Silicone head Gaskets

>>>I'm a great fan of these silicon gaskets. Haven't had a single blown
gasket since switching
back in '98 (these gaskets are used on the turbo metro according to at least
one person).
Even on the 13:1 race motor and on the 1380 rally motor.

>>>Mike

Are these the gaskets that require a groove to be machined into the block
around each barrel or are they a simple substitution for the "regular"
gasket?