Gearbox & Freewheel

Three engines, three gearboxes

The Aceca was never built to one transmission specification, and the chassis prefix tells you most of what you need to know before you so much as lift the tunnel cover.

• AC-engined cars (AE chassis): the AC 2.0-litre Light Six was mated to the Moss four-speed gearbox, the same well-known British unit, with its distinctive rearward gearlever, used in a number of period sports cars.
• Aceca-Bristol (BE chassis): from 1956 the Bristol 2.0-litre engine came with its own Bristol four-speed gearbox, running closer ratios than the AC-engined cars along with a smaller-diameter Borg & Beck clutch. This is the box with the freewheel.
• Aceca 2.6 (RS chassis): the late Ruddspeed Ford-engined cars used an AC alloy-cased gearbox with Triumph TR3 gears.

An A-type overdrive was offered as an option, operating on the upper gears, a welcome thing on a car that is perfectly happy to cover long distances, and worth establishing the presence or absence of when you are looking at a car.

The freewheel explained

The Bristol gearbox carries a freewheel device on first gear, a feature inherited from the BMW-derived Bristol design. In practice it lets the transmission overrun, freewheel, in first, which historically allowed smooth, near-clutchless changes up out of first and softened the shock of taking up drive.

To a modern driver used to a conventional box it can feel odd, because the car seems to coast when you lift off in first, and it catches people out, but it is working exactly as designed. It is part of the Bristol cars' character, and once you have driven a few miles with it you stop noticing.

Fitting an overdrive

Overdrive was a factory option on the Aceca, and owners still fit units today. Two practical routes are discussed on the AC Owners' Club forum, and they differ in what has to be altered under the car.

• Derek Hughes overdrive: this unit replaces the non-overdrive tailshaft and uses the existing gearbox mounts, so the mounts themselves do not need altering.
• Bristol 405 type overdrive gearbox (BWCR12): this route would require the gearbox mounts to be altered. That is the practical difference between the two conversions.

The open question is whether the fibreglass gearbox tunnel has to be widened where the overdrive unit bolts on. Nobody who has fitted an overdrive to an Ace or an Aceca replied to the forum thread. The only response came from an owner who had fitted a Derek Hughes kit to a Bristol 400, a different car with a different tunnel. On that car the conversion needed a new fibreglass tunnel, obtained from a specialist, and he noted that one could also be fabricated in aluminium. He was explicit that he did not know whether the Aceca uses a remote gear linkage, and his conclusion was that he would expect fabrication to be needed either way. That is an informed guess about a different car, not an answer.

As of September 2026 the tunnel question remains open. If you have fitted an overdrive to an Ace or Aceca and can answer it, please get in touch.

For the car's fibreglass bulkheads and body structure, see the body and ash frame article.

Source: AC Owners' Club forum, "Fitting an overdrive gearbox to an AC Aceca Bristol, gearbox tunnel width", started 18 December 2025.

Living with it, and looking after it

Whichever box you have: use the correct oil, and note it is not the same specification as the hypoid rear axle, as the gearbox wants an oil that protects its yellow-metal components, keep the level up, and treat crunchy synchromesh, jumping out of gear or a whine as wear to investigate rather than noise to live with.

On the Bristol cars, keep the freewheel in mind when diagnosing any "it will not hold in first" complaint: it may be the freewheel doing its job, not a fault. Transmission behaviour is also easily confused with chassis behaviour, so if something feels wrong on the move it is worth reading the steering and suspension article and the brakes article alongside this one.