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One of my suspension bolts came undone while I was mountain biking recently, sadly I didn't notice. I landed a big drop with only 2-3 threads left engaged, and unsurprisingly it's ruined them.

The rest of the threads (another 20 or so) are fine. It's a blind hole.

I have the right size tap to cut the thread, but I don't know if that's the right thing to do. Don't I risk being "out of sync" with the 20 good threads and causing problems with them?

I feel there should be a tap that you insert deep into the bolt hole, and it then somehow expands into the known-good threads, before being able to be wound out.

If that does exist, please let me know what it's called and where I can get one.

If it doesn't, please give me advice on how best to fix this hole with the existing tap that I have is Frame Tap

Frame Tap

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    $\begingroup$ The tap in the link is a bottoming tap. It shouldn't be used to cut threads from scratch nor to chase damaged threads. Its purpose is to finalize a threaded blind hole after a taper then a plug (possibly just a plug) tap have cut the majority of the threads leaving some malformed threads at the bottom due to their leading taper. Essentially it does all the work at once which is exactly what you don't want. Get a three tap set or just a taper tap and you shouldn't have any trouble. Also see @Jim's reply below. $\endgroup$
    – user21645
    Commented Aug 30, 2019 at 6:44
  • $\begingroup$ Seems a bit poor when Park Tool sell this tool specifically to fix broken threads, not to cut new ones. Anyway, I used it, and it worked just fine. $\endgroup$
    – Codemonkey
    Commented Aug 30, 2019 at 10:45
  • $\begingroup$ Without a taper to lead the tap in and repair the damaged threads, your chance of starting off cross-threaded greatly increases; more so depending on the damage done to the existing threads. In your case, the threads at the bottom were fine; you just needed to repair a few at the top. With a bottoming tap, all the work is performed by the first one or two helixes. You can see from the image that a taper or plug tap distribute the cutting work through several helixes. This helps to line up the cutting teeth to existing threads so you are not cutting anything you shouldn't be. $\endgroup$
    – user21645
    Commented Aug 30, 2019 at 11:01
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    $\begingroup$ This seems like a question better suited for Bicycling. E.g., there may be safety aspects not obvious here that folks there would catch. $\endgroup$
    – Reid
    Commented Aug 30, 2019 at 17:29
  • $\begingroup$ I didn't know that was available; I did think "Engineering" was overstating my question a bit! $\endgroup$
    – Codemonkey
    Commented Aug 30, 2019 at 18:17

7 Answers 7

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I'm not a machinist, but I've tapped a few holes in my time. In my experience a tapered tap will handle this problem as the taper will reach down to the good threads and self align. Use some oil with the tap. I looked at your link and that tap doesn't have much taper so perhaps a more tapered tap might help if you have problems. There are any number of Youtube videos showing how to fix a tapped hole.

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    $\begingroup$ If there's enough meat around the threads it could be helicoiled, but most bike lugs are built to be light, so this may not be possible. $\endgroup$
    – TimWescott
    Commented Aug 29, 2019 at 14:57
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    $\begingroup$ Just went ahead and gave it a go with the tap I had and 30 seconds later I was done. Piece of cake. $\endgroup$
    – Codemonkey
    Commented Aug 29, 2019 at 15:06
  • $\begingroup$ Sometimes the simple solutions are the best! $\endgroup$
    – Jonathan R Swift
    Commented Aug 29, 2019 at 16:24
  • $\begingroup$ @Codemonkey You should wait a day or two to potentially allow someone to draft a better answer and then accept the answer you feel you like the best by clicking on the check mark. $\endgroup$
    – Eric S
    Commented Aug 29, 2019 at 17:09
  • $\begingroup$ My thoughts before I read answers. An extra point may be to remind OP to clear swarf out of the blind hole before re-assembly. $\endgroup$
    – tim
    Commented Aug 30, 2019 at 14:05
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The product that you're describing (an 'expanding tap') doesn't exist, unfortunately.

I had a similar problem a few years back, and actually built my own out of a Sleeve anchor, like the one pictured below.

As you tighten the nut, it pulls a tapered cone inwards, pushing the sleeve apart. I loosened the nut until the sides were parallel, shaved down the diameter using a lathe, except for a small portion near the tip, which I threaded the using a die matching the thread I was trying to fix. I then hardened the very tip of the sleeve, being careful not to harden too much as this would cause it to crack when expanding. If you are cutting aluminium this step may not be necessary.

It wasn't pretty, and it wasn't in good enough condition to use a second time when it came out of the hole, but it formed a rough thread that was sufficient to allow me to align a 'regular' tap from the outside.

