John,
As described so accurately by "Keith EFFCEE"
"This mod was common in Grey motors that engaged in combat on the race track. Due to the poor design of the water circulation throughout the engine, the Grey Motor developed steam pockets in the head when in full war cry, as a result, heads developed cracks and headgaskets felt the need to be elsewhere with great regularity.
To solve this problem the thermostat was blocked off and holes were drilled at even points along the head. A coolant "manifold" was then made up to allow steam pockets to escape and return the water to the radiator."
This is the set up on my race car No 53
regards
Weddo