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radiator air vent thread size
I am replacing an old ITT McDonnell & Miller 1A (Hoffman Specialty) air valve on one of my radiators. I have two valves I can use, a new Maid-O Mist 1/8" angle and a new valve which appears to be the exact replacement for the one I am removing, except, for the thread. The major diameter of the threads on the old valve appear to be .375" whereas the major diameter on the new valve is more like 0.4".
If I look up a 1/8"NPT in the charts it says the proper drill bit is an "R" and the tap a 1/8- 27 NPT. Printed on the Irwin tap (8202) that is 1/8"-27NPT it says use with "Q" drill bit. The "Q" drill bit (which measures about .33" slides right through the existing hole that is already in the radiator?!? So, am I supposed to use an "R" bit even though the packaging for the tap says "Q"?
These replacement valves were in a box in the basement when I bought this house 12 years ago. My concern is that since the thread does not match the existing thread in the radiator I would not want to drill out a larger hole in the radiator just to find out that I have some oddball replacement parts in the basement that you cannot get anymore. To further confuse things 11 years ago I replaced another valve. Somehow I made it work with an existing drill bit I had (markings worn off but measures just under .375") and a 3/8"-24 tap. No issues, but, I apparently was less cautious it seems in those days.
Would appreciate someone who could just tell me for the available air vents you can buy today what is the proper bit and tap. And, just out of curiosity did or do McDonnell & Miller make two different thread sized valves? Which is more common?