rocker arm and cam lobe failure

[RobFrost]

The force on the face of the rocker is greatest there, closer to the fulcrum.

Not only because it is where the spring is more compressed, but also because the leverage the cam has over the spring is less.

It is possible that there is some pressure threshold above which the oil is either excluded altogether, or is thinner than the grain size of any abrasive debris suspended within it, causing a sudden step up in wear within a certain proximity of the fulcrum.

If your spring force is particularly high, a thicker oil will counteract the propensity of the oil to be squeezed out.

The multiple ridges, to me, suggests a failure condition involving the rocker jumping in steps or vibrating. I suggest checking if it is loose on its shaft, or the rocker loose in the cam box, or even the cam box cracked.

Just from a physics point of view, not speaking from experience.






Sent from my SM-S918B using Tapatalk

[Magyar Kiwi]

Hi
What were the marking(s) on the failed rocker(s) from the first rebuild ?
Cheers

[Turbo_pro]

Originally Posted by RobFrost

The force on the face of the rocker is greatest there, closer to the fulcrum.

Not only because it is where the spring is more compressed, but also because the leverage the cam has over the spring is less.

It is possible that there is some pressure threshold above which the oil is either excluded altogether, or is thinner than the grain size of any abrasive debris suspended within it, causing a sudden step up in wear within a certain proximity of the fulcrum.

If your spring force is particularly high, a thicker oil will counteract the propensity of the oil to be squeezed out.

The multiple ridges, to me, suggests a failure condition involving the rocker jumping in steps or vibrating. I suggest checking if it is loose on its shaft, or the rocker loose in the cam box, or even the cam box cracked.

Just from a physics point of view, not speaking from experience.


Sent from my SM-S918B using Tapatalk

After reading this a few times, it feels a little like a word salad.
Is it possible for you to explain your conclusions differently so non-engineers like me, can understand?

[rs-vic]

That DP1120 rocker is the problem. I’ve never seen one. Some kind of cheap aftermarket junk. The original Porsche rockers all have a B or the triangle

[trond]

I believe you could be right.

Originally Posted by rs-vic

That DP1120 rocker is the problem. I’ve never seen one. Some kind of cheap aftermarket junk. The original Porsche rockers all have a B or the triangle

[Magyar Kiwi]

Hi Trond
Can you please add pics of the failed #4 inlet rocker from the previous rebuild ?
Many thanks

[trond]

I have been looking for rocker and/or pictures but no luck. I must have thrown it away unfortunately. Now it might have been a valuable datapoint. Not very clever of me. Sorry

Originally Posted by Magyar Kiwi

Hi Trond
Can you please add pics of the failed #4 inlet rocker from the previous rebuild ?
Many thanks

[trond]

Unfortunately the only photo I have of failed rocker from the previous failure (hardwelded cam, different profile from same supplier). It failed after very short time same as latest failure. Picture is of the wrong side, opposite side of markings... Not very clever of me ..

[RobFrost]

Originally Posted by trond

Unfortunately the only photo I have of failed rocker from the previous failure (hardwelded cam, different profile from same supplier). It failed after very short time same as latest failure. Picture is of the wrong side, opposite side of markings... Not very clever of me ..

What weight oil were you using, and what percentage of standard spring force?

Sent from my SM-S918B using Tapatalk

[Henry Schmidt]

Originally Posted by RobFrost

What weight oil were you using, and what percentage of standard spring force?

Sent from my SM-S918B using Tapatalk

I don't know what oil he was running but I already posted the spring rate.
The issue isn't so much the oil weight but the capacity for the rocker pad surface to trap oil.
Chill casting allows for the greatest oil retention capacity in a cast product and if the cams are chill cast (I assume they are) and the rockers are not, the rocker surface will most likely be compromised.
Now at some point the OP posted that the cams were hard welded and that is an entirely different kettle of fish. Hard welding will remover any surface porosity and the only way for the rocker to survive is to have a reliably hard surface. Relative to the cam that is.
My guess in this situation is simply poor quality rockers (we are seeing more every day) or low quality rebuilding technique.

