Gaskets or brass shims, whatever you want to call them. There is a small soft brass ring in a groove in the cylinder head that serves the purpose.

I always had 55ax-es but somehow the ones that I bought from my LHS always seemed to go off within a year or so with some problem or the other. Not that they would stop running but that they lost reliability. May be the LHS bought them in the time OS was having those liner issue people keep referring to

Ameyam