Which nipple is in and which is out?

[green69rt]

I think I know the answer but thought I would ask.  The picture below shows the two nipples that go to the heater.  Which is the inlet to the pump and which is out?

Edit 440 source pump housing on a 400 block.

[John_Kunkel]

The highest nipple to the rear is OUT, the lower one to the front is RETURN.

[green69rt]

That's what I thought.  Having trouble getting any heat out of my heater so was checking flows.

Thanks John.

[Bronzedodge]

The heater cores have a tendency to trap dirt on the coolant side.  The inlet and outlet are usually even with the top of the core.  Dirt, rust, and scale from bad maintenance gets trapped at the bottom, can't rise up and out.   I remove both hoses at the block and flush in both directions, alternately, with a garden hose - run both hoses over the fender.  A fender cover is your friend here.  Catch the outlet side in a 5 gal bucket.  You might be surprised at what comes out. 

[John_Kunkel]

Municipal water pressure can exceed 60 psi and can burst the heater core. I prefer to blow out the core with an air supply regulated to about 20 psi.

[john108]

See Diagram

[green69rt]

See Diagram

Thanks John, that's the diagram that I used to hook things up.  Only question, in my mind was, I use a 440 Source pump housing so was wondering if the nipples were different.  Looks like they are not.

I am installing a Classic Air system and wondered why I got no heat when I moved the temp control.  Turns out the coolant valve is not moving.  Now I have to figure out why.

[john108]

My coolant valve had a cable from the dash that opened and closed it.  
I don't know how yours works.
My water Pump housing was from Mancini.

[green69rt]

My coolant valve had a cable from the dash that opened and closed it.  
I don't know how yours works.
My water Pump housing was from Mancini.

On my AC all the doors and valves are moved by electric servos, no cable actuators.