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STI v "regular" taps

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  • STI v "regular" taps

    As part of my ongoing continuing edumacation...

    how much of a relative difference are we talking about between a
    STI tap and a "regular" tap of the same given size?

    As example, I have an 8-32 STI tap sitting in front of me, if I use that with the intention of using a 8-32 screw without putting in the insert, will the screw just drop through? Or will it want to start and then bind as it goes out of alignment due to the oversize nature of the now tapped hole? Or will something the next size up want to start and then just bind as it is too big?

    Asked so I can have it place mentally in terms of fit...sort of "got" the various fits in terms of class or more how they relate, is this STI fit just sort of "it is what it is" since it is meant for inserts?

  • #2
    STI Taps

    An STI tap is for installing Heli-Coils or generic coiled inserts. The insert is made of 60 degree diamond shaped wire. The outer part of the wire threads into the STI tapped hole, the inside of the wire is the threads for the fastener. If you thread a hole with an STI tap, the correct fastener will drop through. The tap drill is larger than the major diam. of the bolt or screw. I don't use coiled inserts unless I absolutely have to due to space limitations or drawing specs. I've had a lot of them pull out or come unscrewed. The better alternative is a KeenSert or keyed insert. Those are a solid insert that use a standard tap to install. They have small keys on the sides that keep the insert from turning once it is in place. Those are way stronger and more reliable than Heli-Coils. The STI taps are all specials since they have to have the same thread as the original only in a larger diameter.
    Last edited by Toolguy; 01-18-2011, 06:57 PM.
    Kansas City area


    • #3
      STI tap is a wire insert tap, So its gonna have the same TPI as the screw its designed for, but require a MUCH bigger hole. And as such the screw will likey fall right through, And a larger screw will have the wrong TPI to fit.

      Oversized holes/threads do not cause binding, they cause striping due to lack of thread engagement, Or just no engagement at all and the thread falls out.

      As this PDF states, STI taps are designed to have a 3B Tollerance, But I suspect thats after the insert has been installed.

      Also gives hole size for diffrent thread sizes. A quick look tells me STI taps require a *clearance* hole for the given thread size after insert, meaning without the insert, a screw will just fall right through.
      Play Brutal Nature, Black Moons free to play highly realistic voxel sandbox game.


      • #4
        This a question that is hard to believe.Have you looked at an STI tap and compared it to the screw?
        Just put the math to it. Or don't. It should still be obvious that the minor diameter of the STI is equal to or larger than the major diameter of the screw.