The MGC drivetrain is closely related to the contemporary MGB's, but with specific differences at almost every point that affect parts ordering. The manual gearbox is almost identical to the four-synchromesh MGB gearbox, differing only in the front cover (clutch release arm pivot relocated), main casing profile (to match the C-series rear engine plate), and gear ratios. The rear axle is a Salisbury tube-type shared with the MGB, but with different crown wheel and pinion ratios, a modified differential cage, a more substantial propshaft flange, and sturdier five-stud driving flanges. The clutch uses the same basic Borg and Beck type as the MGB but with a different release arm and gaiter specific to the MGC front cover.
Gearbox Ratio Change, The Key Production Change
The most significant drivetrain production change is the gearbox ratio modification. Early non-overdrive MGCs, up to engine numbers 29G/3200 and 29GA/1400, retained the lower MGB gear ratios, which made the car feel over-geared and sluggish in the lower gears despite the six-cylinder engine's torque advantage. From engine numbers 29G/3201 and 29GA/1401 onwards, higher ratios were introduced on non-overdrive cars, matching the ratios already used on overdrive models.
This change affected the first motion shaft, laygear, second gear, and third gear, four components within the gearbox. The speedometer pinion also changed to maintain accurate speed indication with the revised ratios.
Three Rear Axle Ratios
Three different rear axle ratios were used, changing at the same chassis number breakpoint (4235 Roadster / 4265 GT). Non-overdrive cars used 3.071:1 (14/43 teeth) initially, changing to 3.307:1 (13/43 teeth). Overdrive cars used 3.307:1 initially, changing to 3.7:1 (10/37 teeth), the numerically higher 3.7:1 ratio was a significant improvement that addressed the early car's notorious over-gearing complaint, providing notably better acceleration and hill-climbing ability at the expense of slightly higher cruising revs. Automatic cars used 3.307:1 throughout production.
Automatic Gearbox
The MGC was available with a Borg-Warner 35 automatic gearbox as a factory option, except in the USA, where the automatic was not offered. The automatic is a mildly modified version of the MGB's BW35, with differences in the main casing (incorporating oil cooler unions), torque converter housing profile, torque converter itself (redesigned for the C-series engine's torque characteristics), kickdown cable attachment, rear clutch plate assembly (higher torque specification), front servo (incorporating an exhaust valve), governor valve body, and dipstick tube.
Early automatic cars up to chassis number 1299 used a cable gear change mechanism, replaced from chassis number 1300 by the simpler MGB-type rod change system.