by Hib Halverson
© May 2013— Updated: February 2016
No use without permission, All Rights Reserved
In mid-2012, official GM, finally, acknowledged the LS7 valve guide wear controversy. According to public statements made by GM engineers at the "Corvettes at Carlisle" event that summer and content posted on forum sites by Chevrolet Customer Service on 11 October 2012, the condition affected engines in "...some 2008-2011 Z06es." and was discovered during an investigation into warranty claims involving cylinder heads. The volume of problems with heads spiked from "...practically zero to 6.5 problems per 1000 engines." Chevrolet Senior Customer Relationship Specialist, Evan Sawaya, said in web forum posts, "Through inspection of returned heads, it was determined that a machining error in the valve guide had occurred at our head supplier." According to Sawaya, the most common customer complaint with engines having heads with worn guides due to this "...machining error..." is excessive valve noise. GM stated that the problem was solved in February of 2011 and the solution verified with inspection of 100% of cylinder head production.
In any warranty return head with badly worn guides, much less one on an engine which experienced catastrophic failure, evidence of improper valve guide machining might be hard to obtain. It would be difficult to quantify original guide diameter or the relationship of the original centerline of a worn guide to the centerline of its valve seat, soâ€“how did GM arrive at a "machining error" as the probable cause? We believe it possible that the suppler, Linamar Corporation, determined the cause though an internal investigation, then advised GM of its findings. It is, also, possible that GM could document the problem in warranty return heads which sustained modest rather than severe guide wear. It also could have documented the problem by inspecting replacement heads on the shelves at GM Parts warehouses.
We reached-out to Linamar twice during the preparation of the original edition of this story, attempting to obtain imagery of the cylinder head CNC machining process and to ask questions about the head machining and assembly processes. They did not respond. In early-February 2013, during the preparation of the first revision, we contacted Linamar a third time and, when asked to comment, spokesperson, Kevin Hallahan, stated only that production of the LS7 cylinder head was projected to end about the first of March, 2013.
The Corvette Action Center believes amount of heads with valve guide wear due to machining errors is likely higher than the 6.5/1000 number, in part because it does not include heads on engines which were changed due to damage caused by valve failure after the guides experienced severe wear. Since guides can be worn beyond factory limits in an engine which is not making abnormal valve noise or using too much oil, it's likely there are engines with worn guides of which their owners are unaware. There have been cases of '06 and '07 stock engines with prematurely worn guides. In the summer of 2014, we discovered evidence of LS7 cylinder heads assembled after the date at which GM stated the problem had been solved experiencing premature guide wear. We've documented that even new LS7 heads supplied by GM for warranty replacement purposes may exhibit non-concentricity of the guides and seats.
On 14 January, 2013. GM issued an "Information Service Bulletin" covering LS7 valve guide wear. We don't have the space to reproduce it in its entirety but its stating that GMâ€™s warranty numbers for LS7 valve and head replacement, "...are very low with no indication of an excessive wear issue." conflicts with: 1) facts gathered in the field, 2) anecdotal information posted to web forums and 3) what Chevrolet Customer Service stated in the Fall of 2012.
In the Spring of '13, we asked General Motors to explain some of the conflicts between the bulletin and previous statements about LS7 valve guide wear. GM referred us back to the Fall 2012 statements posted to the Corvette Action Center and other forum sites by Chevrolet Customer Service and declined to comment further.
Further confusing the guide wear issue was a March, 2015 interview we conducted with members of the LS7 Team at GM Powertrain Headquarters in Pontiac, Michigan, GM stated that Chevrolet Customer Service received incorrect information about the "suspect period". In this interview, GM Powertrain spokesperson, Tom Read, restated the suspect period as being from start-of-production 2008 to February 2011 to being from July 2008 to March 2009. Also, the LS7 Team told the Corvette Action Center that the machining error resulted in valve guides with "...excessive inside diameter."
A problem with excessive valve guide diameter is not the same as a problem with non-concentricity of guides and seats. Another key aspect of GM's statements to the CAC in this interview is that the revelation of a problem with improper valve guide inside diameter existing from 7/08 to 3/09, does not address what the Corvette Action Center's research indicates is a more long-standing problem with lack of concentricity of valve guide and seat centerlines.
