Bottom bracket shell dimensions for steel frames
Machined from 1.562" x 0.120" wall 1020 steel tubing. The outside diameter is the the stock tube diameter. The width is 68.5 mm, supplied 0.5 mm oversize to allow clean up after welding. The threads are 1.370"-24, right and left hand; thread length is 0.50". The left hand thread is identified by a radial groove on the face of the shell. The minor diameter of the thread is 1.330"/1.340" and is relieved on the inside diameter to 1.385"/1.395". This threaded bottom bracket shells is 1.37" x 24 threads per inch (TPI) and right and left threaded (also known as British Standard, ISO/English or BSA), making it compatible with all modern threaded bottom bracket spindles (Paragon machine works, 2018).
Standards of frame bottom bracket shell threading
Threads can be ISO, BSC or Swiss threaded, and there is left-hand thread and right-hand thread. Tools used for tapping are called taps.
| Standard | Thread dimensions | Cup outer diameter | Frame shell width | Frame shell inner diameter |
| ISO/British, or BSA | 1.37 in x 24 TPI | 34.6-34.9 mm. Right cup has left hand side threading. | 68 mm, 73 mm oversize, 83 mm for DH i FR bikes, 100 mm Fat bikes. | 33.6 to 33.9 mm (Bike Gremlin, 2018). |
More standards can be found in Ref. 1.
http://atomiczombie.com/BBRAC%20Steel%20Bottom%20Bracket.aspx
Facing the bottom-bracket
Facing the bottom-bracket shell improves the alignment of the bearing parts that are installed in the bottom-bracket shell.
References
1. (Bike Gremlin, 2018): http://www.bike.bikegremlin.com/2016/10/27/bicycle-bottom-bracket-standards-explained/
2. (Paragon machine works, 2018): https://goo.gl/qBHqVF
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