hERCULES 236 strange problem

[alan white]

Chris Craft inboard, Hercules 236 flathead just rebored .060 over. Engine runs fine, but is blowing hot exhaust from a bolt hole on the aft-facing casting of the cylinder head. There are two freeze/casting plugs adjacent to the bolt hole in question, and two other bolt holes NOT blowing (the three form a triangle, assumedly to mount either an accessory or a lifting strap).
No water leaks from this hole. It is as if it is a spark plug hole in it's behavior, except that furthest-back cylinder is definitely firing. New head gasket, bottom end all redone in a good shop.
Is this simply a water jacket (should be)---- but no water issues from same.
Engine has only run by bucket cooling in the shop so far, and run very little (ten minutes). The owner (a very good mechanic) says he remembers no bolt sealing the hole before it was sent to the rebuilder. The rebuilder recommends just sealing the threaded hole with a bolt.
I suppose I'm wondering (and the owner is too) if there is a good reason why zero water is issuing from the hole (it's assumed so far that the backside of the drilled-too-deep threaded hole corroded through to the water jacketing, and yet the frequency of the power impulses are as if only one cylinder, not six. As if there's a direct connection to the cylinder itself--- it even caught a rag on fire.
Ideas?

Alan