Road bike maintenance - chainset & bottom bracket

Road bike maintenance - chainset & bottom bracket

When buying a replacement bottom bracket make certain you either get an identical replacement or one with same length of axle on the right hand (chainwheel) side. Beware also, that some Italian frames have a different thread dimension and direction to the British standard.

Time: Two to three hours depending on work needed
Difficulty: Moderate

Tools required:

  • Crank extractor
  • 8mm Allen keys*
  • Bottom bracket cartridge tool
  • 180mm adjustable spanner*
  • Chainring bolt back nut spanner
  • * Optional depending on needs
When buying a replacement bottom bracket make certain you either get an identical replacement or one with same length of axle on the right hand (chainwheel) side. Beware also, that some Italian frames have a different thread dimension and direction to the British standard.

Checking chainset and bottom bracket
  1. Check the bottom-bracket bearings by grasping the end of each crank and rocking them from side to side. If you can detect any movement, either the retaining cups are loose or the cartridge or its bearing units need replacing.
  2. Check too for bent, worn or damaged chainrings that may cause the chain to jump, or rub on the front derailleur.
  3. Bent teeth on the chainring can cause the chain to jump or catch (most commonly caused by airline baggage handling!) Use an adjustable spanner to bend them back carefully.
  4. Bent (untrue) chainrings can sometimes be straightened by judicious use of a large adjustable spanner.
Removing chainset  
  1. Older chainsets may have a separate dust cap with 14mm or 15mm crank bolt underneath, if so remove the chainset dust cap if fitted and then remove the crank bolt and washer. Other cranks have integral dust cap and bolt - unscrew this.
  2. With both of these types, using the appropriate crank extractor, carefully screw the tool into the crank's threads (with its inner bolt fully retracted).
  3. Now screw in the tool's inner bolt to push the crank off the axle.
  4. Post '97 Shimano Dura-Ace and Ultegra and some Campagnolo chainsets now use a 'one key release'. This is an integral bolt and dust cap which when unscrewed pushes the crank off the axle - simply unscrew the Allen bolt anticlockwise.
Fitting a new bottom bracket  
  1. To remove a cartridge type bottom bracket unit, fit the cartridge removal tool in the splines on the right side of the frame and using a large adjustable spanner, unscrew it clockwise.
  2. For Italian threaded frames unscrew it anti-clockwise (the side of the bottom bracket cartridge will be marked 'It' or 'Italian').
  3. Remove the cup from the left side; all unscrew anti-clockwise.
  4. Grease the threads of the new cartridge and cup with anti-seize compound.
  5. Fit the new cartridge from the right side and carefully finger tighten anticlockwise (clockwise for Italian frames).
  6. Install the left side cup all but the last couple of threads.
  7. Now fully tighten the right hand side of the cartridge close up to the frame, followed by the left hand cup.
Refitting the chainset
  1. Clean the axle ends and the crank tapers with a solvent and let them dry.
  2. Grease the crank bolts and re-fit the chainset.
  3. Tighten the crank bolt - ideally use a torque wrench to 28-40Nm. But if you don't have a torque wrench, tighten almost as tight as you can get with a 10in spanner. Refit dust caps where fitted.
  4. Refit the opposite crank - making sure that it lines up with the other - and tighten similarly.
  5. Check crank bolt tightness after a couple of hundred miles and once every three months or so.
  6. Never ride on a loose crank. Light alloy cranks will wear very rapidly and may never be fitted securely again if ridden loose.
Removing and fitting chainrings
  1. You do not usually have to remove the cranks to change your chainrings. Note the position of any spacers between the rings before removing the chainring fixing bolts.
  2. A back nut spanner to stop the rear nut turning is a great help.
  3. Clean and then grease all the mounting points.
  4. Position the chainrings on with their correct side facing outwards and correctly orientated on the crank. There will sometimes be a mark or catch pin on the chainring to be lined up with the crank.
  5. Assemble loosely and finally, working across the 'spider' (arms of the crank) tighten each bolt a little at a time, until they are all tight.
  6. If you are changing the size of chainring, you may have to adjust your gears - and possibly even your bottom bracket axle length too.

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