Water Pump Inlet/Outlet
 

Which water pump fitting (pass side of block) is the heater return line from the heater core...the smaller or larger one? I'm installing a Vintage Air unit and I'm ready to start the plumbing. Thanks in advance for your assistance!

Back in sbc days, pre LS, the smaller hose was the inlet for the heater core, and the larger hose was the outlet for the heater core. Always figured the hoses were different sizes to keep people from connecting wrong hose in inlet/outlet.

Now as for the LS water pump, I'm totally unsure which is which, but would hope that GM kept the hose sizing, larger vs. smaller, the same, meaning small hose is heater core in, larger hose is heater core out.
So basically, I didn't help you out one bit...LOL

We also had conflicting info so we pulled the lines and ran the engine. Pressure came from the 5/8" or rear port. This made sense cause it was engine side of the tstat bypass plate.

Clarification
 

So just to clarify, the smaller port on the water pump closer to the engine side is the supply to the heater core, and the larger port on the pump, closer to the front of the car is the return from the core ?

Thanks, For the clarification. I am just about to start that process. I just got my taps for the pump to convert to the AN #10 beadlock fittings to match my vintage air fittings on the core. :idea:

yes, 5/8" port is rear closest to the heater core and is the feed to the heater and 3/4" port is on the radiator side return from heater.

Thanks, I appreciate the help.

:thankyou: