CorvetteForum - Chevrolet Corvette Forum Discussion - Z06 Took GM to court over dropped valve, and....

After 20 months, GM decided it will be too costly to go to trial and decided not to contest my claim fours days before the trial day. In some ways, I feel bad that it didn’t make it to trial because it would have established if the court agrees with GM that a track day is considered racing and thus warrantee repair can be denied. GM originally denied my warrantee based on not finding any defective material or workmanship. Then, shortly after I filed my claim in court, and my subsequent submission of my engineering report as evidence, GM changed their position and focused on my track day usage as their argument.

For detail background information on how this all started, http://forums.corvetteforum.com/c6-z...-warranty.html

I will break this post into two parts. The second part (to come) will deal with the legal arguments presented by GM, as well as myself. The first part (this thread) will deal with some of the technical findings from my failure investigation report, which was prepared and submitted to court as evidence. I conducted the investigation myself, tore the engine down completely and examined all internal parts. Some of the findings might be of interest to members of this forum. However, I have noticed in the last 20 months that there is a small group of individuals on this forum who like to mock the original posters when threads on blown engines appear. Accusations from missing shifts (I was accused of that), running engine low on oil (I was accused of that too), to being trolls (yes, I am finally posting photos for the first time on this forum) are only some of the examples. If you are one of those individuals, I suggest you skip this thread because I will have nothing nice to say about GM for the balance, and part 2, of my post. So you have been warned.

For the record, I am an engineer who has been participating in competitive driving since the ‘70s. Have built my own engines, designed, built and competed with my own chassis. Own my own engine dyno and have seen, and investigated, my share of blown engines. Also for the record, my Z06 is a 2007 model purchased new in early September 2006, so it is one of the earlier ’07 modes. The engine is completely stock, no tune, no aftermarket exhaust, not even an aftermarket air filter. It had 11,000 miles (less than 100 track miles) when it dropped an exhaust valve.

The major thing that stood out during my investigation is the extreme wear in the exhaust valve guides. I believe I am the first person to report valve guide wear on this forum. Looking at the broken valve stem, it is pretty obvious that it had an overheating problem. Discoloration and significant material transfer is very evident.

[IMG]http://i1234.photobucket.com/albums/...1/IMG_1042.jpg[/IMG]

GM’s hired independent outside engineer noted in his investigation report the same observation (see photo below).

[IMG]http://i1234.photobucket.com/albums/...1/IMG_1055.jpg[/IMG]

I was looking forward to cross examine this engineer on why he didn’t measure the valve guide when there is galling shown on the valve stem. It should have been the next logical step. Too bad I didn’t get the chance to do that. Besides the broken valve, there is another valve stem (cylinder #5) which shows a small amount of material transfer, indicating the on set of galling (see photo below).

[IMG]http://i1234.photobucket.com/albums/...1/IMG_1047.jpg[/IMG]

Measurement taken at the corresponding valve guide showed a clearance of twice the allowable service limit set by GM in their shop manual. Subsequent non-destructive testing showed the valve to be in perfect condition otherwise. No crack, no run out and the stem diameter is spot on with no wear. The broken valve stem diameter is also spot on. One interesting thing to note is that all the other remaining six exhaust guides are all sitting at the wear limit. For an engine that only has 11,000 miles, I don’t think that is acceptable. The problem with valve guide clearance is that once it is beyond 4 to 5 thou, the ability to conduct heat away from the valve stem starts to decrease dramatically. The problem then starts feeding itself. As the valve stem stays hotter, oil film starts to break down and wear will accelerate which increases the guide clearance some more. As my broken engine indicates, by the time the guide clearance approaches .008” (or before that), material transfer starts taking place. Most likely during wide open throttle runs when the extreme heat broke down the oil film. The valve stem is then like a rat tail file, wearing the guide material rapidly even when the oil film re-established itself.

Another important abnormality observed is the exhaust valve rocker tip. The tip is grounded during manufacturing to provide a profiled contact surface to the lash pad. Instead of grounding the full width of the tip, the sloping side of the rocker tip is not grounded. Hence, a step exists just adjacent to the grounded surface.

[IMG]http://i1234.photobucket.com/albums/...1/IMG_1036.jpg[/IMG]

One would think that if everything is designed and manufactured properly, the lash pad would not make contact with this “step”. As the following photos clearly show, part of the “step” is shinny and contact did take place with the lash pad.

[IMG]http://i1234.photobucket.com/albums/...1/IMG_1035.jpg[/IMG]

[IMG]http://i1234.photobucket.com/albums/...1/IMG_1034.jpg[/IMG]

My estimate is there was an overlap of approximately 0.015” between the edge of the lash pad and the step. Of all the eight exhaust rockers, six of them have a step that is approximately 10 thou tall. One is about 5 thou and there is one that is almost flat (the right one in the photo below).

[IMG]http://i1234.photobucket.com/albums/...1/IMG_1033.jpg[/IMG]

With the step, it will induce a torsional load into the rocker which is resisted by the two needle roller bearings at the ends of the trunnion. All of the intake rockers, in contrast, have the full width of their tips grounded (since it has no offset sloping side) and wear pattern on the tip is perfectly normal. GM’s previous service bulletin regarding a bad batch of rockers in 2007 did not disclose the actual defect involved. All we know is that, the needle bearings fall out when the end retainer breaks off. Is it a retainer defect, or is it a rocker tip defect, which induced torsional load into the rocker that caused the retainer to break off over time? I do know that a rocker tip is not supposed to jump over the edge of the lash pad a few thousand times per minute. I don’t know if newer rockers in my replacement engine have the full width of the tip grounded or are they still partially grounded like ones in the broken engine. I also don’t know if all the owners who found needle bearings stuck to their drain plugs confirmed if the failed rocker was an exhaust one rather than an intake unit.

To be honest, it is sad to see **** like this in an engine, any engine. Take apart a Honda Civic engine, any BMW engines or a Porsche engine and they are like a jewel on the inside. Some parts in the LS7 are very nice, and then you have **** like this like the rocker tip. Does GM actually have QC programs to check parts before they get put inside their engines???

It’s getting late and way passed my bedtime. Will continue another night.