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I'm new to thread repairs and I was just wondering when using a helicoil, why does the helicoil need to be inserted if you have already tapped the new threads into a hole? Can't you just put a bolt in once you've tapped new threads?

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The threads made by the tapper used for helicoil is not going to work with the standard size bolts.

The taps used are specific STI (Screw Thread Insert) tools. But of course, if you decide the old thread is not too damaged you can use a standard tap and try to fix it.

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  • $\begingroup$ Thanks for clearing that up $\endgroup$
    – TMax
    Jan 20, 2021 at 4:00
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Why does the helicoil need to be inserted if you have already tapped the new threads into a hole? Can't you just put a bolt in once you've tapped new threads?

Consider the case of an M5 x 0.8 tapped hole. The helicoil will need the same pitch (0.8 mm) with a larger diameter hole. Note that the M6 is a 1.0 mm pitch so even if the helicoil tap was the same diameter the pitch wouldn't work. I haven't checked but I imagine that all the helicoil taps are non-standard threads.

The next issue is that the part being held in place has a hole for a 5 mm screw so tapping to M6 would require a modification to the part to be held. A related issue to this is that you might now have one odd screw in a set holding a part on.

In my (very limited) experience helicoils are mostly used to repair stripped threads in aluminium or to toughen up threads in aluminium that require frequent un/fastening.

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  • $\begingroup$ thanks that's a good clear explanation, my understanding is if you have a tap of the same measurements you could re-tap the hole if the damage is minor and skip drilling a larger hole is that correct? $\endgroup$
    – TMax
    Jan 21, 2021 at 9:25
  • $\begingroup$ Yes - if you can get the tap to re-engage correctly with the original thread and not gouge out more material leaving it weak. You've already lost some of the original due to damage. $\endgroup$
    – Transistor
    Jan 21, 2021 at 11:38
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Short and simple, if you want to use the same size fastener again, you have to use a helicoil. By drilling out the old thread with the new pilot hole you have made it larger thus the fastener size will go up unless a helicoil is used to take up that change in diameter. Further more, helicoils also prevent the threads from needing to be repaired in the future, because the helicoil can be replaced and it is made from (usually) a stronger material than the base material.

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  • $\begingroup$ ^this. It's usually not desirable to move to a larger fastener $\endgroup$
    – Jonathan R Swift
    Jan 20, 2021 at 16:24
  • $\begingroup$ +1 for this answer. It's exactly the situation I was in with a stripped screw on a 3D printer extruder housing. Not enough material for a larger hole and larger bolt, plus design constraints require same fastener size. M3 helicoils use a 3.1 mm drill for the helicoil tap. There was barely enough material, but it worked. $\endgroup$
    – fred_dot_u
    Jan 20, 2021 at 20:04
  • $\begingroup$ thanks for the answer $\endgroup$
    – TMax
    Jan 21, 2021 at 9:39
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Also consider that the damaged thread / hole has to be drilled out to allow the oversize helicoil to fit to provide the new thread.

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The simple answer is that using a helicoil allows you to repair a thread while keeping it the same size.

Say you have an M8 thread which is completely stripped, you now have an approximately 8mm diameter hole. If you want to restore the original thread size you need some way to reduce the size of the hole again.

You could just drill out the hole to the next size up and re-tap but that is not always desirable, for example if it is one of a set of tapped holes and you want to use the same size bolts for all of them, or if the male thread is part of a more complex part not readily available in other sizes eg a spark plug.

Helicoils can also be useful in materials which are a bit too soft to accept threads or for dissimilar materials. Because of manufacturing tolerances most thread standards tend to put the load on quite a small area of the thread and some materials are prone to galling threads.

Because helicoils introduce a bit of flex they tend to distribute the load a bit better. For example aluminium engine blocks often use some sort of steel insert for to take bolts where they are needed and helicoils are a convenient way to do that.

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