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Fine thread vs. course threads??

Discussion in '1973-1991 K5 Blazer | Truck | Suburban' started by Eric M., Nov 2, 2002.

  1. Eric M.

    Eric M. 1/2 ton status

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    I need to bolt on my rear spring perches (they got knocked off in an accident). I thought I'd purchased grade 8 fine threads, but when I got home, all I had were course threads. I've always heard that course have a tendancy to loosen on vehicles, is that true?

    I am using crush nuts and lock washers and was even thinking of Locktite as well.

    What is the real diference between course and fine threads?

    Thanks,

    Eric
     
  2. Shaggy

    Shaggy 3/4 ton status

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    <font color="green"> You should get the fine threads, not only do they have less tendency to loosen, but you can put quite a bit more torque on them as well. </font color>
     
  3. TX Mudder

    TX Mudder 1/2 ton status

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    It's all a matter of surface area.
    Fine threads will have more contact with a thread than coarse thread.
    It will tolerate more torque.
    Get fine thread.
     
  4. thatK30guy

    thatK30guy 1 ton status Premium Member

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    What the others said..... /forums/images/graemlins/wink.gif /forums/images/graemlins/thumb.gif
     
  5. hammer

    hammer 1/2 ton status

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    I put all course thread on my truck grade 8 haven't broke any yet
     
  6. HarryH3

    HarryH3 1 ton status Author

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    It depends on the direction of the load. In a shear situation, a fine thread and coarse thread have pretty much the same strength. But in a tension situation, the fine thread bolt has a much higher load rating. A coarse thread will fail much earlier due to the reduced surface contact area between the two parts. An excellent example is the 6 tiny studs that hold the spindle to the steering knuckle. They see tension loading, since the spindle fits too tightly into the knuckle to allow much shear load. They're grade 8, fine thread. /forums/images/graemlins/cool.gif

    Also think about the splines on an axle or transmission shaft. They're loaded in a shear direction. Yet the big, beefy looking splines on the output shaft of a 10-spline 465 tranny will fail long before those tiny looking splines on a 32-spline Turbo 400 output shaft.
     
  7. Seventy4Blazer

    Seventy4Blazer 3/4 ton status

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    all i use is grade 8 fine thread or grade 5 fine thread. if i am not mistaken you can only get grade 8 in fine thread!?!
    i dunno, maybe i am wrong.
    everybody has said the right stuff on the surface area and more TQ on them. they are all right on that. i would go with fine thread. 8 or 5 is good in the case of bolting to your frame. from what i understand your frame is about = to grade 5. 8 can be pulled through damaging your frame. i still use 8 though.
    Grant
     
  8. imiceman44

    imiceman44 1 ton status

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    Yes you are wrong but it's OK /forums/images/graemlins/grin.gif
    Grade 8 can come in either fine or coarse.
    I use some coarse for the spring eyes.
    Anyway, the arguments here on the fine versus coarse are close but no Cigars.
    The contact area is very close so this is not the reason.
    The strength issue is actually the opposite, fine thread has less depth and can strip easier, but if you look or think about the angles of a fine thread v/s a coarse, the fine thread doesn't loosen up as easily because the slopes are close to flat, as opposed to the coarse which are sloped and it makes it easier for the nut to slide and release tension.
    And for the torque thing, if you torque them to the same torque reading you will get more tension on because it's much easier to turn in those shalow slopes. So be carefull when you torque down fine threads verus coarse.
    I hope I didn't go too scientific on you guys, but that is what a mechanical engineering course will do to you.
    /forums/images/graemlins/thumb.gif
     
  9. Blazer1970

    Blazer1970 1/2 ton status

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    Coarse threads are:

    More readily available in industrial fasteners
    easier to assemble because of larger helix angle
    require fewer turns and reduce cross threading
    higher thread stripping strength per given length of thread
    less critical of tap drill size
    not as easily damaged in handling

    Their disadvantages are:

    lower tensile strength
    reduced vibrational resistance
    coarse adjustment

    Fine threads provide:

    higher tensile strength
    greater vibrational resistance
    finer adjustment

    Their disadvantages are:

    easier cross threaded
    threads damaged more easily by handling
    tap drill size slightly more critical
    slightly lower thread stripping strength

    In your application, I would use coarse thread SAE Grade 8 fasteners with hardened nuts and hardened lock washers.
     
  10. imiceman44

    imiceman44 1 ton status

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    That is put even better.
    And I forgot to give my recomendation, I second blazer1970's motion, go with coarse thread for the strength.
    Just make sure you use lock nuts, the crushed style not nylocks.
     
  11. hammer

    hammer 1/2 ton status

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    checked a ref machinery book and fine thread is 9% stronger than course. By torque specs
     

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