[rotorwrench]

I've noticed that most if not all the more recent 2-piece head manufacturer's use screws around the plug bosses. Hylomar should work OK since it worked on Rolls Royce Merlin V-12 engines. I would use the more recent Loctite 515 and a bit of #50 silk thread around the bosses to insure a seal. That's how I seal Rolls-Royce cases now. An annular o-ring would have been the best bet but the Robert Roof heads go back a few more years than say the Frick types.

Kerby