Like the earlier poster, I've been following the "dropped valve" story for around 2 years now. If you go to "the other forum" and look around the Z06 section, you have to dig through cat fights and drama, but will eventually find the info.
An overview? GM released an official statement saying that a "machining error" had occured at the head supplier. Officially, they stated that this affected "a small number of cars", and was "contained as of February 2011 with all heads inspected after that date". One milling machine had been setup wrong at the head supplier. Now, exhaust valves have been dropping on LS7s starting with the 2006 model year. Considering how few LS7s are sold each year, how many head milling machines do you suspect the supplier had? Well...."the other forum" asked its members to pull their valve covers and showed them how to measure the amount of wiggle (or play) their exhaust valves showed on a gauge. 40 members have done it so far, and JUST OVER HALF ARE OVER THE SERVICE LIMIT FOR EXHAUST GUIDE WEAR. The numbers, photos, and owner reports are available. This is why quite a few people think that there might be two machines producing heads at the supplier, and if one of them has been setup wrong from 2005 to 2011, then that would mean that half of the LS7 heads in the field are machined wrong. This may be correct, and it may not even be close. Do not know. But, that is where that came from. They continue to have members check and report, so it will be interesting to see where the numbers go.
That forum has -tons- of blown LS7s from cars that were never tracked and were bone stock. One of the cars that received a replacement LS7 under warranty for a dropped exhaust valve had severe misfire at 400 miles. Turned out to be caused by excessive exhaust valve guide wear! ALREADY! AT 400 MILES! Valve was closing crooked! Here is the kicker, This replacement warranty engine from GM had a build date of September 2012. LONG after GM claimed the problem was fixed.
There will always be those who do NOT have a problem. One of my friends with a 2006 Z06 has 92,000 miles on his and NEVER a problem. I assume he got heads from the good machine! LOL! But, guys.....at this time we just do not know the truth. Is it really 50% of heads? I don't know. BUT, I personally have zero doubt that this IS a very real problem. Considering how few Z06 are sold each year, and how few of those buyers post on forums, an alarming percentage of those who DO are showing up with blown motors. I *will not* overhype or try to scare people, but I do feel this is serious enough that everyone should be aware of it. Everyone should NOT ignore it or claim it does not exist. Everyone should do some reading to learn what symptoms to look for (louder than normal valvetrain noise and oil on plug are bad signs), but be aware that some cars have dropped a valve with NO warning signs. If you track your car or go with an aftermarket cam then you are really playing with fire if you do not have your heads done. Bone stock cars are at risk as well. The bad part is, we just do not know how high that risk is.
WCCH is the place that most people and shops alike send their heads to for valve guide and valve replacement. Katech sends their heads there. I do NOT yet own a Z06, but once my Xterra is paid off this fall I plan to buy one to go with my Viper. Yes, even knowing about this issue. Because MY plan is to either buy one that has already had the heads done, OR I will simply budget that in as part of the buying process. Once the heads are done, I will KNOW that I do not have to even wonder if the car is ok. Small price to pay to enjoy such an incredible machine. For those of you who will choose to not worry about it and not do your heads, I am completely cool with that too! Enjoy your cars. But I did at least want to let everyone know that this issue was out there.