How to fit a square taper MTB bottom bracket?

Your MTB is fitted with a square taper bottom bracket and you don't know how to proceed? Don't panic, here is a tutorial to help you with this operation.

• Time: 45 min
• Level: Difficult
• Tools: 8 mm Allen key | 15 mm open-end spanner | Crank puller | Spline tool for square taper / Isis / Octalink | Degreaser | Cloth | Anti-seize assembly paste | Brush | Grease | Gloves | Torque wrench.

Steps

1. Disengage the chain from the MTB crankset.
   
   
2. Using an 8 mm Allen key, unscrew the central bolt that holds the crankset on the bottom bracket on the chainring side.
   
   
3. Remove the crank arm from the bottom bracket using a specific tool called a crank puller:
   • Screw the threaded part of the puller into the thread of the bottom bracket (chainring side), then finish tightening with a 15 mm open-end spanner;
   • Using the 8 mm Allen key, now screw in the opposite part of the puller. Apply force and you will feel the crankset come loose on the chainring side;
   • Retrieve the puller from the crank arm by unscrewing the threaded part;
   • Perform the same operations on the other side to remove the other part of the crankset (side opposite the chainrings).
     
     
4. Proceed with removing the bottom bracket. Start on the side opposite the chainrings. To do this, use a 20-spline tool designed for square taper or Isis / Octalink bottom brackets. As the bottom bracket is generally tightened to 50 Nm, you will need to apply some force to loosen the adjustment cup. Now remove the bottom bracket itself on the chainring side, again using the spline tool.
   Probikeshop Tip: Be careful, on this side, loosening is done clockwise.
   
   
5. Take this opportunity to clean the bottom bracket shell using a quick degreaser.
   
   
6. Use anti-seize assembly paste to ensure effective tightening and prevent any creaking. Apply the anti-seize with a brush inside the bottom bracket shell, on the chainring side only.
   Probikeshop Tip: Do not apply anti-seize to the plastic adjustment cup, which must be fitted dry.
   
   
7. Apply grease generously to the new bottom bracket to prevent water from penetrating inside the bearings. Then fit and screw in the bottom bracket from the chainring side, first by hand and then with the crank puller, using a torque wrench and respecting the tightening torque specified by the manufacturer (generally 50 Nm).
   Probikeshop Tip: Be careful, tightening on this side is done anti-clockwise.
   
   
8. Then perform the same operation on the other side with the adjustment cup, this time with standard clockwise tightening.
   
   
9. The bottom bracket is now fitted. We will proceed with refitting the crank arms, starting on the side opposite the chainrings. Begin by greasing the bottom bracket / crank arm interface, as well as the crank arm clamping bolt. Tighten the clamping bolt using an 8 mm Allen key, then finish tightening with a torque wrench to 35 Nm, clockwise.
   
   
10. Refit the crankset on the chainring side by first greasing the bottom bracket / crank arm interface, as well as the thread of the clamping bolt. Try to rest the chain on the bottom bracket shell, then insert the crankset making sure to keep the left and right crank arms opposite each other (one at the bottom, the other at the top).
    Probikeshop Tip: To make the operation easier, place the chain on the small chainring. Then tighten the bolt using an 8 mm Allen key, and finish tightening with a torque wrench to 35 Nm, clockwise.