Sounds a little crazy, but I even "lap in" my cylinders now that we
don't do the head gasket thing on the Type 4 air-cooled.

I lay well-oiled 800 grit wet-sand paper on a flat pane of glass, and
true/clean the top of the cylinder with a 3-5 minute semi-orbital
rubbing. Then I clean thoroughly, and load the head edge with a little
valve cutting paste, rotate the cylinder a couple dozen times clockwise,
then anti-clockwise.

I have no clue if this helps seal better, but it makes me feel better.

From Snowtown, DC,
G. Matthew Bulley