The automatic gearbox is one of the rarest and least understood transmission options on the MGB. Offered as a cost option from the start of the Mark II model in late 1967, it was available on both the MGB roadster and GT through to the end of the 1973 model year, with production ceasing in August 1973. The unit fitted was the Borg-Warner Type 35, a three-speed epicyclic automatic coupled to a hydraulic fluid torque converter, a well-proven design also found across a wide range of other British and European cars of the period, which works in the automatic MGB owner’s favour when it comes to sourcing rebuild expertise.
Production History
The automatic gearbox was a genuine factory option rather than a conversion, and cars fitted with it can be positively identified by their engine number prefix. The ‘Rc’ designation within the engine prefix code denotes the Borg-Warner automatic, distinguishing these engines from manual-gearbox equivalents. For the 1968 model year, the applicable prefix is 18GD-Rc-H/L, with engine numbers running from 101 to 240.
From October 1968 on 1969 to 1971 model year cars, the prefix is 18GG-Rc-H/L, running from 101 to 955. For the final 1972 and 1973 model years, the prefix is 18V-583-F-H (home market) or 18V-583-Y-H (export), both running from 101 to 870. These engine number series were separate from their manual-gearbox counterparts, and this distinction is useful when tracing a car’s original specification.
Technical Specification
The Borg-Warner Type 35 is a three-speed unit with internal gear ratios of 2.39:1 first, 1.45:1 second, and 1:1 top, with a reverse ratio of 2.09:1. The torque converter multiplies input torque between 1:1 and 2.2:1 depending on load conditions. Gear selection was made via a sliding lever on the transmission tunnel, operating through the positions P (Park), R (Reverse), N (Neutral), D (Drive), L2 (intermediate hold), and L1 (low gear hold). The selector lever had an Edam-shaped knob with a collar-type detent sleeve that had to be lifted before R or P could be selected.
Cars fitted with the automatic also received a wider, rectangular brake pedal pad and rubber, distinguishing them from manual-gearbox cars.
Final Drive & Gearing
The overall gearing on automatic cars differed from manual-gearbox MGBs because a different rear axle ratio was fitted. The 1968 model automatic used the same 3.909:1 final drive as contemporary manual cars, giving overall ratios of 9.34:1, 5.67:1, and 3.91:1 in first, second, and top respectively. From the 1969 model year onwards the ratio was changed to 3.7:1, altering the overall ratios to 8.84:1, 5.37:1, and 3.7:1. This rear axle ratio change also means the speedometer calibration differs between 1968 and post-1968 automatic cars, a detail relevant if a speedometer or speedo drive component is being replaced.
A special gear selector illumination lamp was originally fitted on the lower facia panel above the selector quadrant, wired to come on with the instrument lights but independently switchable. This feature was discontinued after the 1971 model year, as the 1972 and 1973 model cars received a courtesy lamp as part of the revised centre console instead.