In the Summer of '15, in a follow-up email, we asked GM Powertrain's spokesperson, Tom Read why, if 100% inspection of heads prior to their assembly was the solution to problems with guide wear, heads manufactured as much as a year later lacked concentric guides and seats. Read stated to the CAC in an email reply, "The heads leaving the supplier should always be in spec." and declined further comment on the seat run-out issue.
These conflicting and confusing situations are frustrating and suggest that General Motors may either misunderstand or, perish the thought, chooses to ignore (or a combination of both) the magnitude of the LS7 guide wear problem.
The summary of the rest of the January 2013 TSB is: if the car's engine is under warranty, its owner expresses concern about excessive valve guide wear and the dealer confirms that has occurred; GM will repair the engine, however, if the engine or engine controls system are modified, even if excessive guide wear is documented, GM may deny a warranty claim.
If the engine is under warranty, the owner feels the guides are worn but the dealer disagrees; the situation gets a little more complicated. To resolve the issue; the customer must pay for an inspection. If the guides prove to be worn; GM will reimburse for the inspection and pay for the repairs. If the engine is out of warranty; the customer pays the cost of inspection and repairs. If you want to read the complete bulletin, click here.
With claims against the factory warranty for LS7 head replacement, GM is going to supply new heads. The reason for this is a GM policy that powdered metal guides cannot be replaced and, for that reason, it does not make them available. In February of 2015, during a conference call with members of the GM LS7 Team the Corvette Action Center asked GM to change it's policy and offer the PM guides as replacement parts. Small-Block Chief Engineer, Jordan Lee, told us he thought that was a good idea, however, the final decision went the other way and, in mid-April 2015, GM's Tom Read told the CAC the OE valve guides would not be made available.
Engines out of factory warranties but under service contracts, also known as "extended warranties," and which have demonstrated excessive valve guide wear, may be eligible for either repair of or replacement of their heads. Whether or not and how that happens depends on the service contract's coverage.
In many cases of out-of-warranty LS7 heads with guide wear, the aftermarket does replace guides, however, the replacements are almost always made from bronze alloys, in part, because some of them are good choices for replacement of powdered metal guides, but mostly because replacement PM guides are not available individually. Our research found only two head service vendors which install proprietary PM guides in heads they service. Neither will sell those guides individually.
A Corvette service shop, "American Heritage" in Harbor City, California, sells a GM V8 valve guide made of Moldstar90, a costly, exotic alloy from specialty metals supplier, Performance Alloys. It's composed of 90% copper, 7.5% nickel and traces of chromium and silicon. American Heritage told the CAC that its Moldstar90 guides have lubricity exceeding that of the typical bronze guides, hardness approaching that of PM guides and are an excellent choice for production-based LS7 heads. We contacted a couple of engineers who work in metallurgy to confirm that but neither shared that opinion.
First, we contacted the Managing Director at a manufacturer and GM supplier of parts such as valves, valve guides and valve seats and is involved with materials selection. We asked him if he recommended Moldstar90 guides for LS7 heads. He told us that, while his company uses it for severe-duty valve seats, it is not a good material for guides because it is quite expensive and there are other materials which are better, one being manganese-bronze. The second engineer we interviewed, a Senior Manager at an aftermarket manufacturer of valve guides and seats who makes materials decisions, told us that for street-high-performance and racing engines having less than .750-in. valve lift, Moldstar90 is not a good choice and a waste of money. He went on to tell the CAC that manganese-bronze is the way to go in those applications because it's just as reliable and durable but costs less. Finally, he said that, for valve lifts over .750-in.â€“no one would do that with a production-based LS7 headâ€“then Moldstar90 becomes a wise choice because of its reliability with long-stemmed valves lifting .750 to 1.000-in. What kind of engines have such huge valve lifts? Blown-fuel hemis in Top Fuel Dragsters and Funny Cars.
Our recommendation for LS7 valve guide replacements? Forget Moldstar90. Use less-expensive, but just as durable, manganese-bronze guides from vendors such as CHE Precision and spend the rest of your LS7 performance money on other useful components.