The Midget drivetrain comprises clutch, gearbox, propshaft, and rear axle. The BMC A-Series ribcase gearbox was fitted to the 1098cc and 1275cc; the Triumph single-rail gearbox to the 1500. No gearbox or clutch components are shared between them, and even the rear axle half shafts differ between steel and wire wheel fitments, so identifying the correct combination for the car before ordering is essential.
Clutch Assemblies
The 1275cc clutch uses a 7¼-inch driven plate and diaphragm-spring cover, hydraulically operated via master and slave cylinders. The crankshaft spigot bush at the rear of the engine, which supports the gearbox input shaft, comes in two external diameters, the correct size must be confirmed before ordering. The 1500 clutch is a completely different assembly with its own driven plate, cover, release bearing, release mechanism, slave cylinder mounting, and alignment tools, none of these components interchange with the A-Series clutch.
Clutch Hydraulics and Bleeding
The clutch hydraulic system is one of the most commonly troublesome areas of the Midget drivetrain. Poor disengagement can be caused by air in the system, worn cylinder seals (in the master, slave, or both), or a deteriorated flexible hose that expands under pedal pressure rather than transmitting it to the slave. Bleeding by conventional pedal pumping is notoriously difficult on these cars due to the routing of the hydraulic line and the natural air trap at the slave cylinder, a pressure or vacuum bleeding tool is strongly recommended for any work on the system. Mechanical wear points (elongated clevis pin holes, worn pins, bent release arm) should also be inspected if hydraulic refurbishment alone does not restore correct operation.
A-Series Gearbox
The A-Series gearbox is a four-speed unit with synchromesh on second, third, and fourth gears only, first gear is non-synchromesh, requiring the car to be fully stopped or double-declutched for the change. The gear lever anti-rattle mechanism (often called a "zizzle kit" by enthusiasts) dampens the vibrational buzz transmitted up through the lever and is a straightforward renewal carried out from inside the car.
Note that the front cover of the A-Series gearbox is not oil-tight around a stationary input shaft, so jacking only the rear of the car will cause oil to dribble from the front cover area, this is normal and not a sign of a faulty seal.
1500 Gearbox
The 1500 gearbox is fully synchromesh on all four forward gears. Two front cover types were produced, distinguished by their sealing arrangement. The oil seal type was fitted up to gearbox number 59162, and again from 60001 to 65224. The oil thrower type was fitted from 59163 to 60000, and from 65225 onward.
The front covers are not interchangeable, fitting the wrong type causes an oil leak. This is one of the most common 1500 ordering errors, so the gearbox number should always be confirmed before ordering front cover components or oil seals.
Rear Axle
The rear axle is a semi-floating live axle with a hypoid bevel differential. Two final drive ratios were offered: 3.9:1 (standard, livelier acceleration) and 3.7:1 (more relaxed cruising with reduced engine revs at speed). Half shafts differ between steel and wire wheel cars, wire wheel half shafts have splined ends to accept the wire wheel hubs, while steel wheel cars use a flanged half shaft end. Hub nut thread direction is left-hand on the left side and right-hand on the right side, ensuring the rotational direction of the wheel tightens rather than loosens the nut under braking and acceleration loads.
The rear wheel bearing kit and oil seal should always be renewed when a half shaft is removed.
Crown Wheel, Pinion and Differential
Crown wheel and pinion sets and reconditioned exchange differentials are available. The axle case differs between steel wheel and wire wheel applications, and the axle case specification was revised during 1500 production, confirming the chassis number ensures the correct axle case is supplied. A limited slip differential is available as an upgrade for improved traction, particularly valuable on cars used for autotest, sprint, or wet road conditions where one driven wheel can otherwise lose grip and waste available engine torque.
Propshaft
The propshaft uses universal joints at each end. The 1275cc and 1500 propshafts differ, the 1500 uses a flanged yoke at the gearbox end matching the Triumph gearbox output. Greaseable universal joints offer longer service life than the modern sealed-for-life types and allow the joints to be re-greased at major service intervals.
Five-Speed Conversion
A five-speed gearbox conversion is available, adding an overdrive fifth gear that reduces engine revs at cruising speed by approximately 15 to 20%. The benefits are reduced engine wear at sustained motorway speeds, improved fuel economy on long trips, and lower cabin noise.