[trond]

Yes cams are hardwelded. The cam supplier now wants me to send in a set of rockers to be hardwelded and say this will make the set up work. I understand this will make the rockers more resilient but if it it really a lubrication/oil retention issue then it is still not a good design and I would be better off finding different cams (not hardwelded), The odd shape of the worn cam lobe is puzzling; it is not round any more, but faceted. Also I agree it is not unlikely that the completely destroyed DP1120 rocker may be low quality. There was only that single one marked withDP1120 of the twelve installed, and it was the only one to fail in such a spectacular way. It may be a point that the same DP1120 rocker had survived without any damage in a previous build with stock cast cams.

Originally Posted by Henry Schmidt

I don't know what oil he was running but I already posted the spring rate.
The issue isn't so much the oil weight but the capacity for the rocker pad surface to trap oil.
Chill casting allows for the greatest oil retention capacity in a cast product and if the cams are chill cast (I assume they are) and the rockers are not, the rocker surface will most likely be compromised.
Now at some point the OP posted that the cams were hard welded and that is an entirely different kettle of fish. Hard welding will remover any surface porosity and the only way for the rocker to survive is to have a reliably hard surface. Relative to the cam that is.
My guess in this situation is simply poor quality rockers (we are seeing more every day) or low quality rebuilding technique.

[mikedsilva]

Originally Posted by trond

There was only that single one marked withDP1120 of the twelve installed, and it was the only one to fail in such a spectacular way.

ok... that is something I failed to notice before.
I think that makes it the obvious candidate for the failure... doesn't it?

[Henry Schmidt]

If running your hard-welded cams is your chosen path, perhaps running a set of factory chromed forged rockers. We have them on the self ready to ship.
One caveat: forced rockers can make the money shift very expensive.

[KNIGHTRACE]

I have returned from holiday, I am pretty sure we know what went wrong here. Not wanting to start any drama post. I am trying to fix the problem and we will know in a few days. Kevin, very good post on the thread and helpful. I have the rocker issue sorted out. Taking some rockers to the lab today for hardness testing including webcam hard welded rockers just to verify. William
KNIGHTRACE@MAC.COM

[Magyar Kiwi]

Hi William
What was the outcome of your investigations ?
Many thanks

[Turbo_pro]

Originally Posted by KNIGHTRACE

I have returned from holiday, I am pretty sure we know what went wrong here. Not wanting to start any drama post. I am trying to fix the problem and we will know in a few days. Kevin, very good post on the thread and helpful. I have the rocker issue sorted out. Taking some rockers to the lab today for hardness testing including webcam hard welded rockers just to verify. William
KNIGHTRACE@MAC.COM

This post reads like you have a personal involvement with the specifics of the issue.
Would you care to elaborate?

[Rosco_NZ]

Sounding like there may be an issue with the hardness of the pad resulting from the hard welding process?

[trond]

rockers were not hardwelded, cam was. Rockers were rebushed and ground. They had perfect contact patch/alignment with the cam. But yes an incompatibility issue, OR an inferior rocker branded DP1120.In which case the cams were not the problem. However the previous failure 2 years ago was with hardwelded cams and new OE rockers and rocker arm pins. So incompatible cam/rocker could be the issue or maybe not. I don't know really. And not knowing is painful and frustrating. For now I am leaning towards the DP1120 rocker being substandard caused the problem. Engine building has some risks and both luck and expertise is needed for a good result. I'll move on and see it as another lesson learned. I did like the cams a lot and it is a loss that the build failed

Originally Posted by Rosco_NZ

Sounding like there may be an issue with the hardness of the pad resulting from the hard welding process?

[Rosco_NZ]

I thought the rocker reborn process also included hardwelding … William seems to suggest this …

[Rosco_NZ]

Just a thought .. check the rocker shafts are being correctly located on installation .. if the rocker runs misaligned on the shaft where the shaft is wider due to expansion nuts, this could cause the rocker to bind on the shaft.