radiator cap and coolant overflow tank

[320 Newb]

I've got a 62 L320 with a J15 motor. I just had the radiator recored and am going to put an overflow tank on it when I stick the radiator back in. I know it's got to have the right type of radiator cap on it to get the coolant to siphon from the tank back into the radiator when it cools but what specifically should I be looking for in a cap? Is it the spring mechanism? Will a modern aftermarket cap that fits the radiator do the trick or is there something in particular I need to look for?

[kelowg]

Factory should work id think. Does it have the big spring with the lil spring loaded valve in the middle? If not the 521 cap might fit it.

[datzenmike]

I like to call it 'coolant recovery' because it actually returns it not just catch it. Any cap that was on coolant recovery vehicles will work if it fits your rad. For example the 620 truck and 810, had a recovery tank starting in '78. The '77 280z also had this. The special cap is part 21430-C0500. Just ask for a '79 620 truck rad cap and that will get you one.

 

 

Run a rubber hose from the rad to any container, (hose connections must be air tight) I used a bottled water bottle. Drilled a hole in the cap for the tube. The tube must be below liquid level so half fill it and push tube to the bottom. You can also get one from any car in a wrecking yard that will fit and look nice.

 

With coolant recovery, in theory you will never have to top up your rad. Every time I check mine it's full to the brim.n

[320 Newb]

Thanks for quick responses! I'll try out a '79 620 cap and see how it goes. I'm in Tucson AZ so it doesn't take much driving this time of year to get the overflow to spit up. 

[datzenmike]

IIRC the cap has an outer rubber seal... just like the old style... but in the center is a smaller rubber seal with a spring behind it.

 

http://www.mgb-stuff.org.uk/radcap.htm.

iller neck in context.

 

Excess pressure raises the main seal compressing its spring, allowing the pressure and/or coolant to escape via the overflow tube. The upper seal prevents it escaping from around the cap itself.

After excess pressure and/or coolant has been expelled and the system is cooling down. Only slight negative pressure is required to pull the low-pressure valve down, normally allowing outside air to enter the cooling system. But if lost-coolant recovery is in use and the end of the overflow tube is sitting in coolant in a catch-bottle, this will be sucked up and returned to the cooling system. The upper seal must make full contact with the upper filler neck for this to happen.

[Rjawm]

^ Awesome diagram find Mike

[MikeRL411]

And of course the end of the overflow tubing should not be directed to the ground, but should terminate in an overflow catch can.  Like a Japanese brand beer can !!!!!!!!  You don't need to suck air into your cooling system.  See my setup at the JCCS get together in September [guess what year of the meet].  I have often been the sole 410 / 411 representative at the JCCS meeting.