My bike's frameset has IS mounts. The mounts are 9.6mm thick. I bought a pair of IS to post mount adapters but they don't come with bolts. The adapters are 11mm thick. How long should the bolts be?

The adapters I bought are not Shimano but I saw that Shimano adapters (SM-MA-F180P/S, SM-MA-R160P/S) come with bolts 18.7mm long. Now I'm not sure how thick their adapters are. Are the bolts supposed to thread past the adapter or just short of it? Shimano also has those snap rings; are those needed or just the bolts and washers?

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1 Answer 1


These bolts are usually 18-20mm long M6 bolts. The exact length isn’t important, but going shorter than 15 will likely mean insufficient thread engagement, while longer than 24 or so could potentially result in the bolt interfering with the rotor.

The plastic snap rings aren’t strictly necessary. They act as a second line of defense against the bolts inadvertently loosening. Shimano uses a variety of these secondary safety systems with their products (eg. safety clips on brake pad retaining bolts, pre-applied threadlocker on bolts, fancy spring washers under rotor bolts, safety wire and drilled bolts, and these plastic things) which certainly will help in preventing those what-if scenarios, but the bike won’t explode if you don’t use them.

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    Brake bolts should be torqued up pretty hard. They shouldn't need secondary securing, but if you're really worried a drop of threadlocker should do the trick. And sizing bolts to go almost all the way through the tapped part is a good rule of thumb which works here. +1
    – Chris H
    Commented Mar 18, 2021 at 13:12
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    Most bolts that came with the adapters I've mounted have sufficient length to finish very close to square with the inner aspect of the adapter and no more. There is a very small gap from that point to the rotor--no more than 3mm--thus, a 22mm bolt would be just right in your case. I would be comfortable with a 20mm bolt but probably not less. There is very near 10 threads/10mm thick adapter, and it's best to have them all engaged with this application. The snap rings are to help prevent the bolts from loosening and are optional. Some forks with IS mounts have no clearance to fit these..
    – Jeff
    Commented Mar 19, 2021 at 8:34
  • @Jeff Yea, 22mm might have been a little too conservative. Will edit.
    – MaplePanda
    Commented Mar 19, 2021 at 18:09

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