john_g_46

I decided I'd better move this up front. So far 46 fellas have had their heads checked. 26 have learned that they have a problem as of Jan. 22, 2013. Cases of good and bad geometry have been found in '06s, '07s, '08s, and '09s. One '12 has reported and his geometry was fine.

I am sorry to report that the measuremments outlined in steps 10 and 11 of the procedure outlined on p.17 are of little merit. A new procedure for that data utilizing a tool I've developed is forthcoming and will be mad eavailable to forum members. The witness mark test remains a valuable inspection. As this thread has developed, it has become clear that there is more than one root cause for early guide wear. Major factors include:

1. Bad geometry...discussed at length throughout the thread...causes high side loads on valve stems leading to early wear.
2. Bad surface finish on valve stems. Even perfect geometry generates side forces. A bad surface acts like sand paper.
3. Guide clearance can be out of spec when the LS7 leaves the factory
4. Defective spring(s)
5. Soft or abrasive guides. Errors in manufacturing of the guides can lead to soft guides or abrasive ones. Either way, early guide wear.
6. Heat is a factor from the sodium filled valves. Likely the least significant as there are folks with over 100K miles on the cars and no issues.

When all of the above happen, there is a perfect storm...such is likely the case for one fella that had shot guides at (no, I'm not kidding) 400 miles.

This thread discusses the one of these four that is easy to detect, bad geometry.

When I removed my stock rockers a month ago while rebuilding my heads, I was troubled by what I saw but didn't know how to interpret it. I recognized right away that the shiny wear spot at the valve stem end of the rocker was wrong. It should basically be rectangular. These are very rounded at the bottom as pictured.
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Following some thought provoking conversations on the forum, I realized what I was seeing. If you look closely, you can see a little brown crescent at the bottom of the wear marks on the two left rockers (it shows up better in real life). The little brown crescent was visible on all but two rockers. The crescent is there because that little area was getting HOT ! You'll see why in a few shots below.

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So...Let's inspect the valve train.

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Removing the fuel rail cover is easy...tug on the bottom...

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...and then tug at the top.

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Next remove the five coil assembly fasteners...they're painted orange on their ends.

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Screws removed in this shot

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Here's a handy tool I made.

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It's easy to pull the plug wires off this way.

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Just lay the coil assembly over on the intake manifold.

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Next disconnect the PCV line. There's a tab on the back side. Just lift up on it...

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...and the tube comes right off.

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Next loosen the four valve cover screws until they spin freely. They are captive in the cover and stay with it. Set it out of the way.

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I blued the ends of the two valve stems (I just used a fresh blue Sharpie), pulled the two push rods, then re-installed my new roller rockers and tapped them against the valve stem to make a witness mark. You can do this with all of the valves. With the push rods out, the valves will be closed which is where I've observed the worst contact between the stock rocker and stem. This is what you should see with the stock rockers as well
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I repeated this procedure with the stock exhaust valve rocker...OH OH !! Where is the witness mark? Look closely. The rocker only touches the valve at the very top edge/corner. This is BAD! There are two problems here with the stock rocker. One, at the start of opening the valve, the rocker is imparting substantial side load on the valve stem, and two, the pressure at the edge of the valve stem is sky high due to the contact area being so tiny, basically what's called a point contact. The pressure in this situation is extremely high...generates excessive heat and cooks a little oil onto the rocker...hence the little brown crescent. The moment load (side forces at each end of the guide) is substantially higher that the side force on the valve stem...a perfect formula for wearing out guides in a big hurry.

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An additional test that is very telling is to blue the stem, reassemble, spin the motor, and pull back apart and inspect the wipe mark on the stem. Details for this test are forthcoming..
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Root Cause: I wish this was a little clearer in the photo... The stock rocker has rolled off the end of the valve stem. A line drawn from the point of contact through the center of the rocker radius is no longer parallel to the axis of the valve stem...again, this causes the rocker to impart substantial side force to the end of the valve stem, and even higher side force at the top and bottom of the guide.


When re-assembling, don't forget to put back the push rods and rockers. The rocker bolt torque spec is 22 ft-lbs. Make certain the valve cover seal (it is reusable) is seated properly in its cover. Position the valve cover on the head and run in all four screws by hand. I only tighten the valve cover bolts until I feel them come snug, plus about 1/8 turn. If you've used the YellaTerra rockers and installed the thick gasket do NOT tighten the cover bolts to the spec value! You will strip out the threads in the head. Snug these and call them done. I tried once to cranks them up to the specified 106 in-lbs, but didn't like what I was feeling. Turns out I succeeded at striping one and had to put an insert in the head. Position the coil assembly on the valve cover and run its five screws in by hand. Then torque them to the spec'd 106 in-lbs, which feels fine. When snapping the fuel rail cover on, engage the lower connections first, and then snap onto the fuel rail.

