The MGA transmission, a four-speed BMC B-series gearbox with synchromesh on second, third and fourth gears, was designed for the driving conditions and engine specifications of the late 1950s and early 1960s. Two principal upgrade routes are available for owners looking to update the transmission for modern road use: a five-speed gearbox conversion kit based around the Ford Type 9 gearbox, and the factory-approved close-ratio gearbox option that was already part of the MGA's own production specification. Clutch-related tooling and specialist items for the transmission sit alongside these in the Upgrades & Alternatives section.
Five-speed conversion, Hi-Gear Engineering kit
The five-speed conversion kit for the MGA was developed by Hi-Gear Engineering and is designed around the Ford Type 9 gearbox, which is supplied separately from the kit itself. The Type 9 gearbox adds a selectable fifth gear under the control of a clutch and gear lever, and in addition to the fifth gear it carries synchromesh on all forward gears including first, unlike the original MGA gearbox, where first is non-synchromesh. The kit is aimed specifically at owners of non-overdrive MGAs looking for lower motorway cruising rpm, improved fuel economy, and a 10% higher gear at the top of the range for more responsive acceleration and safer overtaking. The Hi-Gear kit includes an alloy bellhousing, a propshaft matched to the conversion, a gearbox cross member, a speedo cable, the necessary fixings, and comprehensive instructions for DIY installation.
Parts may vary depending on the specific MGA model being converted. Optional backplates are available separately for MGA owners who have fitted an MGB engine to their car, since the MGB B-series engine differs from the MGA B-series in bellhousing interface and the conversion kit must therefore match the actual engine fitted. The five-speed conversion is covered in detail under the Five Speed Conversions node.
Close-ratio gearbox, factory option
A factory-approved alternative to the five-speed conversion is the close-ratio gearbox option, which was available as a factory-fitted option during MGA production from engine number 15GB/1379 in March 1959. When fitted, the engine prefix becomes 16GB-Da as a marker of the different installation. The close-ratio gearbox retains the standard four-speed configuration but gives tighter spacing between gear ratios, first becomes 2.445:1 rather than the standard 3.64:1, second becomes 1.62:1 rather than 2.214:1, and so on. The close-ratio gearbox was originally supplied primarily for competition-specification cars; close-ratio internals are for practical purposes identical to those of the Twin Cam gearbox.
The close-ratio gearbox is covered under the Gearbox - Internal Components node.
Clutch-related upgrades and specialist tooling
The Upgrades & Alternatives section also covers clutch-related items including clutch alignment tools. The clutch alignment tool replicates the gearbox input shaft and is essential equipment for any clutch replacement, ensuring that the clutch driven plate is correctly centred against the flywheel and aligned to the spigot bush in the rear of the crankshaft during refitting. Full detail of the clutch alignment tool is covered under the Clutch Tools node.
Ordering considerations
For owners considering a transmission upgrade, the principal decision is between a five-speed conversion (materially different to the factory arrangement, requires fitment work including propshaft and cross member) and the factory close-ratio option (same four-speed configuration, tighter spacing, direct swap for the original gearbox). The five-speed conversion brings synchromesh on first gear, a 10% higher top-end ratio and the full benefit of an additional gear, at the cost of deeper installation work. The close-ratio option retains period-correct factory specification. Both routes are available through MGOC Spares; the Hi-Gear five-speed kit is supplied without the Ford Type 9 gearbox itself, which is sourced separately, and owners with MGB engines should note the optional separately-available backplate when ordering.