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I have recenly changed my crankset from FSA comet to Shimano SLX. I have a pretty large BB shell (92 mm) and there is a press-fit BB installed, so it doesn't protrude. So how can I figure out if I need any spacer or not? (FSA had one spacer on the drive side).

Thank you!

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The literal answer to how to figure it out is pull up the exploded view diagram and/or manuals from si.shimano.com of your crankset model.

Most Shimano 2-piece cranks (until very recently all of them, but new XTR is a departure) handle preload via the cap on the left side and crank attachment via the pinch bolts on the left crank. This sort of design has no use for the sorts of spacers you sometimes have to add and take away from the other main design, where one bolt does everything and you may need to mess with spacers to make sure bearing preload "just happens" in the manner intended.

All BB92 is doing is putting the outside bearing surfaces in the same planes as they'd be in with a 68mm or 73mm shell plus a regular outboard Shimano mountain BB.

Tldr, worrying about whether Shimano cranks need spacers isn't really a thing unless you're doing something unorthodox, at least not yet.

Note that a number of Shimano cranks do have little spacer-like rings that go on the left side and are more part of the crank, there as part of the sealing and/or bearing contact design. They live in a groove on the bearing side of the spindle attachment area of the left crank, and in some instances it may be helpful to know that if your crank has that groove, it's supposed to be occupied by that ring.

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