Turning a .375" hole into a threaded hole - insert or?
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    Default Turning a .375" hole into a threaded hole - insert or?

    I have a transmission case - cast aluminum - that has a .375" dia. blind hole, 1" deep. I want to make it a threaded hole.

    I can use a threaded insert (NOT a Stanley, lol) and 'Helicoil' it to 3/8-16. But, I'd prefer the hole end up 5/16-18 anyway, and I'm wondering if it's better to thread the hole to accept an 11mm threaded steel rod, Loctite in a rod, then drill and tap the filled in hole for 5/16-18?

    This would leave a minimum wall thickness of about .030 per side - is that enough?

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    Quote Originally Posted by GregSY View Post
    I have a transmission case - cast aluminum - that has a .375" dia. blind hole, 1" deep. I want to make it a threaded hole.

    I can use a threaded insert (NOT a Stanley, lol) and 'Helicoil' it to 3/8-16. But, I'd prefer the hole end up 5/16-18 anyway, and I'm wondering if it's better to thread the hole to accept an 11mm threaded steel rod, Loctite in a rod, then drill and tap the filled in hole for 5/16-18?

    This would leave a minimum wall thickness of about .030 per side - is that enough?
    Application and load please, more info needed.

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    It's a front pump bolt on an automatic transmission. The load is all tension, no shear, and needs to be 'as good as' a normal 5/16-18 bolt will hold.

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    One of these maybe?

    McMaster-Carr

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    Looking at the linked insert specs, they want more than a ⅜" O.D. thread for a 5/16" I.D. thread. The "max strength" version is 9/16" O.D. thread.

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    Use Keenserts, I prefer them over helicoils.

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    Quote Originally Posted by GregSY View Post
    I have a transmission case - cast aluminum - that has a .375" dia. blind hole, 1" deep. I want to make it a threaded hole.

    I can use a threaded insert (NOT a Stanley, lol) and 'Helicoil' it to 3/8-16. But, I'd prefer the hole end up 5/16-18 anyway, and I'm wondering if it's better to thread the hole to accept an 11mm threaded steel rod, Loctite in a rod, then drill and tap the filled in hole for 5/16-18?

    This would leave a minimum wall thickness of about .030 per side - is that enough?
    I did some quick "napkin" calculations of the yield strength of a hardware store 5/16 bolt and compared it to the yield strength of a common aluminum hollow casting boss about the size I think you are dealing with. It looks like (and it agrees with my recollected strength of small thin hollow aluminum bosses) the bolt would be about three times as strong as the boss as the boss currently stands unthreaded.

    This would be easy to roughly test by just turning an AL cylinder approximating the boss and the coiling it or loctiting in a stud and then , using a spacer and nut, testing to failure. I believe the result will show the casting will fail with only minimal torque on the nut. I trust a direct test much more than calcs.

    Strength would be quite a bit better if the thin boss is only thin at its "depths" and ends more or less broadly at the surface. Hard to verbally explain that picture.

    I don't think your boss is beefy enough to come close to the strength of a low grade 5/16 bolt.

    Coils are much better than tapped holes for strength in aluminum. That is why they were developed 80 yrs ago---not as a repair but as an better way to seat bolts in aircraft engines and the like.

    Denis

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    Those things have always looked kinda weak to me...but may they well be plenty strong.

    Here's the catch - the trans case is quite thin around the hole....there's not a lot of extra meat for the OD of the insert. I can enlarge the hole to .390 or so but beyond that I think I'd have a strong insert anchored in very little aluminum.

    The factory bolts are 5/16-18 and thread directly into the aluminum case. They have a full 3/4" depth of engagement which helps. Most inserts are 1.5X the diameter which is fine because they are steel - but that doesn't help their engagement into the aluminum. That's why I'm thinking a steel rod threaded into the case a full 1" then tapped for at least 3/4" thread would be the strongest. But I'm unsure if the wall is thick enough.

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    Denis

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    For comparison try getting the CORRECT Loctite and make a similar test in aluminum. Just insert a smooth stud that fits the boss recess and use Loctite to glue it in. I would bet on the Loctite in the situation you describe. Henkel tech support will help you if you call.

    Denis
    Last edited by dgfoster; 10-24-2019 at 10:41 AM. Reason: clarification of proposed test

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    I prefer the Time inserts. When you get done it looks pretty.

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    My general fear of loctite is having it loosen up due to heat. And oil. And the difference in expansion between steel and aluminum.

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    Quote Originally Posted by GregSY View Post
    My general fear of loctite is having it loosen up due to heat. And oil. And the difference in expansion between steel and aluminum.
    Sure, gluing something in place goes against my knee-jerk intuition too, but that is where Henkel, based on their engineering expertise and decades of application experience, can advise you with authority. Major manufacturers successfully use Loctite (or competitors') products in OEM applications all day every day.

    I'd trust them a whole lot more than I would trust some farm-boy engineer wannabe like me.

    Denis

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    Quote Originally Posted by GregSY View Post
    I have a transmission case - cast aluminum - that has a .375" dia. blind hole, 1" deep. I want to make it a threaded hole.

    I can use a threaded insert (NOT a Stanley, lol) and 'Helicoil' it to 3/8-16. But, I'd prefer the hole end up 5/16-18 anyway, and I'm wondering if it's better to thread the hole to accept an 11mm threaded steel rod, Loctite in a rod, then drill and tap the filled in hole for 5/16-18?

    This would leave a minimum wall thickness of about .030 per side - is that enough?
    You don't need to have much wall thickness at all, bolt is taking the load assuming that it is enough long. Just like heli-coil that is a wound spring with zero axial strength.

    If you end up making your own insert make it with tighter fit than bog standard bolts to get maximum strength.
    I'd make the threaded sleeve in advance as its easier to drill and tap the bushing when its not mounted in transmission case.

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    When I look at how fragile the casting is, and porous, it's a wonder it works at all. I milled some of it and you can pretty much feed through it with a 1/2 end mill as fast as you want...it cuts away like it has almost no strength. Still thinking of ideas, appreciate all the comments.

    I should add that I'm adding this bolt as a 7th...there are already 6. But I realized I could add one more in a pretty important location so I want to do it.

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    Just installed some threaded inserts for 5/16 -24, od is 3/4 - 10. NC was the same OD thread size.
    We have some self tapping inserts at work but offhand I don't remember the OD.

    Dave

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    Find a fine thread that has 3/8 as the root diameter size. Peruse the special taps
    section in the MSC catalog.

    Tap the existing hole.

    Make a steel or aluminum OD/ID threaded sleeve and loctite that in place. Use one of the
    green 600 series type.

    Use your 5/16 bolt, jam-nutted onto the threaded sleeve for installation. Remove the
    assembly tool right away.

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    Use a 1/4 bolt in that hole only give yourself more room


    Sent from my iPhone using Tapatalk Pro

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    Misremembered the od. it was 1/2-13

    Dave

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    Given the thin wall, all the above options sound lousy. Jim rosen is on to something, I was thinking along the same lines as I was reading through the thread.

    how about making a custom 4140 pre hard stud with a 7/16-16 thread on one end, (7/16-16 taps are available and the minor diameter is .375), and step it down to the 5/16-18 for the attachment end? maybe just do that in all 7 positions?

    for this application you probably don't even need the tap, just cut some flutes in your large end with a cutoff wheel and send them in.


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