The Stovebolt Site Forums The Shop Area Engine Shop Relief valve in radiator tank questions

My radiator developed a small leak at the thermostat in the top tank of my rad. I took the wire lock off to check it out the gasket was broken causing leak.On examining the thermostat itself the rubber seal on the bottom of it is rotted half off,can you still get these thermostats and who supplies them.The part #is AC-476-4#. can I just seal up the top part and run it this way or a new thermostat must be used. Any ideas Thanks biddaboy :confused:

On my 54 GMC with the thermostat in the stock radiator I had the same problem. Mine went out on the Grapevine outside of L.A. NOT A GOOD PLACE! When I finally got home I had the radiator shop remove the thermostat altogether. I connected the overflow tube to an overflow bottle and now it works like a newer truck. Cost was thirty bucks.

where did you connect the overflow tube on the rad,did you remove thermostat housing altogether andpatch the hole where it was. Thanks for any info you can provide Biddaboy

The thermostat you guys are referring to is not a thermostat at all. It is a pressure relief valve. If pressure builds up in the radiator, the valve activates and antifreeze travels down the overflow..

I need a new one are they available Part#is AC-476-4#,or can I bypass it Thanks for any help Biddaboy

I used to have a '51 GMC 350 w/248 and radiator with the 4 lb. relief valve in the top tank. I pulled it out, got some high-temp sealer and was done. Connected the overflow to an aftermarket recovery tank. I never lost coolant after that.

If I take the valve right out I have an open hole in the tank,can I leave it in and just seal around the top of valve then run overflow tube to a recovery tank Thanks Biddaboy

what the heck R U guys talking about? :confused: the rad cap? [which should be 4#, and should be available at any auto parts store] never seen any kinda "relief valve" in a task force rad beyond the cap, which when it gets above its pressure rating, lets the pressure off thru the "overflow" - if the cooling system is in good repair, and you don't overfill the rad, that function will not be used, but if it runs too hot, it'll [censored] a bit of water when you shut it off and the temp goes up a bit

w/ a good rad and such, a 7# cap will work too - specially w/ a small block

Bill

Bill I am talking about an older rad 49-54 with a relief valve on the backside of the top radiator tank which the overflow tube is connected to.

hmmp, well I've never seen such a thing - what kinda rad cap does it have then? my 46 had a no pressure cap, so no pressure in the rad - so this must have a sealed cap? seal up the relief and put on a 4# cap? guess GM had a hard time figgerin out the pressurized rad thing!

Bill

There should have been a round brass disk holding it all in.........just leave out the valve and "glue" in the disk and put the retaining ring back in place in front of it.
Connect your overflow (did your metal tube break off? or is it missing?) to a recovery tank; your local supplier should have one, if not, there is always J.C. Whitney............
You can use clear vinyl tubing from the hardware store, it's the same stuff they use in swamp coolers............it's nice because you can see the coolant in the hose and which way it's going; either to the tank or back to the radiator.

engine view w/recovery tank

The vinyl tubing yellows with age, but is cheap to replace...............sorry about the poor quality of the pic, it was scanned from a chem. photo.

No, I don't own this truck anymore.............

Thanks GMC 248 thats what i needed to know,one last thing what poundage cap are you using on your system. Thanks alot you guys this board is great Biddaboy

Squeeze: This is one of those GMC thangs.

The pressure relief valve in the radiator top tank is standard on all '47-'55 GMCs. The GMC radiator, unlike the Chevrolet truck radiators are not only different but use a sealed cap, not a pressure relief cap. There is no overflow on the orginal radiator cap neck. Instead, GMC used a little "button" style pressure relief valve located at the exact expansion level in the top tank to allow excess coolant out and create the necessary head space for proper pressure (4 psi) operation.

The valves are indeed available at any radiator shop that deals with truck radiators as they are still used in some truck radiators today. I do not know the part number but will try to grub it up and will post it here if I can find it. It's a common part at a good truck radiator place so DO NOT throw the old one out.

