How do I determine bottom bracket size
I found this diagram online which I hope will allow me to determine the size of the bottom bracket by measuring the width of the shell.





I found out that the bike is an old Centurion.  I was not able to find any series number engraved on it anywhere so I haven't been able to determine the exact model or how old it is but just in case anyone wants to know; I concluded that the bottom bracket size is 125x 68 but that a 127 size also works.  Thanks to the mechanics at IMartin in Los Angeles who took the whole crank assembly apart to measure the bracket when I asked them if they knew what size it was.