Yesterday I was installing a BB-MT501 HollowTech II bottom bracket into my ~2006 Trek 520 frame. This frame has a 68mm BB shell.

I was following the directions on Shimano's website: https://si.shimano.com/en/dm/LAFC001/install_remove_bottom_bracket

My process:

  1. Bike frame BB shell was chased and faced by LBS
  2. Cleaned out and dried BB shell
  3. Applied light coat of grease to BB shell threads and to the BB cups
  4. Used a 2.5mm spacer on the non-drive side and 1.8+0.7+2.5mm spacers on the drive side.
  5. Threaded in both cups by hand almost all the way to ensure there is no cross-threading
  6. Used a calibrated high quality torque wrench (Precision Instruments) to torque the non-drive side to 26lb-ft which is on the very low end of the 35-50Nm given by Shimano.
  7. Used the same torque wrench to torque the drive side.

I was able to torque down the non-drive side successfully but when torquing the drive-side, both the 1.8 and 0.7 spacers cracked.

What did I do wrong?

I took everything apart and it doesn't look like the BB shell or the actual bottom bracket was damaged. Thankfully.

What should I do now? I'm thinking it might make sense to have metal spacers instead of plastic but I don't know if those exist.


  • 2
    You've probably observed that the drive-side BB is left-threaded. Does your torque wrench work in this non-standard direction of rotation? My Wera Click A can act as a ratchet in both directions, but only limits torque in the clockwise direction
    – dobiwan
    Commented Jun 13, 2023 at 11:24
  • 2
    Yes: torqwrench.com/3/8-drive-micrometer-click-wrench---m2r100f
    – Dan
    Commented Jun 13, 2023 at 16:21
  • Hello, I've encounter the exact same problem following the exact same procedure, with the same BB. Did you end up doing? Commented Jul 20, 2023 at 10:20
  • @IsidoreIsou, I ended up getting some metal 2.5mm spacers from a local bike shop. It wasn't easy to find, I called about 8 shops before finding one that had them in stock. The metal spacers installed just fine. I don't know why Shimano makes them out of plastic, seems like a poor choice IMO.
    – Dan
    Commented Aug 1, 2023 at 19:12

3 Answers 3


That is a very weird situation because it sounds like you were taking more care than most and did everything right. If the torque wrench were somehow reading way under that could explain it, but it sounds like you were using a good one in good condition so that's doubtful.

In this case I think you should write if off as a defect in the spacers. Metal 2.5mm BB spacers do exist, but there's nothing going on here that says you need something above and beyond the normal plastic ones.

  • The only possible thing is that it's a 20-100 lb-ft torque wrench so it could have been less accurate at the very low end. It's my only reversible one so that's what I used.
    – Dan
    Commented Jun 12, 2023 at 19:03
  • 1
    That can affect the accuracy some, but people use arbitrary amounts of torque all the time with the same spacers, and seeing them break is abnormal regardless. I think it's very unlikely to be the tool's fault. Commented Jun 12, 2023 at 22:14

I wouldn't be too worried. As Nathan said, you did everything right. By any chance do you remember what order you had the 3 spacers in? My guess would be that the two thinner spacers were being pulled apart as you torqued the BB down. Try putting some grease under the BB where it contacts the spacers next time--that'll ensure the BB slides over the spacers instead of dragging them along due to friction. Plastic spacers are fine, but I do like to use aluminum ones here. They may be available at your LBS, and they are definitely available for cheap online.

  • I used the order that the instructions pictured. From the BB shell to the cup face: 1.8, 0.7, 2.5. Grease is a good idea and I'll look for some metal replacements. Thanks.
    – Dan
    Commented Jun 12, 2023 at 22:16
  • 1
    @Dan Hm, sounds like there was just a lot of friction and the torque ripped the spacers apart. Perhaps the 2.5 was stuck to the BB cup and the 1.8 was stuck to the BB shell, and the poor 0.7 was caught in between. Don't try undertorquing it. It should all work fine even at the full 50Nm.
    – MaplePanda
    Commented Jun 13, 2023 at 3:32
  • It's ironic, I tried 35Nm at first because I was worried that the plastic spacers wouldn't be able to handle the full 50. My plan was, assuming the 35Nm wasn't an issue, to split the difference and torque it down to ~43Nm after both sides were first at 35Nm. Is there a reason to go all the way to 50?
    – Dan
    Commented Jun 13, 2023 at 16:25
  • 1
    @Dan Ah, by undertorquing I meant like deliberately going <35Nm. Really, your BB just needs to be tight (hence the fairly wide range of 35-50). There are some theoretical benefits of leaning towards the higher end (more clamping force --> less chance of movement --> less chance of self-loosening or thread damage), but really anything in that window will work fine. The prerequisite being that your BB spacers stay in one piece of course.
    – MaplePanda
    Commented Jun 13, 2023 at 21:50
  • thanks, that makes sense. I'm only seeing 2.5mm aluminum spacers online, is it safe to just use 2x2.5mm spacers rather than the 1.8 and 0.7 ones? My inclination is to just order these spacers since the online store I ordered the BB from is telling me to file a warranty claim with Shimano and I fear that could take ages.
    – Dan
    Commented Jun 14, 2023 at 18:10

I would suspect the stackup of spacers to be a contributing factor.

That's 5.0mm of spacer but made up of three rings, means each can compress at different rates. If you'd had one spacer it would likely not have cracked as early.

I would try again with a single 5.0mm thick spacer.

  • Aside - 5mm seems like a lot - are you positive of that measurement ?
    – Criggie
    Commented Jun 12, 2023 at 18:16
  • 1
    Take a look at the install instructions for that BB: si.shimano.com/en/dm/LAFC001/install_remove_bottom_bracket. They recommend 2x2.5mm on the drive side. Then there's a note that says "If using a band type front derailleur and a bottom bracket shell having a width of 68 mm, install the three spacers so that there are two on the right and one on the left as shown in the figure. ". The pictures show the 1.8 and 0.7. I'm not 100% sure since this is my first time doing this and it's sort of a custom setup. But I am reasonably confident.
    – Dan
    Commented Jun 12, 2023 at 19:04
  • 1
    @Criggie Yes, 2 on the DS and 1 on the NDS is the correct setup.
    – MaplePanda
    Commented Jun 12, 2023 at 19:34
  • 1
    Why do they explicitly say to use the 3 spacers instead of 2 for the band-clamp FD setup? I don't understand what difference it makes but it worries me that using a single 5.0mm would have the same problem.
    – Dan
    Commented Jun 12, 2023 at 22:13
  • 1
    @Dan -- there are front derailleurs that mount to the bottom bracket. In this case the mount itself acts as a spacer. Google "BB mounted FD". Band-clamp means the FD is mounted on the seat tube, so you need an extra spacer.
    – g.kertesz
    Commented Jun 25, 2023 at 15:12

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