By adding an expansion tank you are in effect running your system at zero pressure (unless you pressurize the expansion tank) and allowing the coolant to simply ebb and flow with pressure.

With the proper reflief valve, these old radiators work fine using the top tank as the expansion tank.

Hey Kook the part# on the valve is AC-476-4# is this a good number. So your saying replace the valve and use a non pressurized cap. Are these style caps still available,do you have a supplier or part# Thanks Biddaboy

Bid, you can get a higher pressure radiator cap (7# or ?)and get away with it that way, but yes replace the old 4# relief valve and let it run out the overflow tube. These radiators will typically [censored] coolant for a few days after filling the system or radiator while it finds it's level of comfort. That's why the overflow goes to the ground; to let the coolant go. It will often burp coolant when coming off the highway so this is normal until the system "equilabrates" and then it seems to settle down. They all do it this way in my experience.

Be sure to drill a 3/16 hole in the thermostat prior to refilling the engine to prevent any possible air entrapment behind the thermostat (if you drain the system) in the head. This will crack the head and is a COMMON cause of cracked GMC heads in 228-302.

The top tank or the original radiator IS an expansion tank and is designed as such. Adding an expansion tank on the inner fender will handle the overflow but will not properly pressure. Replacement radiators for these trucks do NOT have the original configuration and use an overflow tube off the cap neck designed to work with a pressure cap relief.

I saw your AC number and I have no idea if they can match it from that. It's not an AC Delco part, it's a radiator manufacturer part #.

Yeah, I forgot to mention: I ran a 160* thermostat with an 1/8" hole dilled in the flat part (not the moving part). I also retained the stock "non-pressure" cap, so I guess I had a "zero" pressure system. So what? it worked, even in California heat. The engine never got above 190*.
OCK is right, GMC's are famous for trapping air and cracking the head, so be careful.........

Some of the big parts chains may have it... check with them, if no luck there, try the sites of the companies here(lots of links):

http://woogeroo.home.mindspring.com/autolinks/trucks/ctgparts.html

hope this helps.

-W

Kook, I recently had a GMC radiator rodded out & cleaned. The shop removed the funky top tank valve & soldered in a plug. I installed the radiator using a 4 lb. cap & I installed the original GM overflow condensor tank on it. No problems even here in the hottest of deserts.

Can any of you post a picture of the relief valve on your GMC radiator? I have 2 radiators and I think both of them have a crack in the neck that the pressure relief valve sets in. I took my radiator to a shop to be cleaned and checked out and I was not happy with the guy. My core is clean and pressure tested, but the lower supports are in bad shape and he did not try to fix them, or call me to tell me what he found before he put it all together, he also did not touch the pressure relief valve because he did not know how it worked or repair the overflow tube coming off the valve. I want to put a valve back in but I have not found a new valve yet and the neck looks cracked. I will try to post pics but it is really alot of work for me right now-computer issues. can you braze on a radiator(brakets & the side of the neck not the core)?

Kook,

The brass "cup" that the pressure relief valve mounts in on my radiator is cracked in several places. Would a good radiator shop have replacements for these in addition to the valve?

We can wait for Kook's answer, but my answer would be "yes". Also a good radiator shop may have old donor radiator available to either swap out with you or re-do yours.

How is your truck coming along? I have a spare radiator and I will not sell it, but I would be willing to trade it for something else GMC related. Drop me a e-mail if you are interested.

Give us some updates!!!

Take care!!

Kook is no longer on the board due to some political issues with the moderators. I doubt you will get any pictures from him unless you email him directly.

Phillip,

Thanks for the info, the '54 is coming along, but slowly, too many irons in the fire! Still trying to get it running. I have rebuilt the carburetor and got a new fuel pump, worked on it last night but no luck. Having problems with the fuel pump, will post them in a new thread.