Cyclist
How to replace a bottom bracket
Until the early ’80s, pulling apart your bike’s bottom bracket would probably have resulted in many tiny ball bearings making a bid for freedom across the floor. Nowadays, bottom brackets have disposable sealed cartridge bearings and, as frames have got beefier, bearings have increased in diameter to allow for lighter, stiffer chainsets using larger axles.
Sealed units means replacement has also become easier, which is just as well, as unlike the old-school loose ball brackets, they can’t be serviced and typically have a shorter life – they may need to be replaced yearly if you do serious mileage. And so goes the irresistible march of technological progress.
If your bike is equipped with Shimano parts, chances are it uses the Hollowtech system (SRAM’s GXP standard works in a similar way), which fits the standard threads cut into a traditional style bottom bracket shell.
However, while a traditional bottom bracket sits inside the frame and includes the axle in a single unit, Hollowtech bearings sit outside the frame and the axle is integrated into the crank.
To check if your BB needs replacing, drop the chain off of the smallest chainring and spin the cranks. If there’s a side-to-side wobble, or feeling of grittiness, it’s time for a new one.
With the right tools and know-how this needn’t necessitate a trip to the bike shop – just follow our step-by-step guide to replacing your bottom bracket. Note that if your bike has a new-style press-fit bottom bracket (such as BB30), it’s probably best you do make a trip to your local independent bike shop.
How to remove and replace a creaking bottom bracket
Time Taken: 45 minutes
Workshop Saving: £20
1. Removing the cranks
The left-hand crank is secured by two pinch bolts (the compression cap in the end presses the axle against the bearings while the pinch bolts hold it in place). To remove the crank, first you must loosen the pinch bolts using a 5mm allen key. They can be at a funny angle, and are done up fairly tight, so avoid using a ball-end allen key if possible.
2. Decompress the bearings
Remove the compression cap. Shimano bottom brackets require a special tool for this (for example, the Park Tool BBT-9, £19.99), which integrates the tool with the spanner you’ll use in step 5; separate tools are also available. Keep hold of it and try not to lose it, spares are strangely difficult to get hold of.
3. Release the safety catch
With a small flathead screwdriver, disengage the safety catch – which is located between the two pinch bolts – by gently pushing it upwards. Pull the crank arm off its spindle – it should slide off easily without much force. If you found the safety catch difficult to remove (and full of grit) now is a good time to give it a quick clean.
4. Crank extraction
Lift the chain off the smallest chainring and let it rest on the bottom bracket shell. Holding the spider (where the chainrings join the arms of the crank) pull the whole assembly from the bottom bracket. If it won’t budge, apply some gentle persuasion with a soft-faced mallet. In our experience, if it’s time to replace the BB, your crank will probably be difficult to get out.
5. British or Italian threads?
Using the spanner part of the bottom bracket tool, remove the bottom bracket. For British BBs (marked BSA), turn the right-hand bearing clockwise to remove, the left anti-clockwise; for Italian-style BBs (marked ITA), turn both anti-clockwise (correct directions are usually marked).
6. Clean up the threads
Clean the edges and threads with a rag and some solvent (such as Finish Line Speed Degreaser). It’s important the sides are smooth and even so the bearings sit flush in the frame. Brush the threads with a coat of anti-seize.
If you find your bearings wearing out quickly (within a few months) this could well be the cause. If so, it’s time for a trip to the shop to have the threads chased and faced with the appropriate cutting tool.
7. Fit the new bearings
The new bottom bracket should come with a plastic sleeve. Push this into the right-hand side of the BB cup and – using just your fingers – screw the cup with the sleeve attached into the frame. Once it’s finger-tight, screw in the other bearing on the opposite side. Using the bottom bracket spanner, tighten first the right bearing then the left bearing to 35-50Nm (if you don’t have a torque wrench, this is ‘pretty darn tight’).
It’s quite easy to cross thread the BB here, which is a costly mistake. If the cups aren’t going in by hand easily take them out and try again. They should screw in about 50% of the way without the tool.
8. Replace the chainset
Push the right hand side of the chainset back through the bottom bracket (remember to pop the chain back over the BB) and replace the chain onto the smallest chainring. Give it a spin to ensure it turns smoothly. Push the left hand arm onto the section of spindle protruding from the opposite side of the bottom bracket.
9. Recompress
Replace the compression cap and, using the tool, turn it until it’s barely finger tight (0.7-1.5Nm) – be careful as over-tightening will cause the bearings to wear prematurely. Spin the cranks to check they rotate freely. Replace the safety catch, tighten the pinch bolts to 10-15Nm, and you’re done.
Got the BB bug now? Read our tech tussle: is it time to ditch the press fit bottom bracket?
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