Thermostat housing bypass 1966

[jonzcustomshop]

my 66 60 series has this thermostat housing bypass... (the two hoses "T" off on the top of the water pump)
what is its function?
also I have another one of these that came on my 65 bus, would I have any problem using this on a small 1/2 ton truck?

the reason I ask is that the unit that I pulled off the bus is all cleaned up, and the water pump is brand new, the hoses are brand new.
where does the thermostat go? above this housing, or below it?

thanks for any insight.

[ray_mcavoy]

I believe that bypass setup was used on all of the pick-up and truck 327's. The version used in pick-up truck applications had a pipe plug in the driver side hole of the adapter and only used one hose connected to a 90 deg fitting on the top of the water pump. The medium duty truck applications (like the one you pictured) used the 2 bypass hoses.

The thermostat should be located on top of the adapter. A bypass setup (such as this) allows some coolant to circulate through the engine in order to prevent localized hot spots from forming before the coolant gets warm enough to open the thermostat and allow full circulation.

The small block Chevy water pumps & blocks do have a built-in bypass passage located on the passenger side (just below the lower water pump mounting bolt). I'm not sure if that bypass was blocked off in these truck applications so they used the external bypass hose(s) instead. Or if it was left open allowing flow through both passageways. I think some Corvette 327's used a similar (but not identical) external bypass hose connecting to the top of the water pump.

[jonzcustomshop]

thanks! so I should have no problem just using that setup on my other truck? looks like it will just help things if anything.

[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.