What thread pitch is this?
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  1. #1
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    Default What thread pitch is this?

    I'm trying to figure out the thread pitch for the knob of an airplane's altimeter. I thought it might be 4-40, but it's finer than that. That's the finest standard pitch I can measure. I checked with my metric gage and .5mm looks like a pretty close match (although I'd be surprised if it's metric, but who knows?). The shaft diameter is about .113".

    Any ideas what thread this might be?

    Thanks,
    Jay

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    It may be 4-48, an old standard that you should be able to find a tap and die for from Victor.

    Try your 24 pitch gage and see if it hits every second thread.

    Your .5 mm is 50.8 TPI and .529 mm is 48 TPI.

    Larry

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    4-48 is an ASME thread; it is, as the description implies, 0.112" dia x 48tpi.

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    Kudos for giving virtually no useful information in you question

    'what' airplane
    'what' brand gauge

    BA threads are another instrument thread

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    Quote Originally Posted by gustafson View Post
    Kudos for giving virtually no useful information in you question

    'what' airplane
    'what' brand gauge

    BA threads are another instrument thread
    The British 6BA thread is .110 inch x 47.9 TPI and has a unique profile. A 6BA screw would probably go into a 4-48 tapped hole and be slightly loose.

    So the instrument maker's identity would help figure out what sort of thread it is.

    Larry

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    Hey, Larry.

    This one has a Cessna label on it, but I also have one that is from a Luscombe which appears to be the same size. As you suggested, I tried a 24 TPI gage and it appears too course. I'm going to give a local avionics shop a jingle and see if they have any ideas.

    Thanks for your help.

    Jay

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    The thread reference that I use shows a few in the vicinity of the measurements/estimates;



    Column C is the size designation, D is the name, E & F are the diameter in inches and mm respectively and G & H are the pitch in tpi & mm respectively.

    It's probably obvious that M is the thread angle.
    Attached Thumbnails Attached Thumbnails threads.jpg  

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    Also, you can accurately measure the pitch by putting a dial indicator on the top of the screw and measuring the amount of movement for one turn.

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    For anyone who's interested:

    I have an altimeter that came in a box of parts that came with a project plane I bought. It's labeled:

    Cessna
    United Instruments, Inc.
    Wichita, Kansas
    Made in Japan

    The thread of its adjustment shaft is M3 x .5.

    The altimeter that's out of my Luscombe (the instrument in question) is labeled:

    Type MB-2
    U.S.A.F Stock No. 6080-A80MB
    Aerosonic Instrument Corp
    Cincinnati, Ohio

    The thread of its adjustment shaft is 5-44.

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    Oh for Pete's sake. 4-48 is just as UP TO DATE as a 4-40, a 10-32, a 1/4-20 or any other UNC or UNF thread. It is a UNF thread that is on ALL the charts for fine threads.

    And it would be a really poor tap and die manufacturer that would not make that size. I would expect that most of them do make the tools for that size. McMaster alone lists at least 14 4-48 taps and one die.

    Now, 4-36 or 1/4-27 would be another thing. But that is another subject for another day. I do have the die and tap for one of those and I have encountered the other in a situation where it cost the company some real dollars.



    Quote Originally Posted by L Vanice View Post
    It may be 4-48, an old standard that you should be able to find a tap and die for from Victor.

    Try your 24 pitch gage and see if it hits every second thread.

    Your .5 mm is 50.8 TPI and .529 mm is 48 TPI.

    Larry

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    Piling on to what EPA says, any gunsmith would have that tap size. 4-48 is *not* uncommon.

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    Outside of guns, I find that #4-48 is rarely used. #5-40 is another one everybody seems to ignore. IMO, very good and useful sizes, but I've never worked at a place that would let you use them.

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    You can count threads with a magnifier and a ruler, not having a thread guage shouldn't be a deal breaker. . 4-48 should have 12 threads in a 1/4 inch or six in an eighth.

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    Standard M3

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    "Let you use them"? I thought shops made things according to the print, not someone's prejudice.

    I have seen many fine threads being used and also the odd diameters, like #5.

    So, if a part calls for a 4-48 thread, do you, on your own, just decide they really did not mean that and substitute a 4-40 instead? Or perhaps a metric size? M3 perhaps?



    Quote Originally Posted by Conrad Hoffman View Post
    Outside of guns, I find that #4-48 is rarely used. #5-40 is another one everybody seems to ignore. IMO, very good and useful sizes, but I've never worked at a place that would let you use them.


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