rocker pad experts?

hcoles · 03-19-2011, 06:32 PM

I had this issue with some of my rockers.
Here is a rocker out of a buddies motor, one rocker had this issue, we now can't remember if it was intake or exhaust. The rockers were ground when the cam was ground to 964. The cam surfaces look almost untouched. I think this is after about 5k miles. Not sure if the valve springs are stock, I think they are at least close to stock.

What is probable root cause(s)?

1- grinding into softer material
2- not enough lubrication
3- rocker pad not ground to correct geometry (e.g. surface angle/axis) note surface failure is toward one side.

Observation, it looks like the rest of the surface has been "used" maybe something like brinelling.

The motor had and still has "new" Porsche recommended oil line restrictors. I took them out of my motor when I saw this.

Thanks for comments. I posted about 1 year ago on this also.

BURN-BROS · 03-19-2011, 07:24 PM

could be any of the causes you stated.

oil without adequate ZDDP level

Also I have seen later rockers(3.2-3.6) exhibit more wear than early rough cast rockers.

cgarr · cgarr's Garage

From what I have seen and done with testing is a lack of oil. I ran some rockers in a test stand and held off oiling for only 60 seconds and the rocker already showed major scuffing. Did he ever remove the spray bars? if so I also found a lot of oil loss between the spray bar and the cam tower housing where they slide through if you do not re-seal them.

Something else may be an engine that sits for several days or weeks at a time, the first start up is when things are dry. A slick setup would be some type of pre-oiling system like an accusump where you could pressurize the oil system prior to start up.

Not often but I have also come across rockers that show no pitting but when surfaced you find pitting in the casting so some may have internal casting problems but its not often but do find them. The surface hardness of a perfect rocker from what I have tested is down .030 at a min but any overheating of the rocker surface quickly changes this to nil.

So I would go with lack of oil which causes heat which then reduces the surface hardness.

hcoles · 03-20-2011, 07:44 AM

Craig,
Thanks. I was going to send you an email.
Your conclusion makes sense.
I guess at this point we will run and see what happens.
My car is back with the more open oil flow restrictors, hopefully more oil flying around in there will help.
Re. pre-oiler - I would hope this is not really needed. We are both running Brad Penn.
Next time the engine is out, I'll clean the spray bars, what a pita.
I want to install the new bushing material for the rockers at that point also.
Thanks again for the comments.
-Henry

chris_seven · 03-20-2011, 08:14 AM

Henry,

It has been a few years since I did much practical metallography but the area ringed in red shows the presence of intergranular defects and the surface of the main defect appears to be oxidised.

I believe this is almost certainly a defective casting which has some significant sub-surface porosity that grinding has broken into. I would find it hard to believe that this wasn't visible after grinding and before assembly.

If you look at the depth of the defect it is clear that it has been ground to about or just below its centreline so if you measure its depth you can estimate how much metal was removed.

I don't think cam line oil restrictors can be responsible for this problem as otherwise all rockers should be affected.

I can agree with a blocked spray bar but would this have been checked during the rebuild?

Brinelling is also unlikley as it relies on high local contact stresses causing surface deformation by somethig such as a roller digging in and causing damage. When a roller digs into a surface it normally causes a disruption that lifts material up form the surface at is edges and this is what then erodes and causes significant surfacer damage. The defect pictured looks nothing like brinelling.

If the rocker had bcome hot enough to soften it would have had to be at temperature for a reasonable time - at least several minutes - and this would have caused some discolouration, which isn't apparent on the photograph.

I think it was a bad rocker and probably bad grinding and inspection on one this one part.

hcoles · 03-20-2011, 08:30 AM

It has been a few years since I did much practical metallography but the area ringed in red shows the presence of intergranular defects and the surface of the main defect appears to be oxidised.

I believe this is almost certainly a defective casting which has some significant sub-surface porosity that grinding has broken into. I would find it hard to believe that this wasn't visible after grinding and before assembly.

I saw this type of pattern at the edge of a bunch of my rockers, and I think they were mostly on one side of the motor, except for maybe one. The rockers are viewed by eye after grinding and nobody saw anything - maybe the situation was just under the surface, but why would this happen on a number of rockers.

If you look at the depth of the defect it is clear that it has been ground to about or just below its centreline so if you measure its depth you can estimate how much metal was removed.

