This may be a dumb question, but can someone explain the flow of the coolant when an engine is cold and the thermostat is closed?

Typically if the stat is closed there is no flow through the engine... You could have a stat with a small bypass hole or two that allows some flow. I think high-volume water pumps like some of the Stewart pumps require them as they move much more water than a stock pump.

Well, I understand that since the thermostat is closed, there is no coolant flowing thru the upper radiator hose into the intake. But what is happening with the coolant the water pump is trying to circulate? The water pump is attached to the block so wouldn't it be pulling coolant from the block and circulating it somewhere?

The water pump pulls fluid from the radiator and pushes it into openings on each side of the block. With the stat closed it doesn't go anyhere. It's kind of like having a hose with a shut off valve on the end of it. You never really turned the water off at the faucet. Once the stat opens the fluid flows. back into the radiator.

On the pass side of the V8 block, there is a cast-in bypass hole. The w/pump pumps through this little hole when thermostat is closed.

Also, if the heater is hooked up, coolant will be pumped from the upper intake crossover, to heater core via 5/8 inch inside diameter hose, and return via 3/4 inch inside diameter hose, back to the water pump, directly above the lower hose, if a short water pump is used.

If a long water pump is used, then the return hose is attached at the 12:00 position on the water pump. Since you've stated a 67, I would assume, I hate this word, you have a short water pump, meaning the backside of the pump is touching, or very close to touching the timing cover. The long water pump installation will have a 1 inch(?) space from the rear of pump body to timing cover.

Whew......

I was under the impression that a stock waterpump would build up as much as 45 psi of pressure between the pump and the thermostat when the stat was closed. The passage Everett speaks of may be why the passanger side head tends to run cooler than the driver side. I don't see how fluid gets to the other side of the pump from it without going through the stat and rad core though. The block is full of fluid and the pump builds pressure but I don't think anything is flowing until the stat opens..

The water pump has cast into its body the matching "tunnel" onits supply side, lower rad hose. So the coolant bypasses this route until the thermostat opens.

Yes, the water pump is capable of building a deadheaded pressure of 45 psi. Once the thermostat opens, its all about flow, pressure from the pumping action of the pump, is gone, for the most part.

Water pumps are designed as a centrifugal pump, meaning they are not positive displacement as in the oil pump design. The water pump can be blocked off at the outlet(s) and nothing happens, except for the coolant heating up due to shearing itself. With the oil pump, block off the outlet and disable the pressure relief valve, the pump would break itself instantly.

There is really no great difference in temperature between the pass side & drvr side. But, if you want to improve the cooling effect, block off the intake crossover and plumb in tubing from the rear of the intake up to the blocked off crossover. Now one has removed the hot spots.

....also on the ZZ4 the bypass hole is not drilled all the way through the block....