Early (39-54) GMC Master Parts Books show a reducing bushing to screw into the port near #5 cylinder position on the head that the sending unit then screws into. But the MPB has no information about the bushing. Most 55.2 thru 60 GMCs used a port on the lower thermostat housing for the temp sensor sending unit (and I think, but don't know for sure) did not drill and tap the head between cyls 4 & 5 for a sending unit.

I have a GMC head casting number 2193983 that was used on military and '52 thru '62 302 engines (the big port head). I just measured the size of the hole for the temp sending unit at 1/2" - 14 NPT. I'm pretty sure the head on my '52 truck's 228 engine has the same size hole, but I don't want to drain the coolant just to find out.

Another interesting thing about this head is that it has another 1/2" NPT hole at #6 cylinder that is piped up to the thermostat housing. It is used to get additional hot coolant (and any air pocket) away from the imfamous "hot spot" in the head. Most GMC heads have a flat spot here on the head casting, but it is drilled and threaded on the military heads.
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Last edited by Bill Hanlon; 01/21/2025 5:02 PM.
'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.