I don't think cam line oil restrictors can be responsible for this problem as otherwise all rockers should be affected.

[COLOR="rgb(255, 0, 255)"]I was thinking in terms of if one spray bar hole was a bit blocked that having more oil flying around from the more open restrictors might somehow help. [/COLOR]

I can agree with a blocked spray bar but would this have been checked during the rebuild?

[COLOR="rgb(255, 0, 255)"]I don't think this was done during the rebuild. I assume all sort of carb cleaner was injected but don't know for sure.[/COLOR]

Brinelling is also unlikley as it relies on high local contact stresses causing surface deformation by somethig such as a roller digging in and causing damage. When a roller digs into a surface it normally causes a disruption that lifts material up form the surface at is edges and this is what then erodes and causes significant surfacer damage. The defect pictured looks nothing like brinelling.

[COLOR="rgb(255, 0, 255)"]Okay, no brinelling.[/COLOR]

If the rocker had bcome hot enough to soften it would have had to be at temperature for a reasonable time - at least several minutes - and this would have caused some discolouration, which isn't apparent on the photograph.

I think it was a bad rocker and probably bad grinding and inspection on one this one part.

Can't seem to get the text color to work correctly.

hcoles · 03-20-2011, 08:32 AM

If you look at the depth of the defect it is clear that it has been ground to about or just below its centreline so if you measure its depth you can estimate how much metal was removed.

I forgot to add - I don't understand, can you explain a bit differently? The complete surface is ground on circular curve with the same radius I assume.

chris_seven · 03-20-2011, 09:11 AM

Henry,

The surface defects shown on your photograph don't look much like wear as it is extremely localised and doesn't really show much of a 'rubbing pattern' and this make me question whether or not it is adhesive wear that has occurred.

If a 'chunk' of metal had been pulled out of the surface to leave the type of defect we are looking at then there should be some witness on the cam as the metal pulled out should have stuck to the opposing surface or at least some part of it should be visible. (If it has been pulled out of the surface it must now either be in your oil filter or still in the engine and ther may beother damage associated with these particles)

The rockers you are using are investment cast and do suffer from porosity and other common casting defects.

This photograph was taken using a Scanning Electron Microscope and is cut transverely through a specimen.

If you now imagine grinding away the upper surface you would eventually break into the defect.

Investment cast parts normally have very good surfaces as they are cast into a ceramic mould and this casues the metal in contact with the mould to freeze quickly and without defects. Any gas which has not been removed during melting will tend to form these bubbles below the surface.

This is why I think they are casting defects and this is why I think they should be visible after grinding.

I would be very interested to know if anyone else has experience with defects of this type.

hcoles · 03-20-2011, 09:46 AM

Chris,
I'm following your description re. porosity better now. Makes sense, the surface can be good but it is easy to grind down to porosity. We are not seeing any odd looking wear on the cam surface. I repeat that we did not notice patterns like this or any patterns after grinding.

I'm including a couple more pictures of rockers that came out of my car about 1 year ago after they ran for 35k miles after grinding. These ran with the more restricted fitting, I'm back on the open fitting now.

Another possible cause, I think depending on intake or exhaust and engine side the cam hits in a binding angle or helping to open angle, I think I haven't done a good job of recording the info. on that to see a correlation, it is just a thought.

Thanks for your comments.

cgarr · cgarr's Garage

Just so we know, here is one that did get hot!!!

hcoles · 03-20-2011, 09:58 AM

yes, it looks like you can see where the cam was hitting with the most force, there is a "hollow" area likely became soft - thanks for that picture

hcoles · 03-20-2011, 10:31 AM

here is a not so great diagram of possible force differences as a function of cam rotation, one situation pushes on the shaft and one pulls force off the shaft,
I'm sure someone has a better diagram avail. I made this in Visio in a few mins.

cgarr · cgarr's Garage

Henry, You want me to grind out that real bad one and see how deep it is and heat treat it? It might be a good test rocker to try, just put it in an easy access spot on the engine? I would also like to get to the bottom of this issue and am willing to try anything if you want.

Craig

hcoles · 03-20-2011, 11:25 AM

Craig,
Thanks very much for the offer, I'm not sure I can offer to get that rocker treated and then test. We are getting old and out of energy to horse with this stuff. Right now we are trying to track down a lean condition at high rpm on a car used a lot on the track.

