That hole at the top rear of the head is the hottest place in the cooling system of the GMC six cylinder engine. Mine typically runs 10-15 degrees hotter there that at the thermostat housing.
Some of the military versions of our engines installed a piece of tubing from there to a fitting on the engine side of the thermostat housing. Solves two problems.
1. Gets the hottest coolant immediately back to the thermostat and into the radiator.
2. Solves the "air pocket" problem. When the coolant gets low, like when you drain and refill it, the back of the head ends up with an air pocket. You can get rid of it by cranking open the fitting back there with the engine running and let the air escape.
'57 GMC 102, Original 347 V8, HydraMatic, 3.08 rear gear, added A/C, disk front brakes, HEI, AFB carb, '98 Honda Black Currant paint. T-boned and totaled 10/12
'52 GMC 152 Stake Bed, Original 228, SM420, added A/C, HEI, disk front brakes, '67 Chev 3.55 rear gear. Gets used as a real truck.
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