ad gmc rad preesue valve fix

[54 jimmy]

as many of you know the gmc rad differs from the chevy in that the pressure releaf valve is mounted in the rad and not attached to the cap. i have looked and could not find a replacement anywhere. here is a quick fix if you plan on keeping your stock radiator. first pick shows the what we are starting with.
second is the valve cover removed. there will be a valve in ther mine was removed previous to try to find a new one. the third pic is of the new radiator cap i got at the auto store. if i remember correctly it was for a 79 chevy small block. you need to drill out the rivet in the center of the cap. i would sujest drilling it out from the top not the underside where the valve it at. fourth pic shows what it should look like after you drill the rivet out. the fifth pic shows the parts you will need from the recently butchered radiator cap.

[54 jimmy]

now we will install the valve in the rad. make sure you take note of wthich way the rad cap was taken appart as you need to install the valve the way it would go into the rad normally. the spring will face out twards the cover. you will not the spring fits nicely in the cover. make sure you get a new o ring to seal the cover or it will leak/spray colant all over the place. and lastley install the cover and snap ring. now you done. this little fix cost me under $10.00. works great. i add a recovery tank and hose like on a newer truck so i wouldn't lose any colant.

[54 jimmy]

30 views and no one has a comment. i'm shocked. come on good or bad lets hear it.

[solidaxel]

When the rad om my 55 1st series was 50 years old I bought her a new rad and hope it only lasts for twenty five years, as that is the only guarentee I want!

[tmoble]

what was the pressure rating on the cap you pulled the spring from? looks like it might have been a 16lb cap. You old honeycomb radiator won't like at all. neither will your heater core. I think you find these old system had like 6lbs.

might depend on the new installed height of the spring though. I'd get one of those pump pressure testers and check it.

[1project2many]

what was the pressure rating on the cap you pulled the spring from? looks like it might have been a 16lb cap. You old honeycomb radiator won't like at all. neither will your heater core. I think you find these old system had like 6lbs.

might depend on the new installed height of the spring though. I'd get one of those pump pressure testers and check it.

Similar thoughts to mine. '79 truck was likely 12-14 psi system but may have been higher. OP, maybe you've already done this but the depth between the seat for the valve and the seat for the cover should be compared to the radiator the cap is intended for. Too much pressure isn't good for an old radiator. I removed the pressure valve and fitting from my GMC radiator and I use a more modern cap. No worries about having to fab up a replacement valve on Sunday when I'd rather be driving the truck. BTW, congrats on figuring out the valves were similar in the first place.

[54 jimmy]

it is a 12 pound cap. i talked with a local rad guy and he said that it would be fine. the rad will handle it as well as the heater core. they haven't made rads or cores any different in the 70's as the did in the 50's. mine has been working fine no leaks or any problems.

[1project2many]

It's not that the core is different. It's that the metal has been oxidizing for a much longer timeframe and the core is that much closer to failing. Hey, if it works then it works. But not everyone gets lucky.

[tmoble]

that looks like a honeycomb core to me, and yes they did change the way the make radiators. sometime around 1960 they stopped the honeycomb deal and went to tube and fin construction which is still use today. I'd be checking around on this if I were you. Honeycomb cores don't like much pressure, especially old honeycombs

[54 jimmy]

i guess that is why the old timer at the rad shop said i could use my rad with the smallblock i have. my rad is not a honeycomb. it is a tube and fin. so is the heater core. i guess i got really luck that they are not the honeycomb ones. thanks for the info.

[mknittle]

COOL and easy too.

[dwcsr]

I think the ingenuity of your fix would be appreciated more over at Stovebolt.com. Its geared more toward original restoration than this site is.