Disc brake mount/adapter bolt length 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?
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 includes many of these secondary safety systems with their products (eg. safety clips on brake pad retaining bolts, threadlocker, fancy spring washers under rotor bolts, and these plastic things) which certainly will help, but the bike won’t explode if you don’t use them.