Mopar Muscle Association UK

Afternoon chaps
Question about coolant system, my 76 D100 318 truck has no expansion tank, assuming that its important within the system or can be run without (doubt it) truck expansion tanks are made of unobtainium so if I need one I was going to for a universal unit so next Question is fitment height important ie outlets higher or lower than rad
Cheers Doug

Most of our old cars didn’t come with expansion tanks and will run fine most of the time without. I’m a big fan of fitting them to keep the system purged of air and it certainly helps keeps temps down in traffic. Positioning isn’t that critical I don’t think but the higher the better seems to make sense. I use one from a Kawasaki Mule, it’s big enough and a sensible shape for ease of fitment.

That’s uncanny timing, just sorting fitting this today!

I was also wondering not only about height, but distance from the rad for the return.

Next to the rad is ideal but as long as you’re not fitting it in the boot I’m sure it will still do it’s job.

ideally you would also run a cap that is designed to let the coolant that goes into the tank back into the radiator as it cools.
otherwsie there is little or zero return..bsically just enough to fill the space above the end of the spring in the cap and the pipe, as it all cools down.

if yours has been drving about without a tank it probably has a standard cap at the moment.

presume one exists for your radiator, mopar called it the coolant reserve system, it was a unique cap .
Most just got standard, and have a vent pipe and standard cap, allows coolant out when pressure exceeds cap rating, avoids blowing the pipes off.
but seels when pressure is lower than rateing...

presume the return style cap allows inward travel when the pressure reduces to a certain level.. i.e when cold. i can imagine how it might work, likely to pressure activtaed diaphragm or some bi-metallic arrangement like the choke . but have never had one so can't say

retro fit of this stuff to pre 73 cars covered in mopar action about 20 years ago.

seem to rember a robust coil spring in the radiator pipes to stop them collapsing when cold.

this one might do the job

https://www.ebay.co.uk/itm/153855269721

or at least this is the style used with the 2 pipe bottle used on 1973 cars later versions probably exist and i have no idea if it will fit a truck radiator

1 pipe into bottle that runs to the bottom as a syphon tube
1 pipe into bottle just at the top

first pipe allowed expansion and suck back into radiator
second pipe was overflow onto the ground, bscially a vent so the bottle could over flow, and not collapse under vacuum.

i.e the bottle was never at radiator pressure like on a modern car, thats a different set-up again. just a 2 tank radiator really

keep in mind mopar used 2 lengths of spring in the radiator caps. obviously long filler spout and short spring results in a cap that vents at way below its stamped rateing. you need the correct cap with the correct spring length. I suspect some people with overheating issues have been caught out this way.

might be easier just to run standard 16 lb cap and leave the pipe vented to the road where it is.... pre 1973 style.

if you top off radiator it will vent until its finds its HAPPY place usually just below the bottom of the filler.
a nicer upgrade might be a thermo/viscous fan/hub. fan works when its needed at low rpm and somewhat free wheels when you hit the peddle.
has a heat activated spring and oil based setup in the hub. hotter= stiffer= more fan.

ooops too many words..apologies.... i think ive had to much coffee

Dave

Was going to go down the aftermarket expansion tank but an original one popped up on ebay got to get a cap but they're a common Mopar unit so available new