Replacing a Threaded Bottom Bracket on a Mountain Bike

Introduction

• Bottom bracket: the bearing that cranks revolve around, a consumable part that wears out over time.
• Essential skill to learn how to replace it.

Types of Bottom Brackets

Two Major Styles

1. Threaded
   • Bottom bracket cups screw into the frame.
   • Simple approach.
2. Press Fit
   • Bearings fit directly into the frame without screwing in.
   • Streamlined design, but requires precise tolerances.
   • Effective when installed correctly; movement can cause creaking.

Bottom Bracket Widths

• Common widths: 68mm, 73mm (road and mountain bikes).
  • 68mm: for some road and mountain bikes.
  • 73mm: more common for mountain bikes.
  • 83mm: typically for downhill bikes (extra stability).
  • Fat bikes may have up to 120mm width depending on drivetrain spacing.

Buying a Threaded Bottom Bracket

• Comes with spacers to adjust chain line and fit correctly.

Thread Direction in a Bottom Bracket

• Pedal threads tighten towards the front of the bike, while bottom bracket cups tighten towards the rear.
• Drive side (anti-clockwise), non-drive side (clockwise).
• Design prevents damaging threads in case of seized bearings.

Parts of a Bottom Bracket

• Components include:
  • Right and left cups (drive side and non-drive side) with bearings.
  • Bearing covers, spacers, and a plastic shell to keep water away.
• Different designs for downhill bikes and other variations.

Tools Required

• Grease: General bicycle grease or anti-seize for exotic frames.
• Bottom Bracket Tool: Traditional spanner or socket.
• Torque Wrench: For precise tightening (30-55Nm).
• Allen Keys: For crank removal, may need special tool for preload cap.

Preparing the Bike

1. Positioning: Use a work stand or a secure lean.
2. Remove Old Bottom Bracket:
   • Turn off clutch on Shimano derailleur, cage lock for SRAM.
   • Remove chain guide and lift chain out of the way.
   • Undo pinch bolts on left crank; remove preload cap.
   • Remove non-drive side crank first.
   • Clean cranks before reassembly.
3. Remove Old Cups:
   • Drive side loosens towards the front (clockwise), non-drive side loosens towards the rear (anti-clockwise).
   • Clean area around the bottom bracket shell afterward.

Installing the New Bottom Bracket

1. Clean threads on both the bottom bracket shell and the cup threads.
2. Apply grease to threads of the bottom bracket shell.
3. Install new cups:
   • Verify orientation (drive side and non-drive side).
   • Start threading in reverse to align threads correctly, then screw in.
   • Use torque wrench to tighten cups.
4. Replace Cranks:
   • Clean axle and put a bit of grease on it.
   • Align and guide cranks into place.
   • Use preload tool on Shimano cranks, tighten pinch bolts (12-14Nm).
5. Final Steps:
   • Reinstall chain and chain guide.
   • Turn the clutch back on for Shimano.

Conclusion

• Replacing a threaded bottom bracket is a straightforward process when done carefully, particularly with thread alignment.
• Comments and suggestions for additional videos are welcome.