Thread Size for petcock on SLC radiator?

Joel K


I want to replace the top petcock on the radiator with a barbed fitting. Wanted to confirm what size it is, I think it is a 1/2” NPT but not sure.

Thanks for the help!


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Joel, I'm pretty sure it's a 1/4" NPT, but I replaced the radiator with a custom one so I can't check. Whatever you do, don't use plain brass because it promotes electrolysis in aluminum. I used a zinc-plated piece.

As Scott mentioned, it is 1/4" NPT threads in the radiator ports for the petcocks. That is the size of compression fitting I used in mine for the hard plumbed vent line I ran back to the expansion tank (1/4" NPT to 1/4" compression 90 degree elbow).

Joel K


Thanks, this saved me from ordering the wrong parts for the bleed line.

I definitely did not understand how these are measured. I found this picture from an article which show a 1/4” NPT is actually about .5” in diameter. I thought it was 1/2” NPT since I put a caliper on the fitting and it measured about 1/2”, but not the case...

Learn something new every day.
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