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I'm looking to replace the FC-C201 on a 2000 Diamondback Crestview, which uses an RD-C201, FH-RM40-8 rear hub (135mm), and CS-HG40-8I cassette. I've measured the bottom bracket at 68 x 122.5mm. The crankset is 48/38/28 170mm and the chain line is 47.5. If I replace it with a Shimano M361 48/38/28 crankset with a chain line of 50, what size bottom bracket should I use in order to keep the chain straight?

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Because M361 is no longer showing up in current year, first you dig through the archived Specifications .pdfs on si.shimano.com until you find it, like so: enter image description here

We see 123mm gets you to 50mm chainline. Since we're only talking about modern symmetrical cranks and BBs, to go in 2.5mm on the drive side you need a 5mm shorter spindle (2.5mm per side). So, 118mm should recreate the chainline pretty closely.

If you just use the spindle you've got, it's likely to work fine after the front derailleur is re-adjusted, albeit with wider than necessary Q-factor and chainline. You should plan on needing to adjust the FD either way.

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