always been problematic and even more so in the past few years since their QC REALLY slipped. I always magnflux and X-ray them before use in any high-RPM race engine.

The RSR/962 rockers are pretty good and those are always my preference when the customer has the budget. I've used them since 1976 with excellent results.

I don't envy you doing the install/adjustment with the engine in place,....it was bad enough with an RSR or 935. Thankfully, once they are set, you should enjoy more durability and less headaches.

 

Well Stevie, et al:
I'm still unsure of this procedure, as I did mag the rockers twice and, REMEMBER, BOTH the intake and exhaust break simoultaneously. The math would say that rocker failures upper and lower in same cylinder, at the same time; three times in a row? Ahh well, What I will do is post the results before my post traumatic stree disorder kicks in!

 

Both rockers at the same time means the intake and exhaust valves must be touching each other on overlap.

Did you crank up the boost? Boost pressure and the resultant exhaust pressure helps float valves.

 

why after 20 hours? why not bent valves (slightly) then. cam timing has been same and consistant on both sides, why #6 three times after an hour of running each occurance. Boost has been low at 1.1 for a 962 engine. Has run as high as 1.4 without problems. then #2 on south side of engine breaks both rockers right at the shaft - exactly identicle fractures? Remember, no microscopic signs of mechanical interference....perfect leakdown....teardown shows NO indications of touching or rubbing or clearance problems....switched heads with same results too. I do appreciate your input though, thanks anyway. Mark Never saw boost break rockers like this...thanks to all

 

Were, or do you know if, all the rockers involved were purchased around the same time? If they were all from a given casting batch it could simply be slightly off material properties that when stressed do not have the margin of safety needed. That would explain the failures after running for a short time.

 

Can we collectively poll everyone we know and find out about the suppliers of rockers for Porsche 911 engines?? Am I the only one with this issue?? Mark

 

Hi Mark,

A few thoughts,.....

I don't know who makes the rockers for Porsche. Based on what I've seen for the past year or so, I think the manufacture has been outsourced since the QC has REALLY fallen off. I prefer using older ones and simply rebush & regrind the pads.

For peace of mind as well as doing the proper forensics, I would remove the other rockers and both X-ray & magnaflux them. Many years ago, Clay Dopke used to sell X-rayed and fully polished cast iron OEM rockers for race motors and he told me how many failed the initial QC checks before undergoing the polishing. It was amazing.

Unless there is a metallurgical/fatigue issue at work here, the only way these break is when the valve makes contact with the piston or the opposite valve. I've seen a fair amount of valve float-related problems, but broken rockers isn't usually the direct result. The valve had to hit something to transfer force to the rocker arm.

Some piston manufacturers are not perfect and I'd ensure that the valve pockets, compression heights are identical. There are many reasons why piston-to-valve clearances can change and that would be where I'd be spending some time.

Call me, anytime.

 

Colin Belton from 9M in England has machined from solid billet rockers that are DLC coated. I would check there. They also have revised geometry so you can run a larger than .525" camshaft which is about the practical limit of the stock rockers. However, if you are breaking the rocker ams with the regularity posted above, I would have to look at something mechanical in the valvetrain.

 

Stevie:
I used his rockers...they were the original rockers on this engine. They lasted the 20 hours. He's not around anymore?

 

Yeah,..I never thought they were any better than ones without the polishing,..

I even hate to mention it, but dare I say it: we have used the early 911 forged steel rockers which are very strong and present a little less mass at their tips. Not in the same league as factory RSR/962 ones but far less expensive and in some cases, a better solution than the OEM cast ones.

Clay retired to Alabama and now raises horses (HP of a different kind).