There is a document called "Bottom Bracket Standards" on the internet. https://wheelsmfg.com/bb-standards
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 https://www.sheldonbrown.com/cribsheet-bottombrackets.html 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 veloduo.co.uk 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.
|2020.05.25||Male External and female Internal||Roc Robertson|
|2020.05.25||Thread terminology||Robin Tivy|
|2020.05.26||The metric numbers must be minor diameter of the internal threads||Robin Tivy|