I was braking in a downhill, the gradient was steep so I was using both brakes mostly the rear. When suddenly the rear brake shifter went totally soft. I did a full-stop using my front brake only and took a look at the rear, this is what I saw:

missing bolt

The bolt holding the brake cable clamp on the caliper is missing. I had never touched at this bolt. Looking at the front brake, it's TorX head for which I do not even own a key. I looked everywhere around the place the brake gave up but could not find the bolt. It could have been lost way before: I put the clamp back in place and managed to ride using the brake too without it loosening again.

The brake is a Tektro R539. How easy is it to find a replacement. Should I just measure the front one and look up on eBay? Is it worth trying the local bike shops?

Edit, pictures of the front nut:

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Edit 2: Went to the hardware shop. By putting the nut against a benchmark it looked like an M3. They were selling none of these though.

  • A well stocked LBS could have some fitting replacement. And learn to use both brakes at all times.
    – Carel
    Commented Jun 15, 2021 at 14:18
  • @Carel I will bring it to my LBS then. And I almost always use both brakes (exception being when I am using one hand for feeding or drinking), with a different balance depending on incline and urgency to brake. Commented Jun 15, 2021 at 14:42
  • The Tektro product page for R539 is here: tektro.com/products.php?p=44 The Tektro USA web site is tektro-usa.com and has some resources and contact info. I would try calling them or the main site if you are outside the US and find out the specs for that bolt you need - length, diameter, threading, strength grade.
    – Armand
    Commented Jun 15, 2021 at 15:25
  • Just keep in mind that "not in stock" isn't the same as "don't have it". The last time I had a similar problem, the LBS brought out a small bin labelled "random *^(%" and started digging. They found several, and sent me on my way with what I needed and a couple extras. If they have a tip jar, use it!
    – Andrew
    Commented Jun 17, 2021 at 14:32

1 Answer 1


Time to pick up a set of torx keys! They’re handy to have around as general tooling.

Once you do, simply remove the other bolt, and you can find out everything you need from it. No need to go specifically to a bike shop either; most hardware stores should have bolts like this.

My guess is M6, approximately 25mm long. You’d want to pick a head shape that best fits into the recess, and probably a 10.9 strength rating for safety.


Upon further inspection of the photo, this approach may not work. You can see how the threaded part is recessed below the area where the bolt head would sit. I imagine you need a bolt that has a slight shoulder to take up that recess before the head actually contacts the brake. Again, inspecting the other bolt (and grabbing a photo) is key.

  • Thanks man. I will unscrew that front one and show up at my hardware shop. I know little about bolt specs, better not to trust eBay. Bolts I see online are sold by packs on 50 though, no clue what to do with the excess. Commented Jun 16, 2021 at 10:15
  • Have added pictures of the front nut/bolt. It has a washer, which does not seem to be removable. It's ~1cm long, 7mm diameter at the head, 3mm diameter at the shank. Does it look somewhat standard? Commented Jun 16, 2021 at 11:02
  • 25 mm seems way too much given the photo. It's about half of that. Commented Jun 16, 2021 at 11:42
  • @Learningisamess That thing is holding your brake on?!? Yes it’s a standard bolt, but it does look a little bigger than M3. Hard to tell from photo though, you know best. Vladimir is correct.
    – MaplePanda
    Commented Jun 19, 2021 at 23:00
  • 1
    @MaplePanda Went to the LBS, they had a spare bolt of this kind. Given to me for free. Amazing chaps. Commented Jun 20, 2021 at 0:30

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