The head doesn't "stop" at the right place, nor is there a head gasket,
per se. The threads on the barrel ARE the actual seal, and the head is
screwed down a specific distance, plus or minus the amount of rotation
necessary to make the head to barrel flange angular alignment correct.

Cy Galley wrote:
The Cylinder head is screwed on the cylinder like a big jar lid. I always
though that they used the old head with a new barrel, screwed it on tight,
then jig drilled the mounting holes is the flange of the barrel. I don't
know how one would index a new head with and old barrel. I suppose it could
be done with shim gaskets, but if you didn't get the right shim, the head
would not stop at the right place.