71.5mm bottom bracket
 

Hello everyone. I got this frame locally and was told it was an older Schwinn but I am not sure. Any help in identifying would be nice. I am planning to make a vintage single speed but ran into a problem with the bottom bracket. This measures 71.5mm which is a weird size. It came as a bare frame so no info or parts to reference on it. I dont know how to proceed to take the next steps. Only thing I can think of is get a 73mm bottom bracket and somehow space out 1.5mm. .75mm from each side. any help would be appreciated.
https://cimg2.ibsrv.net/gimg/bikefor...44f1c1bf26.jpg
https://cimg3.ibsrv.net/gimg/bikefor...2a95e658fe.jpg
https://cimg4.ibsrv.net/gimg/bikefor...0ce0f35ec7.jpg

Spacer is your quick-and-simple solution.

Plan b involves mounting a dremel-type tool with a cutoff disc to the left-side crankarm and using that go around the shell to cut a ring off, repeat for the other side.

You'll need a fair number of thin discs, and you'll have to arrange the mounting so that moving the tool inwards does not change the cut line.
I used a bit of plate that the nose of the tool went through, and an arm whose outer surface I knew was 90 to the axle - loosen the clamps ever so slightly, give it a tap, tighten, grind another .2mm into the groove.

When the groove is almost through, pull everything off and knock the ring off; tidy up the surface with a file, and repeat for the other side.

This way you can go straight to 68mm, which might be worth the trouble.

Someone who took facing the shell seriously?

i wonder if reaming the shell wouldn’t be a better solution if slower? With it being BSA the target would be 68mm. This way you could control the symmetry better?

Since you're using a square taper crank, if you use a cartridge BB you'll be fine with using some spacers with a 73mm cartridge. Myself I have lots of spacers and always optimize the chainline of a given crank to where I want it to be, not necessarily where it's supposed to be according to a book. Cartridges are quite versatile in that way, in that they are self contained units and do not rely on the shell spacing for the bearing adjustment. This takes knowing ones own equipment of course, what rings/cog combos are ridden most. So it need not necessarily be .75mm on each side, get an assortment of spacers if you don't have any. Biketoolstec. has tons.