Steve,  Chris is correct, I had a "vulcano" with my newly built motor!   Pressure test revealed nothing, but with the engine at idle and looking in the through the radiator cap it kept producing small bubbles about every few seconds.  Hard to get a good look with that great fan whistling past your ear!  The engine ran perfectly and after about 3 - 5 minutes suddenly I had coolant all over the screen.  It kept building up pressure until it vented in a big way.  Stripped down and nothing obvious except that my ":expert" engine builder was very "Fast".  No attentions to detail in cleaning off the plates which must have let a small leak between the water jacket and the combustion chamber.  Used tracy,s O rings, but yet to start due to day jobs in the road.  I have filled the water, so to see how much ends up in the sump!! Regards,  Neil