TandemGeek

As mentioned in one of my earlier posts, Shimano uses a proprietary spline pattern called "Octalink" for it's bottom bracket axles and cranks. Several of the major players in the crank and bottom bracket part of the industry created a competing standard that they made available without a licensing fee called ISIS (International Spline Interface Standard). Unless a tandem comes with Shimano cranks, it will most likely have the ISIS splines or, if it's an older tandem, a square taper bottom bracket, neither of which are compatible with Shimano's cranks. You can read more about ISIS and Octalink here: http://en.wikipedia.org/wiki/ISIS_Drive

Bottom Line: If you don't presently have a Shimano Octalink crankset and are planning on switching over to Shimano you must also factor in the cost of two new Shimano Octalink bottom brackets.