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An online search for "BB tolerance" or "BB standard" results in many pages and technical drawings showing the tolerances to which bottom brackets are made, and to which bottom bracket shells should be made. One notable missing standard seems to be the good old 1.37" × 24 TPI threading of the BSA/ENG/ISO BB.

I've only been able to find two sources that in any way document the supposed tolerance of a BSA BB shell. One is Campagnolo's (PDF link), which says the shell width ought to be "67.2 mm - 68.8 mm". The other is a vague table on "Wheels Mfg" (website link), which lists a diameter of "33.6 mm - 33.9 mm" for the shell diameter. Where are these numbers taken from? Why don't both sources define both values?

I'm not a mechanical engineer so I've not thought to check any engineering-specific documents where these tolerances might be listed, but I'm willing to bet it is documented somewhere; What are the tolerances to which a threaded BSA BB and shell must conform?

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    The column in the Wheels Mfg doc does say “Frame BB shell inner diameter”. To me, this seems to clearly mean that the diameter is for the shell, not the BB cups.
    – Weiwen Ng
    Aug 18, 2022 at 15:36
  • Right, don't know how that flew over my head; I've edited the question. Still, the question remains for the BB cups and what the source is for these numbers.
    – jayded-bee
    Aug 18, 2022 at 16:17

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This questions actually veers into some perhaps unintended territory.

BSA and ISO BBs are virtually and practically interchangeable but not literally the same thing. BSA is much older and ISO is based on it.

ISO publishes a living document that lists the information you seek, but it's paywalled.

I presume BSA is old enough that somewhere a public domain version of the drawing could be found that shows tolerance data. But that may be here nor there since I believe most threaded shell BBs of the last many decades are actually made to the ISO spec, which again is discrete from BSA.

Note that there are a few different tolerances in question. One is deviation from nominal width (i.e. many external crank BB makers spec a simple max deviation of 68 or 73 +/-0.1mm). Another is allowable deviation per face, and another is allowable axial deviation, which normal people don't really have the ability to measure and are going to show up on the high level technical diagrams if anywhere.

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  • You are correct! "A 1 3/8 (1,375) in x 24 t.p.i screw thread of ISO basic profile has been adopted in this International Standard. The basic pitch diameter of this thread differs by only 0,003 mm from that of the I,37 in x 24 t.p.i. B.S.C. thread, but, due to the smaller crest truncation of the IS0 profile (see figure I) interference may occur at the major diameter when an IS0 cup thread made towards its upper limit is assembled with a B.S.C. shell thread made towards its lower limit on major diameter." (Taken from ISO 6696:1989 document preview, freely available.)
    – jayded-bee
    Aug 20, 2022 at 9:17

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