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just a quick question , my mountain bike has 3x drivetrain with SR Suntour XCM Crankset , and Im thinking of getting a 1x Shimano XT Crankset do i need to replace the bottom bracket ? and what bottom bracket should I change it to?

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It appears that all 1x Shimano XT cranksets are Hollowtech 2, two piece cranksets which require outboard bearing bottom brackets specific to Shimano's Hollowtech 2 system. Shimano's compatibility chart points out that 1x drivetrains require different frame spacing. The rear dropouts of a compatible bike would be 142 mm or greater, allowing space for the larger number of rear sprockets.

If your mountain bike has the more common 135 mm spacing between the rear dropouts, you could use one ring of a compatible crankset. There are several tips for doing so on this site and others.

To answer your question specifically, yes cranksets require a specific bottom bracket, especially so the new offerings from SRAM and Shimano and others. The Suntour xcm utilizes a cartridge bearing bottom bracket with either square taper or Octalink spindle interfaces. The 1x XT crankset utilizes a different type as described above.

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When changing to another crankset, do I need to change to the brand's specific bottom bracket?

You need a bottom bracket that matches the crank, and there has to be a bottom bracket option that will fit in the frame.

Shimano MTB crank specs are here. The 1x XT cranks, FC-M8100-1 or FC-M8120-1 will work with a threaded bottom bracket of 68 or 73mm width (which you presumably have) and a Hollowtech external bottom bracket.

However, note the FC-M8100-1 chainline is 55mm and requires rear spacing of 142 or 148mm and the FC-M8120-1 chainline is 58mm and requires rear spacing of 148mm. Your 3x drivetrain almost certainly uses a chainline of 47.5mm, so the XT crank will not work. This is going to be true of any modern 1x drivetrain.

if you really want to convert from an older or lower-end triple to a 1x (which is possibly not a good idea as you are throwing away much of your gear ratio range) the easiest path is to just remove the outer and inner chainrings, and possibly fit a narrow-wide ring in the middle position.

https://productinfo.shimano.com/#/spec/MTB/Front%20Chainwheel

  • A rear spacing of 142 mm is not "Boost". Boost spacing is 110 mm in the front, 148 mm in rear. 142 mm is "standard" and increasingly referred to as "non-boost" in informal settings. The tongue-in-cheekly named "Super Boost Plus" is 157 mm in the rear, which is the same OLD as downhill hubs, though the non-driveside flange is placed more outboard on a Super Boost Plus hub than on a DH hub. – Paul H Aug 7 at 0:05
  • For reference: images.app.goo.gl/nbTna7XHHAEVXdvq6 – Paul H Aug 7 at 0:06

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