How to Replace the Bafang BBSHD Nylon Gear (Step by Step)
Replacing the nylon reduction gear inside a Bafang BBSHD mid-drive motor fixes the grinding noise or loss of drive that happens when the stock nylon teeth wear down or break. The job takes roughly one hour with basic hand tools and a replacement gear (most BBSHD motors use a 42-tooth gear, but verify tooth count on your old gear before ordering). The procedure requires removing the motor from the bike, opening the drive-side housing, swapping the gear, and reinstalling everything with proper torque.
Tools and Prerequisites
| Item | Purpose |
|---|---|
| Replacement nylon gear | Bafang BBSHD-specific; match tooth count exactly |
| T25 Torx driver | Six motor cover screws |
| 8 mm Allen wrench | Chainring bolts and crank arm bolts |
| 10 mm Allen wrench | Main motor shaft nut |
| 5 mm Allen wrench | Four internal cover plate screws |
| Small flathead screwdriver | Pry gear cover if stuck (locate pry slot first) |
| Pick or snap-ring pliers | Remove C-clip on gear shaft |
| High-quality lithium or marine grease | Reassembly lubrication |
| Clean rag | Wipe old grease and debris |
| Torque wrench (recommended) | 4–5 ft-lb for cover screws; 12–15 ft-lb for chainring bolts |
Before starting: Disconnect the battery completely. Unplug the motor harness from the controller. Work on a clean, well-lit bench with the chainring side of the motor facing up.
Step-by-Step Replacement
Step 1: Remove the Motor from the Bike
Unbolt the motor from the bottom bracket shell. On most frames, this means removing four 8 mm or 10 mm bolts (check your frame’s mounting hardware). Slide the motor out of the bracket and place it on your workbench. Do not skip this step — attempting to open the motor while it’s mounted risks damaging the housing or pinching wires against the frame.
Step 2: Remove Crank Arms and Chainring
Use the 5 mm Allen wrench to remove the crank arm bolts. Pull both arms off the tapered shaft. On some models, a puller tool is necessary if the crank arms are stuck. With the arms off, unscrew the four 8 mm chainring bolts and lift the chainring and spider assembly away from the motor.
Step 3: Unbolt the Motor Housing Cover
The large circular cover on the chainring side is secured by six T25 Torx screws. Remove all six screws and lift the cover straight off. If the cover resists, check for a small pry slot at the edge — insert a flathead screwdriver gently and twist. Do not hammer or pry aggressively at the gasket seam; forcing the cover can crack the aluminum flange.
Branch point — stuck cover: If the gasket appears fused or there is visible corrosion around the mating surface, apply penetrating oil around the seam and wait 10 minutes. Carefully cut the gasket loose with a thin utility knife before prying. Replace the gasket during reassembly if it tears.
Step 4: Remove the Old Nylon Gear
Inside the housing, locate the nylon gear seated on its steel shaft. A small C-clip (retaining ring) secures the gear. Remove the clip using snap-ring pliers or a pick — take care not to launch the clip across the room. Slide the old nylon gear off the shaft. Note the orientation of any spacers, washers, or thrust washers on each side of the gear. Photograph the arrangement before removing anything.
Step 5: Clean and Inspect the Motor Cavity
Wipe out all old grease and plastic fragments from the housing. Inspect the steel pinion gear (the smaller gear driven by the motor rotor) for chipped or worn teeth. Stop/escalate threshold: If the steel pinion has broken, chipped, or heavily worn teeth, do not install the new nylon gear. Damaged steel teeth will eat through the new nylon gear within a few miles. Stop here and replace the entire secondary reduction assembly (steel pinion and shaft) or send the motor to an authorized Bafang service center.
Spin the main motor shaft by hand. It should rotate smoothly with no gritty sensation. If you feel grinding, the main shaft bearings are failing — replace them before continuing, or the vibration will shorten the life of your new gear.
Step 6: Install the New Nylon Gear
Apply a thin layer of grease to the gear shaft. Slide the new nylon gear onto the shaft, placing any washers and spacers in the exact order and orientation captured in your photo. Secure the gear with the C-clip. Critical detail: Install the C-clip with the tapered side facing toward the gear. The taper acts as a ramp that keeps the clip seated under load. Press the clip fully into its groove — it should snap in and sit flush. Rotate the gear by hand; it should spin freely without wobbling or binding against the housing.
Step 7: Reassemble the Motor
Place the housing cover back onto the motor, aligning the locating pin or notch on the cover with the corresponding slot in the housing. Ensure the rubber gasket sits flat in its groove — a pinched gasket will leak oil and allow debris into the bearings.
Tighten the six Torx screws evenly in a star pattern. Torque to 4–5 ft-lb maximum. The aluminum threads strip easily above this value; if you do not have a torque wrench, tighten snug with a hand driver and stop.
Reinstall the chainring and spider assembly. Torque the four 8 mm chainring bolts to 12–15 ft-lb in a cross pattern to ensure even seating. Reattach the crank arms and tighten the crank bolts to 30–35 ft-lb.
Step 8: Verify the Fix Before Full Reassembly
Reconnect the battery briefly with the motor still off the bike. Spin the cranks by hand through several revolutions. You should feel smooth rotation with no grinding, clicking, or intermittent resistance. Listen for gear engagement — a normal spur-gear whine is expected, but a repetitive click or clunk indicates the gear is sitting at an angle or the C-clip is not seated fully in its groove.
Success check: If rotation feels smooth and the only sound is the motor’s normal gear whine, the repair is complete. If you hear any irregular noise, remove the cover and inspect the gear alignment, spacer orientation, and C-clip groove. Once verified, mount the motor back into the bike frame, torque the mounting bolts to manufacturer specs, and tension the chain.
Common Installation Mistakes
- C-clip installed backward. The tapered side must face the gear. Installed backward, the clip can pop off under load, allowing the gear to walk off the shaft. Always verify the clip is fully seated and the taper faces the gear.
- Over-torquing cover screws. Aluminum threads strip at approximately 5 ft-lb. Use a torque wrench or stop at firm hand-tightness. Stripped threads require a helicoil or housing replacement.
- Pinched or torn gasket. A gasket that buckles during cover installation will leak grease and allow contaminant entry. Inspect the gasket before final tightening and replace if damaged.
- Reusing gear after debris contamination. If the old gear shattered, bits of nylon can lodge in motor windings. Flush the housing with electrical contact cleaner and compressed air before installing the new gear. Unseen fragments can cause overheating.
- Skipping the bearing check. A rough main shaft bearing creates vibration that accelerates wear on the new gear. If spinning the shaft by hand reveals any grit or roughness, replace the bearing now — it is easier than repeating the full disassembly in 200 miles.
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Ryan Williams has spent over 8 years testing, repairing, and writing about electric bikes. He has personally ridden and reviewed 150+ e-bike models from brands like Lectric, Aventon, Rad Power, Super73, and dozens more.
Before founding EBIKE Delight, Ryan worked as a bicycle mechanic for 5 years at independent bike shops across California, where he specialized in e-bike conversions and electrical system diagnostics. He holds a Certificate in Electric Vehicle Technology from the Light Electric Vehicle Association (LEVA).
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