NOrrTH

This article explains in great detail the problem. I'm new to this, bought my Z last week, so I don't know if this article is well known here or not.

http://www.corvetteactioncenter.com/...-engine18.html

And here is the TSB from GM to dealers

http://www.corvetteactioncenter.com/...e-wear-43.html

LFZ

Wow....breaking news.

erikszr1

NOrrTH

If this is a well known article, then the mods can delete this thread.

As I said, this is all new to me and I thought the article explained the issue very well. Perhaps it would be of interest to other new Z owners.

Dan_the_C5_Man

It will be deleted, primarily because it is against forum rules to post links to another Corevtte-related site with advertising .

Jake5670

AzDave47

OP, this has gone thru the forum before, but it is good that you were able to find it on your own. Hib has contributed much to the understanding of what is going on. He even had heads warranty replaced in 2013 I believe, had them checked before they were installed and some of the valves were not concentric with the seats by more than the factory allowance. This was well after the Feb. 2011 timeframe that GM says they started doing 100% head inspection.

The head contractor, Linimar, also got the C7 Z06 e-dif contract to deliver them ready for install complete with the proper fluid level. The first few thousand e-difs were delivered with low fluid level and at least one CA owner picked up his C7 Z06 at the factory and only got as far as Texas before the dif gave up.

Great QC, don't you think?

bearcatt

Excellent article laid out with good images, also mentions the John Hughes gauge block.


Moral of the story, send your heads to American Heritage or similar.



.

ATC399

Good read again...With updates...Thanks for posting OP..

Michael_D

There are not many journalists that get the tech side of the article correct, because they don't understand it. Hib is one of the few that does.

FrankTank

Unbelievable

BRISLS1

The TSB seems a bit demanding to get any kind of help. From what I have read here on the forum, none of these warnings are visible to a driver -

<li>Is the Check Engine Light ON? Does the vehicle exhibit any starting/running concerns?</li>

<li>Have you experienced any concerns or difficulties that would indicate an engine problem?</li>

<li>If the concern is noise related, ask for a description of the type of noise heard?</li>

None of this seems like something you would get to get the tech to inspect the valves. The car sounds like a sewing machine even when it is good.

Bill Dearborn

Nothing wrong with a sewing machine sound. Used to be a complement. There are other things at work in the valve drop situation. I do not equate the number of people who find loose valve guide clearances as being part of the valve drop situation. Loose guides can cause a valve drop but they have to be really loose. Way out of spec. However, most loose guides being reported are barely out of the GM service spec which was written with the idea if the engine was apart the valve clearances should be reduced before reassembling the engine. The main problem people are trying to avoid is dropping and exhaust valve. Intake valves only drop at a rate one tenth of exhaust valves. If you look at the most popular current solutions to valve drops they include both replacing the guides and replacing the stock exhaust valve and using the stock intake valve. Right there is a reason to wonder why replace two things? Failures have one cause not multiple causes. The reason they recommend replacing both is they aren't sure what the primary failure is. In fact it may not even be either of those two things. Nobody knows. All we know is there has been a disproportionate number of exhaust valve drops on LS7 engines. Some of that may have come about because of what Hib found when he had his new heads checked before they were installed. A machining error that caused guides to get far to loose thus weakening the two piece exhaust valve causing the valve head to fall into the cylinder. Maybe all that is required is to make sure the guides are machined correctly and use all stock parts. Or maybe tired valve springs along with high revs caused a valve head to bounce off a piston.

Bill

AzDave47

Hib's replacement heads had all guides within spec. What they did have was valves that were out of spec regarding concentricity with the valve seats; that would lead to valve head fatigue and failure. My exhaust valve guides were more out of spec than at the top, my intake valves were more out of spec at the top rather than the bottom and all guides were in spec at the middle of the guides so all guides had an hourglass wear pattern.

outhouse

Factually False Bill.




Problems are often dynamic.


IN this case offset valves hitting the edge promotes failure. KEY WORD "promotes" that does not mean instant failure. It only takes out of spec guides to promote fatigue.


Now we also have a heat issue to deal with as guides that are worn excessively also have heat transfer issues, meaning the valves themselves are not able to dissipate the normal 75% of the combustion heat that normally goes into the head and out through the water and air cooled by the radiator.


So in this case Bill we have heat and offset valves promoting fatigue.






Bill these motors are built well and many will hold up under this abuse without letting go, but many are not holding up.













I'm sorry but we do know. I know.


Because GM will not admit it due to libel reasons means nothing here Bill.




Fact.




So lets start there and ask why.




Why do engines drop valves in racing conditions? #1 reason is loose guides.




Bill the sheer number of videos showing valves move back and forth with fingers with springs on all show excessive wear.




Bill why does GM list a tolerance specification for valve guide wear?

outhouse

Which is typical out of high mile or performance applications.




It is not acceptable out of engines with low miles that are not abused.

Dan_the_C5_Man

To "outhouse"

1. Bill doesn't respond to others on this matter - he post with the facts as he understands them, and his word is final. Did you see a hint of an un-answered question in his post? No. He knows what he knows, and no one is going to convince him otherwise.

2. Can you chill a bit on all of the "white space" in your posts? It eats up a lot of real-estate - just a suggestion.

stefuel

"Problems are often dynamic."

Can you say "the butterfly effect"?

outhouse

Its funny I wrote out, "I know you wont respond so my statement in purely rhetorical in nature" And I deleted the paragraph hoping he would "man up" and talk this out like an adult.


This is common in speed shops when dealing with weekend warriors who's keyboards outweigh their actual talent.


I'm new here and appreciate the heads up Dan.

outhouse

Sorry Man. This forum has a different setting then all the others I'm in.


My eyes are terrible up close and I don't have readers on. Ill try and get better at this




Thank you