I was busy re-positioning the brake lever (left-to-right on the handlebar) by loosening the bolts that clamp the brake lever master cylinder onto the handlebar.

When I tightened the bolts I noticed the bottom bolt wouldn't tighten properly, upon further investigation I noticed the internal / female thread was badly stripped. Even so, because of the bolt at the top, if you tighten the top bolt all the way and tighten the bottom bolt as until it get to the stripped part, you can actually tighten the unit enough so that it sits tightly on the bar even if I try to forcefully move it.

So, I have two questions:

  1. Is it safe to leave it as is with the partially stripped thread at the bottom? (I'm guessing no because it would eventually rattle loose)
  2. Would it be safe have the stripped thread re-tapped to a larger bolt size or would it be safer to replace the entire unit?


Edit: Photos added enter image description here enter image description here enter image description here

Despite my best attempts the photos don't show the stripped thread very well.

  • 1
    Is there enough room to insert a longer bolt and affix a selflocking nut?
    – mikes
    Commented Sep 12, 2016 at 11:54
  • Unfortunately not, the thread hole doesn't go all the way through.
    – Willem
    Commented Sep 12, 2016 at 12:14
  • 1
    You could possibly repair it with a helicoil - bollhoff-armstrong.co.uk/en/uk-bal/helicoil-threaded-inserts/…
    – HandyHowie
    Commented Sep 12, 2016 at 13:26
  • normally if you have to ask "is it safe..." its not :)
    – Mauro
    Commented Sep 12, 2016 at 14:24
  • What is the make/model/year of the bike? Is the R1 you have asked about in the past? Commented Sep 12, 2016 at 20:41

2 Answers 2


After examining your added photos, here is how I would do it:

Remove the entire assembly, and clamp it well in a vice. Obtain the proper drill size for the tap you will need, from a really rough guess that looks like M6x1.0 -- but you'll need to measure the bolt (use the clean one) to be sure.

Slowly and carefully drill out the stripped hole, using minimum pressure and a lot of lubricant like WD40, kerosene, or PB blaster. Note that the "breakthrough" is going to be at an odd angle, so it's essential you proceed slowly, clearing out chips frequently, as the bit will want to "walk off" as you break though.

Retap using the proper tap. I suggest you get one with a long lead; there's no need for a bottoming tap as you will be tapping through. Be sure to reverse often to break up chips, and be particularly methodical on the last stretch, as this will have walked off some and you will only be tapping partial threads at an angle.

Now order a bolt 5mm to 10mm longer than the original from your favorite fastener supply. I think you'll be golden. Perhaps not as pretty, but I think under the handlebars not particularly visible. Note that (despite common misconceptions) only 3-4 threads bear all the load on an engaged fastener. It's like a barstool with 3 legs vs one with 25. Only 3 legs bear the total load. You can doubt me if you wish, but I know a lot about barstools...

The only other option is to use a threaded insert "helicoil", but this kit is not inexpensive, and I'm concerned how much additional "meat" you have in that cast aluminum ear. The helicoil process also needs to be approached in a thoughtful, slow, methodical manner. In this case the nice thing is that you won't have to drill through, and you can re-use the stock fastener.

Or you could price out a new housing/master cylinder, or maybe even get a used one? Seems "last resort" to me, but saves some labor and machining if you're not comfortable with drilling and tapping metal.

So, to clarify THAT ... I have a lot of helicoil kits, and if I had the right one, that would be the proper repair. If I didn't, (and I'm usually wrenching 30 minutes after I was supposed to meet up for a ride), I'd blow through the thing, tap, and use a longer bolt (I have shelves upon shelves of this stuff). Bottom line is that the helicoil kit is liable to be $50-60, whereas the tap and drill and longer bolt closer to $15. Shadetree (but sound) or sooper-professional; the choice is yours...

  • Do you mean retap at the same size but deeper so as to get past the stripped area or tapped to one size larger? Is there any danger to using the helicoil?
    – Willem
    Commented Sep 14, 2016 at 12:09
  • Yes, I meant drill all the way through and tap the same size all the way through. My only concern with the Helicoil (or going larger) is that appears to be aluminum, and there is probably sufficient "meat" to go +1, but I'd still be leery on something so important. The other advantage of a threaded insert (Helicoil is a brand) is that you still use stock hardware and don't have one "odd" bolt.
    – SteveRacer
    Commented Sep 15, 2016 at 11:49

A thread repair kit: helicoil, timesert, quicksert would be the correct way to repair it.

Since this is a pretty low torque application, it's possible to repair with the epoxy like JB Weld or quicksteel, filling and then drilling and tapping the hole.

Another repair might be to drill and tap it to a slightly larger size in a different thread system, assuming you don't have room to drill and tap to the next size up in whatever thread system it's currently using. For example 6 mm is .236". You could probably drill and tap a stripped out 6mm threaded hole to something like.25x28tpi. This would be confusing to the next guy that works on your bike since you'd have to use an SAE wrench for this one bolt while all the others a metric. You would become the DPO (Dreaded Previous Owner) to him.

  • I'm not really in favour of tapping it to a larger bolt size for the exact reasons you mentioned. With both of the other options, would it be safe? This is a racing motorcycle so a failure while I'm on circuit could be very dangerous.
    – Willem
    Commented Sep 13, 2016 at 16:01
  • I would want to do the correct fix and use a thread repair kit. Especially since it's the brake. How many bolts hold it on? If it were just 1 stripped out of 4 total I might be tempted to try JB Weld.
    – Eric
    Commented Sep 13, 2016 at 20:04

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