The only markings on the shank are the mfr. name/location and the pitch marking of 26 tpi. It's taper is similar to a pipe tap. The small end is chipped but looks like it was .250 diameter when new, the large end diameter is .500
I acquired this unique tap quite a few years ago, it was found in the bottom of dad's toolbox along with other rusty, banged up bits and taps. I never saw one like this during my tenure in a machine shop and don't know what application this tap would be used for other than thread repair of damaged 26 tpi threads. I spent some time looking online for info, and it appears 26 tpi is a BSC standard used for bicycles and on vintage motorcycles. I could find no info regarding the taper.
"British Standard Cycle (BSC or BSCy) is a British Imperial screw thread standard. Unlike other major British imperial thread standards (British Standard Whitworth and British Standard Fine) the thread runs at a 60 degrees rather than a 55 degrees angle. All sizes 1⁄4 inch and larger use 26 threads per inch (tpi), making them similar to 1 mm ISO threads, which are 25.4 per inch and also run at a 60 degrees angle. It was originally used with both bicycles and motorcycles.[1] However it is now believed[by whom?] to be obsolete in motorcycle manufacture. In the bicycle industry it is still found on virtually all bottom bracket threads and the wheel axles of low-end models manufactured in China,"
I acquired this unique tap quite a few years ago, it was found in the bottom of dad's toolbox along with other rusty, banged up bits and taps. I never saw one like this during my tenure in a machine shop and don't know what application this tap would be used for other than thread repair of damaged 26 tpi threads. I spent some time looking online for info, and it appears 26 tpi is a BSC standard used for bicycles and on vintage motorcycles. I could find no info regarding the taper.
"British Standard Cycle (BSC or BSCy) is a British Imperial screw thread standard. Unlike other major British imperial thread standards (British Standard Whitworth and British Standard Fine) the thread runs at a 60 degrees rather than a 55 degrees angle. All sizes 1⁄4 inch and larger use 26 threads per inch (tpi), making them similar to 1 mm ISO threads, which are 25.4 per inch and also run at a 60 degrees angle. It was originally used with both bicycles and motorcycles.[1] However it is now believed[by whom?] to be obsolete in motorcycle manufacture. In the bicycle industry it is still found on virtually all bottom bracket threads and the wheel axles of low-end models manufactured in China,"