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I have this ubiquitous cheap Wal-Mart bike (http://www.walmart.com/ip/26-Roadmaster-Granite-Peak-Men-s-Bike-Black-Green/45089317z), which has a square taper bottom bracket.

I'm trying to buy a new crankset (http://www.amazon.com/Shimano-Tourney-FC-M131-Crankset-42-34-24/dp/B000F5HXHI), and it requires a 123mm bottom bracket. I can remove my left side crank arm but nothing else, so I'm having trouble guessing the spindle length. Anyone know? It's not listed in the manual either.

The closest approximation I got was by measuring from the tip of the left spindle to the centerline of the bike. That's 82mm. But if you double that, that's 164mm, which seems to be quite a bit more than any bottom bracket that I've seen.

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    Even with both cranks off and the axle removed it's hard to measure, since there's really no defined point to measure from. You don't measure tip-to-tip, and the two ends are often asymmetric, not tapering down to the same point. I've never seen a description of how to measure accurately. – Daniel R Hicks May 3 '16 at 11:19
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The only measurement for a bottom bracket you can do with the bottom bracket still on the bike is to do the shell width, which is the width of the frame where the bottom bracket installs.

In order to measure the spindle length, you need to remove the cranks and then measure the spindle length. There is no guarantee that the spindle is symmetric or the width of the cranks on both sides is the same. In any case, you'll need to pull off the cranks to replace them.

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Pull off both crank arms, then measure the spindle of the BB, that's what the Shimano Crank is saying it needs. When you measure spindle length, you need to only measure to the ends of the square taper. Some spindles have threaded bits that extend beyond the square taper. Don't include those in that measurement. If the Bottom Bracket you have isn't the right size, you will need to replace it with one that is the right size. Always remember, the crank determines the spindle length, not the frame.

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