I am doing the same thing with my Trek 1000 and I am having touble with the BB. The threads and size seem to be mismatched. The old BB is a TH BB-7700 (68 mm). The new unit, Shimano 105 BB-5500 is a 70mm unit marked "36 x 24T." It will not thred into the frame. Is there a different size I should buy?