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Internal bottom brackets : what size do you need

With internal bottom brackets, every model of every make of chainset is designed to work with a specific-length bottom bracket, which determines how far out from the frame the chainset sits. If it’s too close, the chainrings will rub on the frame; too far out and the front derailleur will struggle to reach the outer chainring.

Aligning the chainset correctly minimizes that angle of the chain in the outer rear sprockets, reducing chain wear. Before removing your old bottom bracket, inspect it to see whether it is the right length. Even new bikes sometimes come fitted with the wrong-length bottom bracket, so it’s worth checking rather than automatically replacing it with the same length again.

Internal bottom brackets what size do you need

  • Check the chain line by shifting into your middle chainring and middle sprocket, and looking along the chain from behind the cassette. The chain should run straight, without an obvious kink where it meshes onto the sprocket and chainring teeth.
  • Check the gap between the chainstay and chairings – there needs to be 2–3mm (around 1/8 inch) clearance to allow for chainring flex under pressure. If the chainrings rub on the chainstay, they’ll wear it away.
  • Shift into the smallest chainring and check the back of the front derailleur does not touch the frame. You need enough clearance here to adjust the front derailleur.
  • Shift into the middle chainring and the smallest chainring at the back. If the chain rubs on the bottom of the outer chainring in this gear, the chainring needs to move outward.

If your current bottom bracket is the right length, replace with the same length. Estimating what length to use to correct any of the above problems can be tricky – it’s worth taking your bike to your bike shop for help. Take your cranks off to measure the size of your current bottom bracket; once you’ve done that, you can measure your bottom bracket while it’s still in the frame. Bottom bracket shell width (A) is measured across the part of the frame that the bottom bracket fits into. Measure only the frame, not the flange of bottom bracket overlapping the edge of the frame. For example, if the frame is blue, measure across only the blue part, not across the silver bottom bracket. The two most common sizes are 68mm and 73mm. If your frame measures up any other size, measure again; there are other widths, but they are very unusual. The total axle width (B) runs from one end of the axle to the other (including the amount that sticks out each side) and is measured to the nearest millimetre. The differences between sizes are quite small – for square taper axles, Shimano make 107mm, 110mm, 113mm, 115mm, 118mm and 122mm lengths. These may seem like a lot of similar sizes, but the difference between one size and the next will radically affect your shifting.


Square taper and splined (internal) bottom brackets. Removing and refitting chainsets and cranks:

  • Crank bolt spanner – 8mm or 10mm Allen key, or (for older bikes) a 14mm socket. For all styles, you’ll need one with a long handle – around 200mm – so that you can apply sufficient leverage. The bolts will be firmly fitted, and need to be firmly refitted
  • A crank extractor – choose the correct type for your crank. Those with a smaller head are designed for smaller cranks, those with larger heads for splined cranks
  • Grease for refitting the crank bolt threads

Removing & refitting bottom bracket cups:

  • A splined bottom bracket cup remover. For ISIS bottom brackets, you’ll need the version with the larger hole in the middle, to accommodate the fatter axle – this will also fit fine on Shimano square taper and splined versions
  • A big spanner to drive the bottom bracket tool. You’ll need lots of leverage here, so a long-handled spanner will help. If you’re struggling, find a suitable length of tubing and slide it over the spanner handle to increase its effective length

One-key release kit:

  • Circlip pliers, small peg spanner, or the reverse end of the special tool (TLFC20) that Shimano make for holding the backs of chainring bolts steady while you tighten the Allen key (It took me years to work out the function of the other end of my chainring bolt tool)

See also bike maintenance tips, tricks and techniques “One-key release”