So how do you change the heater hose nipple?

[Schurkey]

Not really. 1st thing we need to agree on is there's no air, which is compressible, in the system (water is not).

Sure. Pascal's law applies to fluids, which can be liquids or gasses, so I don't know why this is important.

In my 88, the return line to the radiator is just below the cap so (not the "cool" side, that's the bottom of the radiator),

You have a cross-flow radiator. The rad-cap side IS the "cool" side, because the upper rad hose (hottest) is on the other side. In order to get to the rad-cap side, the coolant has to flow through the water tubes...which is where the heat is removed. By the time the fluid has gotten to the rad-cap side...it's been cooled.

it's seeing the same pressure as flow coming from the water pump, when the T-Stat is open.

No, because there's restrictions to flow all over the system. Each restriction reduces pressure somewhat.

Yes, the closed thermostat is the biggest restriction.

If the pressure going through the heater core, was lower, the pressure from the radiator will apply too much backpressure, thus stop flow though the heater core.

The pressure on the cool tank of the radiator (rad cap pressure) is significantly less than rad cap pressure PLUS water pump pressure. There's no way to get stagnant- or reverse-flow.

The only reason for the restricted fitting is to help the heater core supply heat to the cabin when the T-Stat is still closed

No. See above. With the thermostat closed, you'd have the maximum values for rad cap + water pump pressure, and the maximum value for rad cap pressure - some amount of water pump suction. With the thermostat closed, you'd have maximum pressure-drop across the heater core circuit. A good share of that pressure drop happens at the restrictor on the inlet side--either a restriction in the quick-coupler, or a restriction pressed-into the heater core inlet tube.

AND help the engine warm up quicker with less flow back to the radiator, again until the T-Stat opens up.

Hmmm. Perhaps. I hadn't considered that. I'm going to guess that's a secondary feature, but not the primary reason for the restriction on the inlet side of the heater core.

And Pascal's Law does apply to any sealed system, whether static or dynamic.

www.britannica.com/science/Pascals-principle

Pascal’s principle | Definition, Example, & Facts

Pascal’s principle, in fluid (gas or liquid) mechanics, statement that, in a fluid at rest in a closed container, a pressure change in one part is transmitted without loss to every portion of the fluid and to the walls of the container. The principle was first enunciated by the French scientist...

www.britannica.com

in a fluid at rest in a closed container, a pressure change in one part is transmitted without loss to every portion of the fluid and to the walls of the container.

www.en.wikipedia.org/wiki/Pascal%27s_law

Pascal's law - Wikipedia

en.wikipedia.org

A change in pressure at any point in an enclosed fluid at rest is transmitted undiminished to all points in the fluid.

Pascal's Law (or Principle) applies to fluids AT REST. Coolant in a running engine is NOT AT REST, IT'S FLOWING. Pascal's Law is modified by fluid flow--it's not strictly applicable to a dynamic system.

Again--consider A/C systems. With the compressor off, and the system stabilized, you have fluid (gas) pressure the same on the high side as the low side. (Given a reasonable charge of refrigerant, you can even tell the temperature of the system based on the pressure.) Pascal's law fully applies when there's no fluid flow.

But with the compressor running, which produces fluid flow, you've got 30-ish psi on the low side, and maybe 300 psi on the high side. Obviously, the pressures are WAY different even though it's a closed system, and it's due to 1. the pump/compressor creating fluid flow; and 2. A bigass restriction in the TXV/Orifice tube. Pascal's law takes a flying leap on dynamic systems. That is, Pascal's law is not repealed, but it's modified by the moving fluid and any restrictions to fluid flow, where you have a base pressure within the system, modified by pump output pressure (base pressure + pump pressure) and pump suction (base pressure - pump suction) across one or more restrictions. For example, an A/C system that's under-charged will have a low low-side, and a low high-side; an overcharged system can have a high low-side and a high high-side.

 

[someotherguy]

Y'all making my head hurt. But it's good. I say go with whichever fitting you choose and if it works, it works; if it sucks, don't be afraid to post back here with real-world info on whether or not it worked for you. Ignore anyone with the "told you so's" and just be happy you shared useful information. None of us are perfect and the best thing is to not be afraid to admit you got it wrong.

Richard

 

[isle_cruiser]

Y'all making my head hurt. But it's good. I say go with whichever fitting you choose and if it works, it works; if it sucks, don't be afraid to post back here with real-world info on whether or not it

I was trying to avoid this debate...my bad haha

I'm going with homemade from the parts box. It will have a reducer, to an extent. I'll report back

 

[Erik the Awful]

The tl;dr of all the debate we've had:
If you want to swing for the Dorman fitting, it works. If you want to use a hose nipple with a restriction, it works. If you want to use an unrestricted hose nipple, it works.

 

[dwcopple]

Well, got it out today without issue. The quick connect aluminum hose part is dumb though, so returning the Dorman part and adding in a flushing TEE and a length of hose to a barb. I may install a copper pipe cap inside the heater hose with a 1/4" hole drilled in it.

 

[dwcopple]

Got her buttoned up today. Used a 3/4" nipple × 1/2" NPT dorman fitting.