I have an old tandem. It has the bottom bracket in it that is really wide which takes the press in cups for one piece cranks. I was wondering if there is something that allows these bottom brackets to be converted to take threaded cartridge bottom brackets, so that I could put in square taper cranks and match the cadence of the front and rear crank arms (they do not rotate at the same speed right now). Any help would be greatly appretiated. I imagine a really good machine shop could hook me up with a couple custom peices if I am that commited, but I dont know about that.

Are you *sure* that the bottom bracket shell is meant for a 1-piece crank, and not for an eccentric bottom bracket? An eccentric is a thingy that most tandems have allowing you to adjust the distance between the two bottom brackets, and thereby adjust the tension of the sync chain.

If the inside diameter of the shell is just over 2 inches exactly, then it's meant for a 1-piece crank.

Either way, there is a part that will do the job. For a 1-piece crank, https://sheldonbrown.com/harris/opc.html, and scroll down to "One Piece to Euro Conversion Kit". It fits into the 1-piece shell and allows you to put in a standard 68 mm ISO BB.

cool thanks, i will check.
I am pretty sure it is one piece, it is an old sears mixie tandem, but I will make sure before ordering anything.
Thanks alot