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First of, these are the sort of brake mounts I'm talking about (though I believe the diameter is standard whether the mounts are removable or welded to the frame/forks):

enter image description here

What is the diameter on the shaft of the mount (marked in the picture with a C) where the brake arm is attached?

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Are you sure these are standard? I'm far from an expert in things bmx, but I got my daughter a WeThePeople bike last christmas, just searching the web site of the shop we bought from. They have kits like the photograph above (unfortunately they don't list measurements), but the kits seem not only to come from different manufacturers, but to cover different years. Might this imply non-standard sizes? Maybe worth giving them a shout (they're up in Wigan), they were really helpful to me. Winstanleys bmx – PeteH Jun 7 '13 at 14:20
Thanks Pete - I don't think the mounts themselves are standard, they're definately specific to the frame they're going on but pretty sure the section the brake arms fit on to are standard. I've never seen any choice in this respect when buying brakes - not that I buy that many - however have recently bought a new frame, so I know the brake that was on my old GT frame definitely fits my Failure frame and that brake itself was a replacement for what originally came with the GT. Will try to measure when I get home. – Nick Holt Jun 7 '13 at 15:33
up vote 1 down vote accepted

I measured several sets that I have and got the following results. These were used with Shimano, Tektro, Avid and one with "no name" brakes. This not to say that some other brands don't have a unique size. The male threads that go into the frame were either M6 by 15mm long or M8 by 10mm long. The shaft or studs (part labeled as C) were all just under 8mm, 7.93, 7.90, 7.89 etc. The threaded female hole were all M6. The M8 by 10mm posts all came off of rear brake mounts. The use of the larger diameter and shorter length may be due to the smaller diameter of the seat stay tubing.

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