This inspection is easy to perform. By following this procedure you can tell immediately (at 10 miles or 10,000 miles) if you are going to have rapid guide wear out...even before it happens. However, if your cause of wearout is a combination of 2 and 3 above, sorry, there is no way to easily detect those.

John

HyperX

Very awesome post! Thanks!

clogan

Interesting diagnosis...but how would a person fix this? Looks like bad geometry from the outset. Would anything short of new, aftermarket heads fix this?

Forgive me if this is a dumb question; I never did know much, and have forgotten most of that.

CL

AzDave47

oicw

Very interesting find.

But this still doesn't explain why a car with 400 miles could have worn out guides?

john_g_46

You're right! It IS bad geometry! I did a mechanical layout and two things help. Move the rocker pivot a little closer (basically impossible) or raise the pivot a little...there's surprising bang for the buck in raising the pivot. You'd need some sort of custom shim. Don't know how much room there is to raise the pivot before running out of hydraulic lifter travel.

GM has admitted to improperly machined heads being out in an undetermined number of Z06s. They say they don't how many or which cars. New GM heads ought to be OK.

If you are still under warranty, make GM fix it.

In my other thread...Rebuilding my heads..., I mention that I used Roller Rockers to fix this. As you can see in the first photo with bluing, the roller rocker stays on the end of the valve stem...not centered as well as I'd like, but still comfortably on.

John

john_g_46

Actually, it does,

The amount of side load on the guides is very sensitive to this geometry. If I move the pivot 0.010" towards center and down, the guide loads quadruple. Once the guides start wearing, the loads increase asymptotically.

John

erick_e

This theorie was mentioned once before, and I concur that this could be part of the problem.

But if it were entirely true only stock cars should be failing, because pushrod length needs to be checked when installing aftermarket camshafts, and this wear pattern would be identified right then.

john_g_46

Might want to consider that rebuilding your heads won't re-position a pivot that was machined into the wrong place. Hopefully it's true that this happened to a limited number of heads and isn't a design problem.

In my other posts I mention that I believe the sodium filled exhaust valves and steel guides are a bad combination exacerbating what was already a bad situation.

I'll elaborate a little more...the only posible sources of excessive side loads are the valve spring (not likely) and the way the rocker pushes on the end of the valve stem. Poor surface finish could be an issue. But without side loading, you won't get wear.

John

erick_e

I agree. I just think that it would have been identified by the wipe pattern, when checking the pushrod length, after installing a aftermarket camshaft. Thiis would eliminate a lot of engines that have been cammed.

john_g_46

The wipe pattern is what tipped me off to my geometry problem. Then bluing and looking at the valve contact confirmed it.

John

Javvy

P●ss poor machining by a supplier, could be just one machine in the process?
Any heads could could be crap! !!

john_g_46

Right on, brother, right on! And clearly having the rocker going off of the edge of the stem is a very bad thing.

phipp85

Thanks for the informative post. Looks like a very simple inspection.

tigertank

wow in a dream world GM would read this and do voluntary inspection off all engine lol wont hold my breath

GMuffley

AzDave47

...except the 2012 heads that failed at 400 miles, and any other "100% inspected" heads that aren't right.

jimman

Could be looked at oppositely, self adjusting, therefore forgiving the side load. Would need to measure the resultant side force to confirm. Interesting theory and leaves a lot of concern with the aftermarket guys just changing out valves and guides.

bktmbill

Had my 08z with 7700mi. in dealership. heads were replaced. exhaust valve wear greater than .010. they reused intake rockers but replaced all exhaust rockers. I stopped by after heads were removed and did wiggle test on old heads and yea they had excessive play. I did also notice that old exhaust rockers had bad wipe pattern on tip, almost to the point of a sharp burr as well as only contacting about 75% of the tip. mentioned this to service tech and manager but they really did not have much of a response. The question i have is why would only exhaust side geometry be off due to bad machining and not the intake side as well. I was also wondering what aftermarket rocker would be a good replacement. Thanks for write up and excellent illustrations.

john_g_46

"Would need to measure the resultant side force to confirm."
The resultant side force is very easy to predict. It goes with the TAN(contact angle).


"Could be looked at oppositely, self adjusting, therefore forgiving the side load."
I've looked carefully at the geometry. It absolutely is NOT self adjusting. The wear only accelerates once it gets started.

"leaves a lot of concern with the aftermarket guys just changing out valves and guides."

As I wrote above, changing out guides and valves doesn't fix this. Steel and bronze run well together, but if the geometry isn't fixed (getting the rocker to stay on the end of the valve stem) then new guides and valves are just a temporary repair. As seen in my original post at the top of the thread, the roller rockers I got lucky with do this.