SBC water pump bypass

[pat #25912]

For what purpose is the water pump bypass?

On my '59 (350 cid) the past owner has one heater hose connected to the bypass fitting on top of the water pump and the second heater hose connected to the heater hose fitting on the intake. There is a plug in the side of the water pump where I think the first heater hose should go. Would I be messing up the plumbing if I plugged the bypass hole on top of the pump, and plumb the first heater hose to the side of the pump? Thanks in advance.

[Joe L.]

Re: SBC water pump bypass

pat-----

The bypass on Chevrolet small block waterpumps serves 2 purposes. First, it provides for faster heater functionality, especially important in cold weather areas. Second, it provides for some coolant circulation should the thermostat fail shut. In my opinion, the second function is, by far, the most important function but I might not think that way if I lived in Thunder Bay, Ontario, Canada.

Small blocks use either internal or external bypass. Most all small blocks with cast iron manifolds and all after 1967 used internal bypass. In this arrangement, bybass is provided via a 3/8" hole in the block and left side waterpump mounting leg. Other small blocks, mostly those with aluminum manifolds and before 1968, used external bypass. This involves a connection between the upper fitting on the waterpump and the inlet manifold coolant crossover passage.

In your case, I would highly recommend doing exactly what ytou suggest: plug the waterpump upper bypass fitting with a pipe plug; connect the 5/8" heater hose to the fitting on the right side of the intake manifold; and, connect the 3/4" heater hose to the fitting on the right side of the waterpump. That will work perfectly and it's the arrangment you should have.

Don't worry about the pipe plug on the upper waterpump boss. MANY SERVICE small block waterpumps have the fitting and it is to be plugged for applications (like yours) that don't require it. ALL GM NEW SERVICE waterpumps sold for small blocks for the last 25 years, or so, have had the upper fitting on the waterpump. Also, mid-1972 through 1982 Corvette small blocks were ORIGINALLY BUILT with a waterpump with upper bypass fitting which was plugged with a pipe plug. Very unusual arrangement for a PRODUCTION engine, but that's how it was done.

[Pat #25912]

Re: SBC water pump bypass

Thank you Joe, that is what I was hoping to hear. To plug the waterpump bypass, I will use a plug with a square recess. This will allow me to use the correct motor-cross mount (no cut-out). Since I live considerbly south of Thunder Bay, the faster heating should not be an issue. Thanks again.