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Thread: 8 x 32 or 8 x 24 ?

  1. #1
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    Default 8 x 32 or 8 x 24 ?

    Hello folks, How do you decide the proper thread for a project when given two choices? Such as 8 x 32 or 8 x 24. Does a fine thread give more holding power? Also I have been told when the amount of thread goes into a piece that is the width of the bolt no more holding power is had. True or false? While on the subject could some one explain aircraft fit. Thanks guys & gals.

  2. #2
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    A finer thread does leave more core diameter so the fastener is technically stronger if that is a thing. Plus the spiral ramp angle of a finer thread is longer so it'll be more resistant to vibration making it loosen up. Also the finer pitch with this lower spiral angle makes it so you can achieve the tension you want in the bolt with less torque. So all in all there's much to be said for the 8-32 over the 8-24. Plus when I look at an 8-24 they just seem so crude to my eyes. Yes, that's certainly a personal taste thing.

    Yeah, it's true that the amount of thread found in the thickness of a nut is pretty well optimum. Any longer a thread engagement does not add to the holding power of the threaded fastener.

  3. #3
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    The tensile strength of the bolt vs the material it is in makes a big difference. A grade 8 socket head cap screw in 6061 aluminum would need to be installed to about six times diameter depth to take full load without stripping out the aluminum.

  4. #4
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    Quote Originally Posted by BCRider View Post
    A finer thread does leave more core diameter so the fastener is technically stronger if that is a thing. ................................
    It's technically stronger in shear (sideways loading) strength. It is weaker in tension (lengthwise loading) strength. If fine was stronger in both cases and more resistant to loosing under vibration, why would they make coarse threads?

    You can see it here by comparing chart E506 and E507.

    https://www.hobson.com.au/files/tech...apped-hole.pdf

    Steve

  5. #5
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    I believe I am pretty well traveled
    and I have never heard of 8-24.
    I do believe it exists, but it sounds odd.
    Is it a model engineering size that the British made up?

    -D
    DZER

  6. #6
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    If you can get 3 diameters of thread engagement, the 8-32 would be best for most applications, 2 diameters in steel. 8-24 is probably a special size.
    Last edited by old mart; 06-17-2019 at 01:03 PM.

  7. #7
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    You're probably not asking specifically about #8 screws , but if you were I'd say 8-32 is a NC (National Coarse) thread and there's seldom a good reason to use a coarser thread that's not a commonly used size.

  8. #8
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    Quote Originally Posted by SteveF View Post
    It's technically stronger in shear (sideways loading) strength. It is weaker in tension (lengthwise loading) strength. If fine was stronger in both cases and more resistant to loosing under vibration, why would they make coarse threads?

    You can see it here by comparing chart E506 and E507.

    https://www.hobson.com.au/files/tech...apped-hole.pdf

    Steve
    Your link didn't open for me. Not sure why.

    The screw itself has a larger cross section core so the screw should be stronger. So are you talking about the pull out strength or the tensile strength of the screw?

    And yeah, I didn't consider it at the first reading but 8-32 is already NC. So 8-24?

  9. #9
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    Quote Originally Posted by Doozer View Post
    I believe I am pretty well traveled
    and I have never heard of 8-24.
    I do believe it exists, but it sounds odd.
    Is it a model engineering size that the British made up?

    -D
    British Model Engineer threads are, probably without exception, finer than comparable diameter UNF, NF and BSF threads.

    Something you most likely know, and you are simply being our usual Doozer.

  10. #10
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    Aircraft fit?

    I have to plead ignorance, but I'm wondering if it's what you need to use if you make something out of aircraft grade billet aluminum?

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