I just bought a set of sheaths and installed them on a customers car. They are my first set. My machine shop advised me to surface them right out of the box. He has had issues with some aluminum heads and especially the stealths in not being flat in the middle. He blew a head gasket on the dyno his first set. Sure enough, these were .004 low between the middle chambers on BOTH heads. I’m glad I listened. I cleaned up the bowls, hand lapped the valve to check the seal and bolted them on. Everything has worked well and them run great. Customer wanted higher compression and they work at 10:1 just fine. I’m happy. The only thing I will do on my next set differently is enlarge the oil drain back hole in the heads and make a slight groove from the gasket rail to the hole for better drain back. I’m having trouble getting a gasket to seal with all the oil that stays in the head.