Heater core

Turbo4whl said:

My reply might be too late, but I would not use teflon tape. More reasons than @Bextreme04 stated but I'll save that unless someone asks.

PST pipe sealant as stated is good. My choice is #2 Permatex, but I'm sure someone would disagree. Also, I would not use a silicone sealant unless the package specifically calls out, good for antifreeze, like an anaerobic sealant. For a anaerobic to work well, both metal connections must be clean. The #2 Permatex will work on rusty threads or threads with old sealant still present.

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I would certainly use teflon tape, but l would also certainly use brass fittings.

Dave M said:

I would certainly use teflon tape, but l would also certainly use brass fittings.

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Teflon tape can work well with brass pipe fittings, but does anyone really know why? I do.

The OP has no brass fittings to seal on his manifold/ water pump.

Factory service manual says the heater hose line should run from the water pump to the heater core. This should be the 5/8" hose (the smaller hose) because this is the pressure side pumping hot coolant to the core. The larger hose, which is the 3/4" return line, can run from the heater core to either the coolant port on the intake manifold next to the thermostat or to the port on the radiator.

The factory service manual specifically shows on truck with automatics the 3/4" return line runs to the radiator and on trucks with manuals it runs to the port on top of the intake. Not sure why this is. I don't see what difference that makes based on what type of transmission you have. I'm just parroting what the engineers specified. It seems like the return line going to the radiator would cause it to loose more heat than if it just went back to the intake.

I have two squares, a 73 C20 auto and a '80 K25 manual. On both I run the return hose to the intake and they work fine.

I love it. This post is crazy. Some of you may remmeber, I just completed my heater core replacement.

It started by me having to bypass the heater core on the side of the interstate. I was in the middle of no-where when antifreeze started gushing out from under the passenger side dash.

The smaller 5/8" hose ran from the back of my intake to the bottom port of heater core. The larger 3/4" hose ran from the port on the radiator to the top port on heater core.

I took the smaller 5/8" hose off of the bottom port of the heater core and (after pulling the 3/4" hose from the radiator fitting) simply snugged that 5/8" hose on to the radiator port. It was tight but it worked. It bypassed the heater core. I drove around like that for a month or two. When i got that heater core back in, i ended upswapping in new lines, but

The smaller 5/8" line attaches to the bottom port on the heater core at the fire wall and runs to the intake.

The larger 3/4" line attaches at the top port on the heater core at the firewall and runs to the port on the radiator.

So I got the heat working
Thanks for the help - it was just a matter of getting the hose in the right place
But a couple things
As you can see from the pictures, the heater core hose that goes to the intake is jammed right up next to the gas line
Is there something I can do to change that - like a fitting that comes off the manifold at a 90 degree angle?
Is there any danger of the gas line getting dangerously hot from the heater core hose?
Also I used plumbers Teflon tape - is that good enough? Should I redo the fittings with paste or high temp tape? Does it matter?

I'd see if you can get a 90° fitting for that fuel line. It's not going to explode yet, but rubbing is NG and I suppose vapor lock is a slight possibility in the warm weather.

I guess a 45° for the heater hose may be an option too.

rwljpl said:

What sealant do you recommend for these plugs? They’ve got a gray sealant but I don’t know what that is. I plan to do the work this weekend.

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Permatex brush-on gasket sealer. #80062

Blue Ox said:

I guess a 45° for the heater hose may be an option too.

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I agree. What size is the pipe thread in the manifold? 3/8" pipe is about 5/8" on the outside. 1/2" pipe is about 3/4" outside.

That's a little nugget right there...a good part to have! ^^^

Thanks guys - because of this forum I got my truck’s heat working again
I got the heater core hose to the right spot on the manifold, then got the right manifold fitting so the heater hose and gas line no longer touch
Really glad to have the wealth of info available here!

We'll send a bill, rest easy.