The gearbox is a small epicyclic (planetary) reduction unit that clips onto the drive end of the motor. A sun pinion on the motor shaft turns three planet gears, which run on metal posts inside a toothed outer case. The output gear carries the toothed pulley for the drive belt.

Lotus designed it. The gears are glass-filled nylon, made by Mikron (UK) Ltd of Huddersfield. Owners describe the small planet cogs as Delrin (acetal).

The gearbox snaps into the plastic “finger” end assembly of the motor (the gearbox holder) with six latches over retention ramps. An O-ring seals the joint.

These are Hoover's internal factory numbers, from the Hoover assembly sheets (Final Assembly, Op. 12–13).

This follows the Sinclair Vehicles service note Notes on Replacement of Output Gear (Part No 101036).

1. Remove the boot and free the drive belt. Discard any shim washers between the road wheel and the large pulley, and don't pull the splined sleeve out along the axle.
2. Free the motor from its clamp.
3. Push 1.5 mm wire (or matchsticks) under the six latches on the motor end plate, so they lift clear of the ramps. Then tap the sides of the gearbox while pulling it off. Take care of the small wires on the motor. Chas made a dedicated pulling tool for this job.
4. Lift the three planet gears off their posts and pull the posts out with cloth-protected pliers. Tap the output gear out through the cover. Check that both small bearings stay on the motor shaft.
5. Refitting:
   • Posts fully home, domed end outwards; they mustn't stand proud of the cover face.
   • Large bearing fully seated in the cover and on the output gear.
   • Location key in the end-plate groove, O-ring in place.
   • Press until all six latches click home.
6. Re-tension the belt. Correct tension is normally reached when the motor brushes are roughly horizontal, with the red lead at the rear. See Drive belt.

• Sinclair said the gearbox only needed greasing when dismantled for service. That assumed a new vehicle on a normal service cycle.
• On any C5 that hasn't had its gearbox serviced (which today means almost all of them), open it, clean out the old grease and regrease it before regular use. After 40 years the original grease will have dried, hardened or moved away from the teeth, and the plastic gears depend on it. Restorers do this as standard practice.
• While it's open, inspect the three planet gears, the output gear and the cover's internal teeth for cracks, chipped or worn teeth and debris.
• Use a grease that is safe for plastics (nylon/acetal), such as a lithium or synthetic grease labelled as plastic-compatible. Don't overfill: the factory quantity was 5.0 ml. Grease the inside of the cover and the three planet shafts.
• After that, check it whenever the belt or motor is off, or if it gets noisy.

• 1985 production stop. Production halted for about three weeks over the plastic moulding attached to the gearbox. Which? (June 1985) reported a “major gearbox fault” on all three of its test C5s.
• Motor tilt. Under load the motor can twist on its support, so the belt runs out of line and wears the output pinion. See Drive belt and Motor support.
• Starting from rest on the motor alone overloads the gearbox. The motor's stall torque is very high. Always pedal off first.
• Higher-voltage conversions (24 V and up). The three small cogs can shatter under hard acceleration and wreck the motor. Accelerate gently, or fit a metal insert. C5Alive is reported to have stopped selling its 24 V kit for this reason.
• Motor runs, but no drive. First check that the gearbox hasn't come off its lip on the motor end.

Original gearboxes now mostly come from donor vehicles.

• Metal gearbox insert by Chas: an alloy insert with the toothed ring and large bearing fitted. You reuse your own three cogs and posts. It cost about £65 + £3 postage in 2013. A four-cog version was shown in 2014. Owners report it surviving 36 V and even 60 V use.
• Brass planet gears have been fitted by at least one owner.
• Planet gear posts: a match for the originals was found on eBay at about £5.30 for 10.
• Replacement cogs cut from Delrin bar stock.
• 3D printing is not strong enough for gearbox internals.

Motor · Drive belt · Drive gear · Rear axle · Motor support · Thermal trip · Heat sensor

• Sinclair C5 Owner's Handbook (Sinclair Vehicles, 1985), pp. 30–32
• The Unofficial Sinclair C5 Service Manual, “Notes on C5 Motor/Gearbox Assembly” and “Known Problems”
• Sinclair Vehicles, Notes on Replacement of Output Gear (Part No 101036)
• Hoover assembly sheets, Final Assembly Op. 12–13
• The Engineer, 17 Jan 1985, pp. 36–37; Which?, June 1985
• ESG parts and price list, December 1991; MATCH parts price list, January 1991; MATCH-AURORA order form
• c5owners.com forum (closed to new posts, still readable):
  • Metal gearbox insert: SINCLAIR C5 CHAS, May 2013
  • Metal gearbox insert (4 small cogs): SINCLAIR C5 CHAS, Jan 2014
  • Moulding the 3 gearbox cogs: Dan, SINCLAIR C5 CHAS, Aug 2013
  • Gearbox posts: Dan, Jan 2014
  • Hello (brass planet gears): Bluebird85, Jan 2014
  • 24v with voltage booster: SINCLAIR C5 CHAS
  • 36 volt C5 and 60 volt C5: kamztrad, 2013–14
  • New project at last and 24v wiring kit wanted: Dan, 2014
  • Hello from Holland: andries.moss, 2014
  • New C5 restoration (gearbox puller) and Help please: SINCLAIR C5 CHAS, Apr 2014
  • Motor (motor and gearbox servicing): SINCLAIR C5 CHAS, Oct 2013
  • What else to 3D print?: Dan, Oct 2013