Copper head gaskets........

The very first time I used coppers they leaked water because I listened to some of the local "experts" that said (A) the spray on sealer was the way to go, and (B) the coppers didn't need a re-torque after the first heat cycling.........both dead wrong.

The spray on sealer (forget which brand) was useless, too thin to fill any scratches in the deck surface. And copper head gaskets (hell any type head gasket truth be told) need to be checked for torque specs on any race engine after the first few heat/cool cycles. The best sealer I have found is the GE silicone with aluminum in it (forget the part number) comes in a calking gun sized tube.

The shop installing O-rings in the block needs to know what their doing too. The O-ring needs to stand out of the block a given amount. Too little & the O-ring won't embed into the copper enough. Too much & you might have a hard time getting the copper to fully seat on the deck surface. A shop that knows what there doing can install the O-ring wire just right so it's hard to tell where the seam is. A crappy shop might cut the wire too short leaving a giant gap that could leak & say "it's good enough".