I would vote for "none". I know some people have done it (i think someone even went so far as to wrap come-alongs around the head and block to maintain some pressure), but I would personally be wary of the headgasket's ability to maintain a proper seal after the pressure has been released. This is the main drawback I've found with this motor, the fact that the headbolts have to be removed to swap a cam... I'm happy with everything else though!