I have been looking online for tutorials on replacing a chainring (or three) for my Scott mountain bike: https://www.thebikelist.co.uk/scott/aspect-40-2012

When trying to take off the chainset so that I can remove a chainring to replace it, the crank bolt holding the chainset on looks different to most of the tutorials I've been watching online. I am wondering what tools and what procedure I would require to remove this so that I can replace my chainset.

The bolt is a hex bolt which has 'CH 12.9' printed on it.

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3 Answers 3


A 14 mm (or possibly a 13 or 15 mm) socket should do the trick for removal. You won't be able to budge the crank arms after removal of these. For this you'll need a Crank puller. One aspect of this tool threads into the center of the crank arms (not the same spot as the fastener you just removed but right around there). Then a second bolt tool with a special head threads into the first part, pushes on the spindle of the bottom bracket which affects the movement of the crankarm off the spindle.

Here's a link to a Crank puller. Here's a couple "for instance" chainsets that are around the level of your current one: Shimano Tourney, SRAM Should you choose to remove and change your bottom bracket, which the specs for your bike state is a Shimano cartridge-type BB-UN26, you will need a bottom-bracket tool.

Cartridge bottom brackets last a long time, so it's likely you don't necessarily have to change it as long a the spindle is in good shape and there doesn't appear to be any grinding or roughness when you spin the cranks (with the chain off is best as you get a better feel for the status of the bearings without the noise and vibration of the chain). One aspect to think about, is if you choose to remove your old bottom bracket, that will open up your options as far as cranksets go. Now days, the bearings are external to the bottom bracket shell, are far lighter than cartridge bearing BB's, and the cranksets are two piece: the right arm with it's chainwheels also has a spindle attached which the left arm slips on to and connects the system together. While they sell and advise the use of special tools for these bottom brackets and an aspect of the crank, one can get by with more common tools used with good sense


There’s a bit of a terminology confusion here. To replace the chainrings, or replace the crank, you don’t need to take the bottom bracket out if the frame. (The bottom bracket is the set of bearings the crank spindle runs in), You have a ‘3-piece’ crank where both the crank arms are separate from the spindle and the spindle is integral to the bottom bracket, so you just need to pull the cranks off.

Some spindles have a hole in the end into which a crank bolt threads, others have a threaded extension onto which a nut threads. Some crank bolts have an Allen key fitting, some have a head for a hex socket - you have the latter. The bolt on the end of the spindle needs to be removed and the cranks pulled off with a special crank puller tool.

The link you provided to the specs of the bike imply the crank is Shimano Tourney, or another low end model. Often these don’t have removable chain rings so you’d have to replace the entire crankset if this is the case with your crank.

  • 2
    For clarity the bolts you need to remove requires a 14mm socket. Unlike the bottom bracket itself and the pedals, both of these bolts are standard right hand thread (lefty loosey-righty tighty).
    – mikes
    Commented May 27, 2019 at 23:00

The ‘CH 12.9’ hex bolt just needs removing with a hex socket, try a 14mm first. The bolt on both sides is right-hand threaded so perfectly normal. It is a hex socket instead of an Allen key fitting, but is doing the same job.

When it is removed you will likely see the square outline of a square taper B.B. spindle, or else be able to identify the spindle type from the tutorial you are following. You then need the cotterless crank puller tool to suit and away you go with the tutorial for pulling the crank.

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