There are only a few ways that can happen and they all involve removing the head. Cracked cylinder or head, warped head or blown gasket.
I would get the head off and take it to a machine shop for checking warpage and/or have it magna fluxed. However, since the cylinder gets enough water in 30 minutes to stop the stroke you could try some dye penetrant and quite possibly find the crack yourselves.
Let's hope it is the head so Chris can get the new ones he has been wanting. If it is not the head, well let's just say that Chris will be on his knees begging for an engine instead of curtains. On the other hand you could pull the plug and still have one more cylinder than Arcy. Actually I think I recall Chris claiming 370hp so running on seven will still leave him with 324hp and that's enough to race Sid and Ralphie.
