Update:So, with the tools kindly loaned to me by Greg, I popped the cranks off (which showed just how much the previous axle was)
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And swapped the BBs over. I'm still not sure on this 73mm vs 68mm thing. The 456 needs a 68mm bottom bracked according to the specifications. I was assuming this was diameter, so there was no chance a 73mm one one even fit in the hole. But somehow, it did. Not all the way, it's a bit of a bodge, but it certainly works! In the future I'll upgrade it to a hollowtech affair, but for now it's fine!
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A bit of tweaking later and the front mech was shifting very nicely across all 3 rings, rear mech seemed fine too, and I was ready to go on the inaugural ride. Covered about 2 miles, got to the first climb, then all hell broke lose! Shifted to the largest rear cog, at which point there was a catastrophic crunching, I came to a dead stop faster than I could unclip, and went over sideways. This is what happened:
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It doesn't look too bad, but it's completely stuck around a sticky-outy bit on the casette. On the plus side, it was a very nice place to break down:
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On the other hand, I then had to carry my not-so-light new ride all the way home, and the rear wheel can't turn at all
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The saga continues. This week's lesson was that you should learn how to set up a rear mech properly,
BEFORE TRYING TO RIDE IT.