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Brake rotor bolts: types and torque specs.

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    Brake rotor bolts: types and torque specs.

    The majority of bicycles will have a standard T25 brake rotor bolt. I use 16mm of rotor spacing so I have to use longer rotor bolts. I learned that M5 bolts are completely interchangeable with the stock T25 rotor bolts.

    I tried some M5X25 SS rotor bolts. The "25" is the length of the bolt shank; they use a T25 key. After some hard braking use (hydraulic brakes with 203mm rotors and an extremely heavy tire and wheel), these bolts worked loose a little and then bent. I now use 30mm Grade 12.9 steel M5 bolts installed 4mm deeper into the hub. This necessitates a 1mm high washer. I use blue threadlocking compound.

    I did an exhaustive internet search for M5 bolts with a Torx head, but I could not find a higher grade steel than the SS. I did find, however, Grade 12.9 M5 bolts in a wide selection of lengths, but they were all socket head type as such: The T25 type have both a lower profile (i.e. more clearance) and a greater ability to handle torque. In my case, clearance was not a problem, so I investigated to see if the socket head (Allen key) would be able to handle the torque spec for rotor bolts. Note that socket head is also sometimes called socket cap, as opposed to button head which yields more clearance.

    From, we have Click image for larger version  Name:	park specs.PNG Views:	1 Size:	27.2 KB ID:	41673
    Last edited by commuter ebikes; 08-05-2017, 07:27 PM.

    That is a large range! I always use Nm, so the range of 18-55 in-lbs equates to 2.03-6.21 Nm.

    Note that the Park Tool table specifically mentions M5 bolts right next to the maximum torque of 35 in-lbs (equal to 3.95 Nm). So can an M5 bolt tolerate up to 6.21 Nm of torque? It can, but only if it is a socket head. If it is a button head, the max is 6.10 Nm--you could strip the bolt!

    From, we see that this M5 bolt can handle up to 12.0Nm! Click image for larger version  Name:	socket type.PNG Views:	1 Size:	53.7 KB ID:	41675
    Last edited by commuter ebikes; 08-05-2017, 07:29 PM.


      The button head type torque spec is only 5.9Nm! What is more, now they are including the length of the shank. The length of the shank is not mentioned for the socket head type. Click image for larger version  Name:	button head.PNG Views:	1 Size:	33.5 KB ID:	41677
      Last edited by commuter ebikes; 08-05-2017, 07:31 PM.


        Looking at the note at the bottom, we see that this data is only for this brand. If you need a higher torque, then you need to find a higher grade bolt. See Post #6 in this thread for more about this (spoiler alert: the bolts in the tables are very high quality).

        Back to Torx (specifically, in this case, T25), as most people know Torx was designed in order to accommodate higher torque. From, we see that a T25 can generally handle up to 15.9Nm, a significant improvement over the Allen type. Click image for larger version  Name:	torx.PNG Views:	1 Size:	24.4 KB ID:	41681
        Last edited by commuter ebikes; 08-05-2017, 07:25 PM.


          Now we see why brake rotor bolts use a Torx head. Park Tool advocates a max torque of 6.21Nm for a T25 brake rotor bolt (in the case of Avid, which I confirmed to be T25).

          Decide to use an M5 bolt and Park Tool reduces the maximum torque to 3.95Nm.
          Last edited by commuter ebikes; 08-05-2017, 07:34 PM.


            So let's say that someone has decided to use M5 rotor bolts. Park Tool says 3.95Nm is the maximum torque for this application. We saw (in the two yellow tables) above that the socket head accommodates significantly higher torque than the button head. Let's assume that there is adequate clearance for the higher socket heads, so we settle on those.

            Looking at, which is a general table for Grade 12.9 metric fasteners, we see that the max torque for a Grade 8 M5 socket cap screw is 10.0Nm:
            Click image for larger version

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ID:	41686
            Last edited by commuter ebikes; 08-05-2017, 07:38 PM.


              So M5 socket head Grade 12.9 bolts can be used throughout the range of brake rotor bolt torque specifications. I still do not see the length of the shank cited; for this reason, I will not consider the use a longer shank to reduce the torque specs above.

              Happily, this type of bolt, in a multitude of lengths, is widely available, and they are affordable! The downside is the reduced clamping surface area, but a washer fixes that.

              If you did not have the clearance for using socket head bolts, you can use button head bolts and tighten them as high as 6.10Nm if need be.
              Last edited by commuter ebikes; 08-05-2017, 08:39 PM.


                I have seen same high tensile bolts with essex based company Bolt World They also do 12.9 grade, in Socket head screws, and hex head screws.


                  There is more to torque values than just what the bolt can take. Consider what the female side limitation is also. For example a SS bolt into an aluminum body. If torqued to the max for the bolt it will surely strip the aluminum side.