Technical Radiator petcock for 1940 pontiac.

I pulled the radiator out about a week ago because it was dumping water. Come to find out that the petcock that was in there, pretty much rotted away.

So, I go into my drawers of parts and look at a normal petcock to look and see if it fits. It's the right size, but it looks like the thread pitch is wrong, and it's not pipe thread?

I went to Napa to see what they had, and they only have the pipethread one, unless they can look it up by make and model. Not sure their computers go back that far. lol

Anyone know where I can find a petcock for this? I'm thinking I might be able to salvage this radiator for at least a little while.

-Mark

 

Your Pontiac must have an unusual style peacock. Most are two piece, the outer part threads into a pipe thread hole in a boss soldered into the radiator tank, and an inner part that is often left hand thread for opening to drain. Are you sure you have all of the peacock removed from the radiator?

If you do, then look at how large the boss or bung is and see if you can drill and tap it for a different or larger petcock. Or, possibly, you could drill/tap and fit it with a reducer bushing, depending on what you find available in new or used peacocks.

If none of those ideas are useable, a radiator shop should be able to remove the old bung and solder in a different one which will accept a common peacock.

 

I think I have it all out. Looks like crap I know, but I haven't figured out where to get a new radiator yet.

 

Maybe it its a fine thread? I'd try screwing in a bolt, a petcock is handy but a bolt should work in a pinch to plug the hole.

 

Look closely at the hex shaped part that is in the radiator. Can you see if it's threaded into another piece, or is it soldered in? It could be that you just didn't remove the part with the pipe thread, you only removed the inner part of the petcock.

But it's hard to tell from the picture, since the steel bracket gets in the way of what we need to see.

also, it might help us help you figure it out, if you can find a bolt that threads into the hole, or almost threads in, and let us know what size it is.

 

It looks like that "nut" is soldered in place. Just looked at numbers and it's a harrison radiator and made for GM. So original? 3R special.

I did get a crazy idea and looked at my brake fittings and it seems to match. Might be a 7/16 brake thread. Now to find a tap to see if that's what it is and clean out the threads.

 

that's real close to 1/8" NPT isn't it?

 

It's bigger than 3/8's and less that 1/2. So I'm guessing 7/16, maybe 20np or 24np

 

I may have just solved my issue for now. I went out to my other 40 parts car and the radiator is the same. So I'll pull that radiator out and I can check the size of the petcock in the other one and at least get the threads right. The other petcock is bad, but it's still there.