CEI CYCLE THREAD “26 TPI”
CEI Thread pdfCycle Engineers’ Institute (CEI) or British Standard Cycle (BSC)
The most common thread on British Cycles prior to 1970.
These are the two different names commonly used for the same thread form. It used a 60 degree thread angle, rather than the 55 degree of BSW and BSF.
For sizes from 1/4″ through 1/2″ by far the most common are 26 tpi, although 20 tpi can be seen as well for 7/16 and above.
Most, but by no means all, fasteners on post-War BSA’s (through the late ’60’s, when it got more complicated) were CEI. Although the thread form and pitch is different, the head sizes on CEI-threaded fasteners use the same wrenches as BSW/BSF. It was originally used with both bicycles and motorcycles. However it is now obsolete in motorcycle manufacture.
In the bicycle industry (3/8×26) it is still found on wheel axles of low-end models manufactured in China.
To learn more on British thread forms check out of article on Whitworth and other British threads.