See Buying a C5 for what to check before you buy.

This follows the typical “budget restoration” written up on c5owners.com, cross-checked against the Owner's Handbook and the Hoover training notes.

• Remove the boot: open it, pull the two retaining clips down off their bosses, spring the wheel arches outward and lift the boot away.
• Remove the battery and the master switch.
• To take the body off, release the harness at the control box and steering stem, the brake cables and grommets, and the four body bolts with bump rubbers.
• The Hoover training course stripped the body, harness, steering, crank set, rear wheels and brake, belt, axle, motor, gearbox and front end in that order. See Hoover Service Network for the three servicing positions.

• Lightly sand the Y-frame and rear frame, treat with a rust converter, and repaint.
• Clean and regrease:
  • the steering ball bearings (head and steering-control bearings);
  • the crank bearings (cotterless chainset);
  • the front hub and right rear hub (cup and cone);
  • the gearbox (5 ml of semi-fluid lithium grease, e.g. Castrol LMB).
• Set all bearings, steering height (15–20 mm above the tunnel) and torques to the handbook figures. See Specifications and Driving and Maintenance.

• Always replace the original brake cables. Replace inner and outer together.
• Front:
  • Generic bicycle side-pull calipers have been used successfully.
  • Hoover's notes warned that normal bicycle brake blocks must not be used on the nylon rim. Use blocks suitable for plastic or composite rims.
  • Set the block clearance to 1.5–3 mm, and make sure the blocks don't touch the tyre.
• Rear: inspect the shoes and the backplate. Reproduction rear-brake bearing brackets are available. Front and rear disc-brake conversions are popular modifications.

• The control box:
  • Check D1, the commutation diode. It's often blown, usually from the C5 being pushed backwards.
  • Check FS1 (5 A) and FS2 (1 A).
  • Check the relay contacts, and look for corroded connectors.
• The pod:
  • Clean the red pod connector pins with contact cleaner.
  • Re-flow any cracked solder joints, especially on the LEDs.
  • Handle the ULA with anti-static precautions. If the ULA is dead, see Modifications for Arduino replacement modules.
• Lights: fit LED bulbs in place of the 5 W W10/5 bulbs. They save power and stop the front housing overheating. LEDs are polarity-sensitive, and the flasher must be an LED type.
• Battery:
  • A modern sealed or leisure lead-acid battery of a suitable size works.
  • A LiFePO4 pack (for example a 12 V 20 Ah golf-trolley battery, about 2.5 kg) dramatically improves acceleration and hill climbing because of the weight saving. Use a charger matched to the chemistry; the original Sinclair charger is lead-acid only.
  • With non-original batteries, fit quick-release terminals and keep an emergency cut-out in the circuit, since a welded relay can make the motor run away.

• Tyre sizes:
  • Front: 12½ × 2¼ (ISO 57-203). Kenda and others make this size.
  • Rear: 16 × 2 (ISO 54-305). Schwalbe Big Apple and City Jet, and Raleigh Chopper-style tyres, have been used.
• Original pressures are 30 psi front / 35 psi rear.
• The plastic wheels need no maintenance, but check the front rim for heat distortion.

• Clean the polypropylene with a strong cleaner and a soft brush. Light sanding removes scuffs. Don't paint it unless you know what you're doing, because polypropylene needs special primers.
• Replace the yellow side stripes with high-intensity reflective tape of a similar pattern; it's about 0.5 cm taller than the original. The stripe also hides the body weld line.
• Decals:
  • “C5” on the headlamp surround;
  • “sinclair” on the boot;
  • the stripes.

Reproductions are available.

• Small plastic parts: many are available as 3D-printable reproductions. See Modifications.

Follow the handbook's commissioning sequence: rolling check, battery installation, then operational check with the rear wheel off the ground. Then do Hoover's 5-minute functional test, including a road test. See Driving and Maintenance.

The c5owners.com budget restoration of about 2014 bought a C5 for £200 and spent about £76 on parts. The parts were rust treatment, paint, brakes and cables, diode D1, contact cleaner, a relay, a key switch, LED bulbs, tyres, tubes and stripes.

• c5owners.com: “What to Look for When Buying a Sinclair C5”; “Restoring a Sinclair C5 on a Budget”; forum threads on LiFePO4 batteries, tyres and gearbox inserts.
• Sinclair C5 Owner's Handbook (1985); Hoover Service Engineer Training Notes (1984); The Unofficial Sinclair C5 Service Manual.