Gearbox & Freewheel
One of the quiet fascinations of the Aceca is that no two are quite alike under the tunnel — the gearbox you get depends on which engine your car was built with, and the Bristol cars have a party trick all of their own.
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.
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.
About the author
A lifelong classic-car enthusiast in North Yorkshire, near Thornton-le-Dale, who owns and maintains an AC Aceca — writing the single-model reference he always wished existed. Read the full bio.