Maintain the gearbox, clutch and overdrive
The Aceca has a four-speed manual gearbox — a Moss box on AC-engined cars, Bristol's own on the Aceca-Bristol, an AC alloy-cased box on the 2.6 — often with a Laycock de Normanville overdrive on the upper gears. Most 'gearbox' faults are really clutch, linkage or overdrive-electrics problems, so diagnose before dismantling.
Tools
- Multimeter (for overdrive electrics)
- Spanner set
- Drain tray
- Torch
- Axle stands
Parts / supplies
- Correct-grade gearbox oil (per manual)
- Overdrive oil to the specified grade
- Overdrive solenoid or relay if faulty
Procedure
Identify your gearbox first, as it sets parts and oil: the Moss box (AC-engined cars), the Bristol four-speed with its smaller Borg & Beck clutch and close ratios (Aceca-Bristol), or the AC alloy-cased box with TR-derived internals (2.6).
Check the gearbox — and the overdrive, if fitted — oil level and condition against the manual, using only the specified grade. The overdrive's hydraulics are sensitive to the wrong oil.
Treat clutch complaints separately: drag, slip or juddering, and many 'won't select a gear' faults, are the clutch, its hydraulics or the linkage — not the gearbox internals. Check free play and operation first.
The Laycock de Normanville overdrive is electrically switched and hydraulically actuated, operating on 2nd, 3rd and 4th. If it won't engage, check the electrics first — the gear-position switches, solenoid, relay, wiring and earths — before suspecting the unit.
Confirm the inhibitor/gear switches allow overdrive only in the intended gears, and never in reverse or 1st.
Leave internal gearbox or overdrive overhaul — bearings, synchromesh, the overdrive's clutch and valves — to a specialist. Diagnose thoroughly, but the rebuild is specialist work.
Warnings
- Use only the gear oil grade specified for your gearbox and overdrive; the wrong oil can stop the overdrive working and damage it.
- Never force the overdrive into a gear the inhibitor switches are designed to block.
- Support the car on axle stands, never just a jack, before working underneath.
Related faults
About the author
An independent classic-car enthusiast in North Yorkshire with decades of hands-on experience on British classics — including the AC Aceca — writing the single-model reference he always wished existed. Read the full bio.