Classic
BMC A-series gearbox rebuild
The A-series gearbox runs from 1950s three-synchro smoothcase units through to ribcase all-synchro boxes and the transverse Mini gearbox that lives in the engine sump. Different problems, same workshop.
12 month warranty
There are really two A-series gearboxes
The in-line box, in the Minor, A35, A40, Sprite and Midget, is conventional. Engine at the front, gearbox behind it, and it either has synchromesh on first gear or it doesn’t. The split that matters is three-synchro versus all-synchro, and smoothcase versus ribcase. Ribcase boxes are stronger and the internals are more available, which is why so many Sprites and Midgets have had one fitted at some point in their life. The all-synchro ribcase is the one most people want.
The transverse box, in the Mini and its derivatives, sits in the sump and shares its oil with the engine. That is the root of most of its problems. Every bit of clutch dust, bearing debris and combustion byproduct ends up circulating through the gears. Idler gear bearings wear, the differential pot joints go, and the box gets noisy long before it stops working. Regular oil changes buy years.
Applications
| Family | Vehicles | Notes |
|---|---|---|
| In-line, three-synchro smoothcase | Morris Minor 1000, Austin A35, A40 Farina, Austin-Healey Sprite Mk1 and Mk2 | No synchro on first |
| In-line, all-synchro ribcase | Sprite Mk3 and Mk4, MG Midget 1275, Morris Minor (later) | The strong one, and the common upgrade |
| Transverse, three-synchro, in sump | Mini 850, 1000 and Cooper, pre-1968 | Magic wand or remote change |
| Transverse, all-synchro, in sump | Mini, Clubman, 1275GT, 1968 on; Metro | Rod change on later cars |
| AP four-speed automatic | Mini automatic | Specialist. Few will touch these |
Scroll the table sideways to see every column.
Note: the MG Midget 1500 uses the Triumph Spitfire gearbox, not the A-series box. It is a common and expensive mix-up when buying secondhand.
Common faults
| Symptom | Cause | Our fix |
|---|---|---|
| Crunches into second | Worn second synchro, the classic A-series complaint | New baulk rings, synchro hub and springs; selector fork checked |
| Jumps out of second on the overrun | Worn dog teeth, worn fork, tired detent springs | Gear renewed if teeth are rounded; fork, springs and balls replaced |
| General whine and rumble, worsening with mileage | Laygear thrust washer wear opening up endfloat | Oversize thrust washers, endfloat set to specification |
| Rumble in neutral | First motion shaft bearing | Full bearing set |
| Mini only: clonk on drive take-up | Worn pot joints or inner CV | Pot joints renewed, driveshafts inspected |
| Mini only: growl on the overrun, glitter in the oil | Idler gear bearings, contaminated shared oil | Idler bearings renewed, differential inspected, oil interval advice |
| Difficult to select any gear | Worn selector shafts, worn remote or rod-change linkage bushes | Linkage overhauled or rebuilt |
| Oil leak at the bellhousing | Rear main or front cover seal | Renewed on rebuild |
Scroll the table sideways to see every column.
What a rebuild includes
Complete strip and clean. Case crack tested. All bearings, bushes, oil seals and gaskets replaced. Baulk rings and synchro hubs assessed and renewed. Laygear thrust washers renewed and endfloat set. On transverse boxes, idler gear bearings, differential and pot joints inspected and renewed as needed. Selector mechanism overhauled. Bench shifted through every gear, warranted for 12 months.
Competition and uprated builds
Straight-cut drop gears, close-ratio gear sets, needle-roller laygear conversions and uprated bearings. We build for road, sprint, hillclimb and circuit, including historic racing where the box must remain period-correct.
No Midget or Sprite ever left the factory with five gears. Where the regulations allow it, a five-speed means fitting a Ford Type 9 in place of the A-series box, with an adaptor bellhousing and the right input shaft. It is a conversion, not an upgrade to the original unit.
Straight-cut drop gears are noisy. Very noisy. That is the trade-off and it’s worth knowing before you commit.
What will it cost?
We price an A-series rebuild once we know which unit you have and what it is doing. Send us the gearbox number and the symptoms and you will have an answer the same working day, with an indicative price wherever we can give one.
Frequently asked questions
Can I fit a ribcase box to an early Sprite?
Usually yes, with the right bellhousing and clutch. Send us the details.
What oil should a Mini gearbox run?
Because the Mini shares oil between engine and gearbox, specification matters more than most people realise. Ask us before you fill it.
Do you do five-speed conversions for Midgets and Sprites?
Yes, usually a Ford Type 9 fitted in place of the A-series box. Worth being clear that this is a conversion. No Midget or Sprite was ever built with a five-speed gearbox, so anything advertised as an original five-speed car has been converted at some point.
How often should a Mini have its oil changed?
More often than the handbook says. It is the single cheapest thing you can do for the gearbox.
Can you rebuild a box someone else has already been into?
Yes. We’d rather see that than an unknown “good used” unit.
Related
How it works
Sending us a unit
You don’t need to bring the car. Most of what we do arrives on a pallet.
- Drain it. Couriers won’t take a unit full of oil, and it’ll leak.
- Crate or pallet it. A gearbox needs to be strapped to a pallet or in a stout crate. A cardboard box won’t survive.
- Label it. Put your name, telephone number and the vehicle inside the packaging as well as on it. Labels come off in transit.
- Tell us it’s coming. Send the enquiry form first so we know what to expect and can book it in.
We can arrange collection anywhere in mainland UK. Ask when you enquire.
What happens next
- 01You send the enquiry.
- 02We come back the same working day with an initial view and, where we can, an indicative price.
- 03If it needs looking at, we book it in or arrange collection.
- 04We strip and assess, then ring you with a firm price before any work goes ahead.
- 05Rebuilt, tested, returned, warranted for 12 months.
No work happens without your say-so, and there are no surprises on the invoice.
Same working day
Send us the number and what it’s doing
The gearbox number, the vehicle and the symptoms. That is usually enough for a straight answer, and photographs are welcome.