One thing that might be of interest is to get the surface tested for hardness. Do you have the setup for that?

-h

911pcars · 03-29-2011, 12:28 PM

Anecdotally, it seems many owners opt to resurface the rockers when installing a new valve train believing there's sufficient hardening on the rocker face. Removing the thin hardened layer exposes the relatively softer base metal in this high load area. Higher spring loads from "racing" springs will exacerbate this condition.

MHO,
Sherwood

hcoles · 03-29-2011, 01:34 PM

When the cam was reground it was recommended to regrind the rocker faces. As far as I can tell this is very commonly done, doesn't mean it is correct.

What evidence do you have that the rocker refacing reveals a softer surface? You might be right. Of course this could be a function of depth. E.g. the cam has enough hardened depth for grind change. I don't have racing springs and yes I agree it is a high load area.

911pcars · 03-29-2011, 02:00 PM

Quote:

Originally Posted by hcoles

When the cam was reground it was recommended to regrind the rocker faces. As far as I can tell this is very commonly done, doesn't mean it is correct.

What evidence do you have that the rocker refacing reveals a softer surface? You might be right. Of course this could be a function of depth. E.g. the cam has enough hardened depth for grind change. I don't have racing springs and yes I agree it is a high load area.

I have no direct evidence, just a hunch, a WAG. A materials engineer might enlighten us. In metallurgy texts, there are reference photos one could compare to provide a clue. As opposed to a crank journal, e.g., there's no spec to use as a reference to know how much can be/has been previously removed from a rocker face. As for reground cams, the depth of any hardened material is also an issue.

Sherwood

chris_seven · 03-30-2011, 07:01 AM

Do you think that the investment cast rocker has a hardened surface?

I would be surprised as it is uncommon to locally harden investment casting.

The materials used for typical investment castings in the seveties were typically manufactured from a Carbon steel and apart from grinding and macining of bores and tjreads for adjusters there are unlikly to have been any other processes involved.

The way in which rocker arms are normally investment cast doesn't really lend itself to adding chills to the moulds so unless rockers are locally hardened in a seperate process there shouldn't be too much of a hardness gradient through what is a relatively thin cross section. If I had to guess I would think these parts are heat treated to a specific conditon and not surface hardened. In a production engine they would then run against a chill cast cam.

I can imagine that resurfacing can cause problems with both finish and accuracy which can lead to wear, also the curvature changes may increase contact stresses.

If you slice a rocker in half through its cross section and boil it in battery acid you should be able to see a hardened layer quite easily.

Cams are quite a different subject. A typical cast camshaft is made using a Grade 17 Cast Iron with about 1% chromium added. Large 'chills' are installed in the mould in the regin of the cams and this causes the molten iron to cool rapidly - hence the expression 'chill cast cam'. The rapid cooling transforms the Grey Cast Iron to a white cast iron which is extremely ahrd and wear resistant. The depth of the white cast iron will depend on the thermal capacity of the chill.

This is the same basic technique used to produce cast iron liners and cylinders.

hcoles · 03-30-2011, 07:14 AM

Chris,
Thanks for the info.

What are your conclusions?

I think of two subjects:

1- the surface looks to have been under stress enough to move material around - I would expect to have very little noticeable wear - maybe the material moves and then stays that way?

2- the area where the "chunks" are missing - is this a localized area that had underlying porosity?

Thanks,
-Henry

chris_seven · 03-31-2011, 05:40 AM

It is really quite difficult as wear is one of those things that is very application specific and difficult to test for or reproduce.

I would question the grinding and the relative contact angles between the cam and the rocker as being relevant.

I would also worry about surface finish.

If I had to guess I would be more likely to blame workmanship rather than metallurgy but I am not 100% confident. (Occam's Razor and all that !)

The porosity can't be good news either as this will not help contact stresses.

The apparent flow of material just seems to say that the surface stresses are high but there is no real evidence of high temperatures that you may expect when there isn't enough lubrication as there doesn't seem to be any discolouration or blueing.

I have to say that I would conclude that new cams = new rockers as this must remove doubt but I am also sure that it is possible to do the job properly when care is excercised.