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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? 


  • PumpguyPumpguy Member Posts: 272
    I can't

    tell you anything about the valves or fittings you reference, but I can tell you about pipe thread sizes.  First of all, there are 2 kinds of pipe threads; tapered and straight.  Both have the same pitch, and typically a tapered male fitting will fit into a straight thread tapped female hole.  Pipe couplings are often straight thread tapped through.  The "correct" hole size for a tapered 1/8-27 pipe thread is "Q" or .332".  For straight threads, the hole size is 11/32" or .344" 

    I assume you will use a 1/8-27 tapered thread tap.  The important thing is not so much the hole size but how deep you run the tapered pipe tap.  The deeper it goes, the larger the hole will be.

    First I would get a fitting that has a 1/8-27 tapered pipe tapping; a tee, elbow, or whatever, and see how your new air vent threads into it; tight or loose, deep or shallow???  Most likely it will thread into it about 2/3-3/4 before it tightens up on the taper.  Then take your tapered thread pipe tap and see how deep it goes into the test fitting before it tightens up.  This will tell you how deep to run the tap. 

    Most likely the 3/8-24 tapped hole was a miss-match, but because of the taper on the male fitting, engagement was good enough to seal. 

    Hope this helps.  Good luck.
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  • ChrisJChrisJ Member Posts: 8,601
    edited March 2013
    wrong threads

    There have been many complaints on here regarding Maid-O-Mist vents having the wrong thread.

    DO NOT retap the radiator.

    Some guys use a die and retap the vent, personally I just use Gortons.

    Proper air vent threads are 1/8" npt and you use a size Q drill bit.
    Single pipe quasi-vapor system. Typical operating pressure 0.14 - 0.43 oz. EcoSteam ES-20 Advanced Control for Residential Steam boilers. Rectorseal Steamaster water treatment

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  • Hap_HazzardHap_Hazzard Member Posts: 1,349
    Don't tap a radiator to fit a valve.

    Ask yourself which one is harder to replace.

    I do, on occasion, run a 1/8 27 tap into a radiator vent tapping to clean it out, but that's just to clean existing threads, not to cut new ones, and you should never use anything other than a 1/8 27 NPT tap, because that's what radiator vents use. Always. If you find a vent with different threads, that should tell you something: this is not a radiator vent. I don't know what it is, but it is not a radiator vent. You'd be wasting your time trying to make it fit because it doesn't belong there.

    The reason you can pass a Q drill bit through a tapped 1/8 NPT or 3/8 NF hole is that the tap removes more material from the walls after it has been drilled. The drill just gets you close to where the valleys will be. In the case of tapered pipe thread, the difference is more pronounced at the opening because the drill is sized to the valley at the minimum diameter.

    Personally, I use an R drill if I'm tapping cast iron, because cast iron is very hard on machine tools. The difference is only .007", and if you run the tap deeper you'll cut valleys all the way and get more threads engaged when you make a fitting. This is for future reference, of course, in case you ever need to drill and tap cast iron.
    1983(?) Peerless G-561-W-S
    3" drop header, CG400-1090, VXT-24
  • Hap_HazzardHap_Hazzard Member Posts: 1,349
    might be wrong, but

    I think he said it was the Hoffman look-alike that had the non-standard threads.

    FWIW, I have yet to find a M-o-M that didn't have the right threads. I think the issue is that they are plating the nipples after machining them and not threading them undersize to allow for the thickness of the plating. To make things worse there is always a layer of crud in ever radiator tapping that makes it impossible to engage more than a thread or two. If I clean out the tapping and run the vent nipple through a die to scrape off most of the plating, everything's copacetic.
    1983(?) Peerless G-561-W-S
    3" drop header, CG400-1090, VXT-24
  • dabrakemandabrakeman Member Posts: 74

    The air vents are steam radiator air vents.  I do know that much.  I am aware of the earlier thread on this forum that mentioned the mismatched Maid-O-Mist threads.  What I find suprising, however, is that I have found the same situation on the newer Hoffman valve.  There is basically no taper in these valves.  I guess they are garbage.
  • dabrakemandabrakeman Member Posts: 74

    major diameter on the old Hoffman valve threads is at least 0.03" smaller than both the new Hoffman and the new Maid-O-Mist replacements.  Neither will come close to threading. 
  • Hap_HazzardHap_Hazzard Member Posts: 1,349
    edited March 2013
    Get a second opinion

    Do you have anything lying around that has a clean 1/8 NPT female fitting? Try the vent in that and see how it fits. Probably the vent tapping on your radiator just has some crud in the threads. If you can get your 1/8 NPT tap in without cross-threading (they are sometimes drilled and tapped at a slight angle, which makes it tricky), try going a couple of turns after it gets tight then back it out. If you just see black stuff on the tap, you have the same crud I've seen. If you clean it all out the vent should fit with a couple of turns of ptfe tape.

    I don't know what that black stuff is--maybe old pipe dope or something--but it's tough. It almost feels like you're cutting threads when you clean it with a tap, but you only see black crud on the tap. When you start to see metal you're done.
    1983(?) Peerless G-561-W-S
    3" drop header, CG400-1090, VXT-24
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