Sleeve Anchor

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    $\begingroup$ Flippin' genius, brother. I've ground a waist into a grade eight bolt so that the bottom few treads got the ones above the waist to worry a thread back into the top part. That was also on a fussy part. Normally just fill and drill and tap. $\endgroup$
    – Phil Sweet
    Commented Aug 29, 2019 at 23:04
  • $\begingroup$ That is a very creative hack. +1 $\endgroup$
    – user6335
    Commented Aug 30, 2019 at 13:31
  • $\begingroup$ Would depend on the material, and it's a touchy method that seems easy to get wrong, but definitely full points for cleverness. I can see this working really well in softer materials... maybe less so for harder steels, etc. You'd really need to nail the heat treatment, I think. $\endgroup$
    – J...
    Commented Aug 30, 2019 at 18:36
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You could drill out the hole to remove the munged threads down to the remaining good threads. This should make it easier for the tap to get into the thread grove correctly. Another thought would be to grind out the damaged threads if you could get a small cylindrical grinder. (Thinking Dremel tool here)

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  • $\begingroup$ Yes. This is the correct answer given that 20 threads are left. And yes, I have been a journeyman machinist for 40 years. You might have to invest in a slightly longer bolt that extends into the hole an additional amount equal to the removed threads (after chasing the remaining threads with a taper tap. $\endgroup$
    – user21645
    Commented Aug 30, 2019 at 6:49
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The most common way to fix mangled threads, if they're really beyond fixing and if there's enough meat to open the hole a bit larger, is to use something like a heli-coil (aka : threaded insert). For a given thread, a helicoil will have a special oversized drill and tap which you use to clear out the mangled threads to a larger diameter and tap. After threading in the helicoil you end up with the desired original thread diameter and pitch.

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  • $\begingroup$ That strongly depends on whether there's enough meat around the hole to support the helicoil. There's a lot of bicycle lugs that, in the interest of light weight, have just enough metal around them to support the threads as is, but would just fall apart if you tried to tap them for a helicoil. So -- it's a judgement call. $\endgroup$
    – TimWescott
    Commented Aug 30, 2019 at 19:10
  • $\begingroup$ @TimWescott Indeed, hence ...and if there's enough meat... $\endgroup$
    – J...
    Commented Aug 30, 2019 at 19:12
  • $\begingroup$ @J... that "meat" may be sufficient for the suspension task, not to be used for enlarging the hole for a heli-coil, which does require a significant amount of "meat' that no longer contributes to the strength - especially so as it is on a mountain bike... $\endgroup$
    – Solar Mike
    Commented Aug 31, 2019 at 7:37
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The product that Jonathan R Swift created for himself does exist in commercial form, for 10, 12 and 14mm spark plug threads. Can save having to Helicoil a head, including all the labor to pull it, etc. Sold by NAPA, Amazon, and others.

https://www.napaonline.com/en/p/SER3689

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Use a 2nd Tap to clean the threads. Take out the tap and apply a thin layer of PTFE tape to the tap. Put the tap back in the hole pack the top of the hole with plastic metal. Let it set and wind out the tap. Flatten off the outer surface of the hole with a file run the tap down the hole again. Paint outer surface, Job Done. Sorry I cannot remember the trade name of the 3M's stuff. But I have posted a link to similar stuff. https://www.maxkote.co.uk/application-areas/metal-repair/

Note; Hand Tap come in sets of 3; taper, second and plug. Taper is the one with the pointed end. Plug is the one with flat end and almost no leading taper. Second looks a little like a plug but with more leading taper at the start.

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  • $\begingroup$ I believe Taper, Second and Plug are EN-BR (British English) terms. North Americans use Taper, Plug and Bottoming. See this $\endgroup$
    – user21645
    Commented Aug 31, 2019 at 0:05
  • $\begingroup$ @Jeeped the English or at least some do use "bottoming" tap... as the name is self-evident ie the bottom of the hole... $\endgroup$
    – Solar Mike
    Commented Aug 31, 2019 at 7:39
  • $\begingroup$ @Jeeped brighthubengineering.com/manufacturing-technology/… $\endgroup$
    – Brad
    Commented Aug 31, 2019 at 22:50
  • $\begingroup$ @Solar Mike your confusing the names a little Mike. Understandably so. Plug in American taps is the same as second in English taps. Bottom Tap in American En is the same as Plug in UK. see the links Jeeped gave and I have given. $\endgroup$
    – Brad
    Commented Aug 31, 2019 at 22:57
  • $\begingroup$ @Brad Experience working with taps in an engineering environment in the UK made sure I knew that we called a tap that went to the bottom a bottoming tap. So thank you for your confusion. $\endgroup$
    – Solar Mike
    Commented Sep 1, 2019 at 5:34
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There are a few options.

In general you need soomething like 1-1.5 times the bolt diameter's worth of thread to get full strength, with a steel bolt in a aluminium thread this may be a bit more but losing 2 or three with 20 left should be fine. In this case retapping the thread to clean up the mangled threads should fix the problem.

If you have a decent depth of thread a taper tap on its own may be adequate but a bottomoimg tap may be worthwhile just for the sake of clearing out any swarf.

If they are really mangled then it might also be worth drilling out the damaged threads completely. This will probaaly require using a drill stop to ensure that you don;t go too far into the intact thread and then retap to ensure you have a clean start to the remaining thread and also clear out any debris from teh original damage and the drilling process. Lubricate with a light oil or cutting fluid and then thoroughly flush out with something like degreasing spray.

It is probbaly also worth using a threadlocking compound to prevent the problem happaning again,m this will also give a bit of extra support to the thread.

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