The PG1 gearbox in the MGF and MG TF carries one of three serial number designators stamped on the gearbox casing, identifying the variant: - C4BP, 4.2:1 final drive. Fitted to MGF 1800 VVC, MG TF 1800 (TF 115 through TF 135), and MG TF 160. - C6BP, 3.9:1 final drive. Fitted to MGF 1800 non-VVC and some MGF 1600 applications. - G4BP, 3.9:1 final drive. An alternative designator for 1.6-litre applications (MGF 1600, MG TF 1600).
For most external service items the serial number matters because production-change breakpoints are identified by serial number rather than by VIN. Confirm the gearbox designator and approximate serial number before ordering items that are known to have mid-production revisions, differential seals, gearcase assemblies, clutch housings, and breathers all have such revisions.
Differential Oil Seals, Left and Right, With Production Variants
The differential output shafts carry oil seals at the points where the drive shafts emerge from the gearbox casing. Two seals are fitted (one LH, one RH), and each seal has early and later production variants: - LH seal early, used on C4BP up to serial 03759 and G4BP/C6BP up to serial 09712 - LH seal later, used on C4BP from 03760 and G4BP/C6BP from 09713 - RH seal early, same serial range as LH early - RH seal later, same serial range as LH later The later LH and RH seals use different part numbers and a slightly different cross-section, which is why the distinction matters. A differential seal protector tool is catalogued separately (and covered on the Drive Shafts page) for use during drive shaft insertion to prevent the shaft splines from damaging the new seal lip. An oil leak at the differential seal end of the gearbox, visible as oil pooling on the underbody beneath the gearbox or migrating down the drive shaft gaiter, is a common high-mileage service requirement.
A weeping seal should be replaced before it produces sufficient leakage to affect gearbox oil level, because running the PG1 low on oil accelerates wear of the synchromesh rings and shaft bearings.
Gearchange Mechanism and Cables
The PG1 uses a cable-operated gearchange mechanism rather than the mechanical rod-and-linkage arrangement of older cars. Two cables carry the driver's gearstick movements to the gearbox selector: - The cross-gate cable moves the gear selector across the gearbox's internal gate (1st/2nd to 3rd/4th to 5th/reverse plane). - The selector cable moves the gear selector fore-and-aft within each gate (engaging the actual gear). The selector cable has model-specific variants, one for MGF, a different one for MG TF, reflecting the different cabin ergonomics and cable routing between the two models. The cross-gate cable is a single specification across both.
A complete gearchange mechanism assembly is catalogued for replacement, including the gearstick housing, cables, and associated linkage. Two variants exist: an earlier specification fitting MGF and early MG TF (up to VIN 2D602323) and a later specification for MG TF from VIN 2D602324 onwards. Worn gearchange cables and stretched linkage are a common cause of vague, notchy, or hard-to-engage gear selection on high-mileage cars, and because the cables are external, they are accessible without opening the gearbox. For many customers complaining of gearbox feel issues, a cable and linkage renewal is the first and often most effective intervention.
Bellcrank Lever Assembly
The bellcrank lever at the gearbox end of the cable system translates the pulling motion of the selector cable into the rotary motion required by the gearbox selector mechanism. Like the selector cable, the bellcrank is model-specific, an MGF bellcrank and an MG TF bellcrank are not interchangeable. Worn bellcrank pivot bushes and sloppy retaining hardware contribute to the same vague-gearchange complaint as worn cables; during a cable renewal, the bellcrank should be inspected and replaced if showing wear.
Reverse Light Switch and Gearbox Breather
The reverse light switch is a mechanical contact switch screwed into the gearbox casing, triggered when the selector mechanism engages reverse. Its sealing washer should be renewed alongside the switch. Reverse-light failure on the MGF and MG TF is frequently diagnosed as a bulb failure when it is actually the switch, a quick test (bridge the switch terminals with ignition on in reverse) confirms which component has failed. The gearbox breather equalises internal gearbox pressure as the gear oil heats and cools.
Two breather arrangements exist: an earlier tube-and-clamp breather fitted to C4BP up to serial 2008562 and G4BP/C6BP up to serial 2014987, and a later complete breather assembly from those serial numbers onwards. The later assembly can be retrofitted to earlier gearboxes and is the preferred replacement specification.