Cylinder Head - O.Ring Service
What's an O-Ring Service you ask?
An O-ringed head means a groove is cut onto the head, the whole way around the combustion chamber. Then a ring is inserted into the groove so that just the rounded edge protrudes from the surface. A receiver groove is then milled into the head. When a pliable material is used, the ring's protrusion pushes the softer copper gasket into the receiver groove cupping the combustion chamber and completely sealing it off.
This is used to change the geometry of the seal. Instead of two flat surfaces being held together by the bolts, you are actually pushing material beyond the flat sealing surface, cutting the chamber off entirely.
What does this mean for you?
This allows for high boost levels and low changes of blowing the head gasket.