[ray_mcavoy]

You're welcome!

Here's a quote from the cooling system section of the 1963 Chevy truck shop manual:

"All engines incorporate engine coolant recirculation bypassing the radiator to keep the coolant moving, thus eliminating the possibility of hot spots during warmup. The 4 cylinder and 230 cubic inch 6 cylinder engines have an internal bypass drilled in the cylinder block. Ohter engines use an external bypass hose. The internal bypass system consists of a drilled passage in the cylinder block indexing with a passage on the inlet side of the water pump housing. The external bypass system consists of a hose from the thermostat housing to the inlet side of the pump housing. Both systems allow circulation of coolant through the engine, bypassing the radiator during warmup, at all engine temperatures. As the engine reaches operating temperature, the thermostat allows coolant to flow through the radiator as well as through the bypass passages."


Since the coolant flowing through the bypass doesn't go through the radiator it will reduce the system's cooling efficiency a little bit. I'm guessing that's why the pick-up truck applications only used one bypass hose. While the medium duty trucks (with their larger radiators) could probably handle a bit more bypass flow and were more likely to be run under heavy loads (that would be more apt to generate localized hot spots) and could therefore benefit from additional bypass flow from 2 hoses.