Here is some new information regarding my tandem's bottom bracket problems.
First, I went to my local hardware store to replace the bent bottom bracket shell bolts. The store owner said that the bent bolts were stainless steel. He recommended replacing these with hardened steel bolts, because they wouldn't bend. These were black in color, similar to what one might find on a seat post clamp. He also suggested that since there wasn't enough room to file open the gap between the shell bolt sleeves, and I couldn't insert a screwdriver to widen the gap, I could use a thin hacksaw blade to cut a gap. Price: $1 per bolt, and a free hacksaw blade thrown in.
Next, I visited my local bike shop. The shop owner said they'd chase the threads to see if that helped. The mechanic at first was unable to remove the fixed cup because the eccentric kept turning. He fixed this by removing the eccentric and applying some friction paste inside the shell. After that, he was able to remove the fixed cup and chase the threads. Interestingly, the shop owner said that in a pinch, an abrasive tooth paste may be used in place of friction paste.
The hope was that chasing the threads on the eccentric would allow the adjustable cup to turn more easily. Also, perhaps the threads originally had not been cut deeply enough. The mechanic cut the threads as far as his tool would go, but I later determined that they still weren't deep enough: it still takes a ridiculous amount of force for the last turn of the adjustable cup. Price: $45 to remove the fixed cup, chase the threads, and get the eccentric to stop turning.
Since the threads in the eccentric were not deep enough for the adjustable cup, my third plan was to file 2 mm off of the inside edge of the cup. In theory this would solve my problem because the cup would no longer bottom out on the threads. In practice, the cup is made out of hardened steel so several minutes of vigorous filing provided no measurable change to the cup thickness.
My fourth plan was to look carefully at the bottom bracket spindles. The rear spindle fits well. It is marked 127-68. I figured out that the first number is the total length. When I measured the width of both bottom bracket shells as 68 mm, I deduced that the second number is the width of the bottom bracket shell for which the spindle is intended.
Here's where it gets interesting. The front spindle is marked 120-68. It is long enough for the crank arms, but the distance between the bearing shoulders (the "B" distance in the diagram below) is slightly too small at 50 mm. To compensate for this, the adjustable cup has to be inserted so far that it is flush with the lock ring, and at this point the cup threads are hitting the bottom of the eccentric threads with great force.
I found another spindle to try. This one is marked 122.5-68, and the "B" distance is 3 or 4 mm larger. When using this new spindle, the adjustable cup no longer bottoms out on the thread, and the bearings in the eccentric can be adjusted properly. The problem is that it looks wrong because the adjustable cup now sticks out about 3 mm beyond the top of the lock ring. My other bikes are all showing about 1 mm of threads.
Here are some new questions for your consideration:
- the old spindle is marked 120-68 F8, while the new one is 122.5-68 CK. Does the 2-letter code indicate the "B" distance?
- is there a source for replacement square taper bottom bracket spindles where one might find a 120-68 spindle with a 52 mm "B" distance?
- is it feasible to shim the fixed cup to reduce the distance that the adjustable cup sticks out?
- is 3 mm of adjustable cup threads acceptable?
