The Stovebolt Site Forums The Parking Lot Big Bolts Radiator Cap -- Right pressure?

What pressure cap should I have on the '65 GMC ton and a half with the 305 V-6?

Thanks,
John

Probably a 14 or 16 lb. cap. Every pound of cap pressure raises the boiling point 3 degrees F., so a 16 lb. cap will let the coolant get above 250 degrees before it boils. A clean radiator with enough swept area should keep the coolant below 230 in normal max-load operation, so you'd have a 20-degree safe zone before it boils. A 50% mix of antifreeze and water drops the freeze protection to -34 F., and raises the boiling point slightly, plus it helps the heat transfer, so don't run plain water, even in non-freezing weather.
Jerry

Thanks Jerry!

The cap on it currently is a 16 pounder. The problem is that I have a patch on the radiator that is leaking a little, plus the front-mounted pump impedes airflow through the radiator. So I'm wondering if going to a 14 pound cap, while lowering the boiling point, will decrease the pressure on my patch until I can get the radiator fixed. Or should I just leave the 16 pound cap and just get the radiator fixed before I try to drive long distance?

Regards,
John

going lower on the pressure should help out by takin some stress off your patch sometimes these ole radiators after so many years aren't ment to handle the high pressure like mondern cars have, i pulled a radiator out of a 65 2 ton chevy and when i pulled it there must have been bout half way full of brown coolant, someone took the top hose off and a mouse made his home in the radiator so i had to take it someplace and have it clean, and they said that they'd clean it and pressure test it... well it failled the pressure test with bright flying colors, the guy said that there were holes absoulutly everywhere,

a good friend of mine back when he was a mechanic pulled a radiator out of a 50's something chevy radiator was prefectly fine no leaks, he pulled it out of a running truck that was driven, he pulled radiator out, put it in the water tank and pressure tested it... ran er up to 16 lbs and holes everywhere....

maybe your rad. its good enough to handle the pressure (besides the leak) but just a little more food for thought

Nate

I'd pull the radiator, have it boiled out, and patched. I've known people to run different thermostats in Summer/Winter. Some of the folks down in Engine/Driveline have even been talking about installing electric pusher fans to help with cooling. Radiator ?????

John does the leak seem to be getting worse or is it staying the same? If it is staying the same perhaps reducing the pressure will help, if it is getting progressively worse reducing the pressure proably won't help. if it's is a crack forming in the tubes as opposed to a pinhole crack may widen with vibration on your trip to macungie. If you have time before your trip pulling it would proably be best route, would be one less thing to worry about.

Well, I guess the firetruck isn't going to make the Macungie trip this year. I was really looking forward to that trip, too. And making the trip to Baltimore for the BMI show probably isn't a great idea, either.

Removing the radiator is easier said than done. It has a cutout in it for the pump -- the shaft mounts to the harmonic balancer, runs through the radiator and to the pump.

The leak is right on the top where there's a fitting for a line to run to the pump. I had removed the line and put a plug in the fitting. But doing all that must have loosened the solder around the fitting. I can probably drive the whole thing to a decent radiator shop and they could most likely fix it right on the truck. That will get me to Macungie, at least. After that, I can do the hot tank thing.

Thanks for the advice. Its one of those things where what I want to hear and what I need to hear aren't the same thing.

John

The old trick when overheating used to be to loosen the cap - removing the pressure entirely. Cars have been driven hundreds of miles that way without damaging the engine. The problem with a leaky Rad. is that it leaks worse when you stop the engine. The heat and pressure builds up then - POP. Just leave it running at the pit stops. You can carry a few gallons of water, can't you?

really didn't wish to talk you out of driving to macungie. perhaps you can try it this week with a lower pressure cap and see how things go.

If you've got enough radiator, which that fire truck should have, try a 7-lb. cap. Just be sure the core is clean, and has plenty of airflow through it. The pressure is just there to handle extreme conditions like max-load pumping situations, or hauling a full tank of water up a steep hill. Unless your engine temperature is routinely running above 220 degrees, you'll never miss the extra pressure.
Jerry

I had my rad recored entirely with a much more efficient core, but I still only run a 7 lb. cap with an overflow tank.

When I took my old rad in and had it checked it failes so badlt it was beyond hope. The old soldered accordian cores just aren't made to be repaired as easily as the finned tube type in modern rads. The bonus is tha it gets up to the thermostat opening temp and doesn't get much hotter, even under load.
Wasn't a cheap fix, but it looks great and works bettr, just one of the few things I dont have to worry about breaking. y waterpump on the other hand,...

Well, the ole waterbus made it to Baltimore and back yesterday (and portions of the trip on the Capitol Beltway, even!) with no issues. JB Weld is marvelous stuff...

Macungie is back on.

Thanks! I have a little breathing room now to get the radiator properly attended to. In the meantime, I'll get my temp gage working ...

Thanks,
John

glad it worked ok

Yep, and that JB Weld sure can save you sometimes!

glad to hear macungie is back on for your chariot. i used jb to repair the pump casing from a freeze condition by the previous owner on the ashton's pump. ill be there with it along with the dump. see you there.