These are really good gaskets. My Cometic failed a couple weeks after getting tuned. I figured it failed primarily due to head lift and the Viton coating getting scuffed away, but after removal i found that the fire rings were cracked:
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I've replaced a ton of head gaskets in my lifetime so far, in both performance and stock applications, and have never personally seen that type of failure before. But after googling around, sure enough there were other reports of similar issues from guys running standard Cometic MLS gaskets in high HP DSM or LS engines.
Cometic makes some badass stopper layer gaskets for the K series and other V8 applications, but nothing for our D series.
Pro Seal is the only brand for the D that I could find who uses a stopper layer. This is an extra pinch of metal that provides a tight ring of tension around the combustion chamber. This helps with head lift issues, and gives a tighter seal under high pressures. I beat on my Pro Seal for a month and a half on 20-22 psi using a GT3271, and never pushed coolant the entire time.
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What head lift looks like:
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Then as fate would have it, 10 days before a track event, fucking tree falls on the car.....
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so its been parked for a while now. Need to get it in the garage, but I have to finish building the garage first lol.