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Bottom Bracket Shell Diameter and threads Update   LinkCheck   Back
PartType: Bottom Bracket
DateAsked: 2020.05.25
Status: Open

There is a document called "Bottom Bracket Standards" on the internet.

In that document, the first row is the familiar standard is "BSA Threaded". In that row, the second column is labelled "Frame BB Shell Inner Diameter". and for that it says the following:

  1.37 In x 24 TPI  
  33.6 mm - 33.9mm

I can't get those numbers to reconcile. Here are my questions:

1. What is the 33.6mm to 33.9mm? Those numbers don't match the inches.

 - 1.37 is 34.789 which does not match either of the 33.6 numbers

 - Sheldon Brown's document has two rows in his first row "British ISO" (I converted each to mm)

  1.370 x 24 tpi 34.789
  1.375 x 24 tpi 34.925

 What is the second row (1.375?) Other websites like just refer to 1.375 x 24 tpi except older Raleigh are 1.375 x 26 tpi. So they don't mention Sheldon Browns first row 1.370"

 - I assume that the diameter of 1.37" is what you get if you stick calipers inside the frame and measure at the point where the threads exist. (to the bottom of And each end of that opening is threaded.


2020.05.25Male External and female InternalRoc Robertson
2020.05.25Thread terminologyRobin Tivy
2020.05.26The metric numbers must be minor diameter of the internal threadsRobin Tivy