Just crossed the 200 mile mark on a brand new Truvativ Giga-X Pipe Bottom Bracket/Crank. Yesterday I noticed that somehow the Drive side BB cup (these are external cups) had unscrewed itself by about 5mm!
Two things baffle me about this:
1) The drive-side BB threads on clockwise, the same direction that the crank turns while riding. One would think that this would lead to *tightening* the BB cup, not loosening it. It's baffling. (I have an Italian threaded BB, if that helps.)
2) The design of the crankset is such that any unscrewing of the BB cup pushes the crank/spider outward, essentially expanding the total spindle width of the crankarms. This is because with external BBs the crankarms, when fully tightened onto the spindle, come into direct contact with the BB cups/races. This means that the non-drive-side crankarm bolt must have unscrewed a little to compensate for the expanded width on the drive side BB cup.
What this seems to mean, bafflingly, is that *both* the drive side BB cup AND the non-drive-side crankarm bolt somehow came loose.
So I disassembled it, tightened the BB cup, then re-tightened the crankarm bolt. This time I tightened it a bit more than I did the last time. But then when rotating the crank I felt a little bit of "indexing" as it spun around (which I didn't think was possible with external BBs, actually). I loosened the crankarm bolt a little, alleviating the "indexing". But I'm afraid to loosen it too much and end up back with the original problem.
Obviously, I am baffled. I guess I'm looking for some insight into (a) whether or not the "indexing" means I've tightened the crankarm bolt too much, and (b) maybe a clue or a theory as to how this could have happened in the first place.