The internal gearbox components are the individual parts within the four-speed gearbox casing, the shafts, gears, synchromesh assemblies, bearings, selectors, and seals. These are specialist rebuild components that benefit from experience with the Y-type gearbox design, but all individual parts are available for the owner or workshop undertaking a full or partial gearbox overhaul.
Shaft Assemblies
The gearbox uses three main shafts: the first motion shaft (the input shaft from the clutch), the mainshaft (the output shaft to the propshaft), and the laygear (providing the intermediate gear ratios). At the junction between the first motion shaft and the mainshaft, a set of eighteen needle rollers supports the relative rotation of the two shafts in the same bore.
The first motion shaft carries a front bearing with a circlip, lock washer, and retaining nut, along with a guard and an oil seal. The mainshaft carries the individual speed gears on their own needle roller sets, 28 needles for the second speed gear and 32 needles for the third speed gear, with synchromesh sliding hubs between them for gear engagement. The laygear runs on 28 needle rollers on a separate layshaft, with thrust washers and plates front and rear to locate it precisely within the casing.
Synchromesh Components
Synchromesh is provided on second, third, and top gears through sliding hub assemblies with springs and balls. The third-fourth speed hub uses six springs and seven balls; the first-second speed hub uses six springs and six balls, with different spring specifications between the two hubs. First gear is not synchromesh, first gear engagement requires the car to be stationary, or a driver comfortable with double-declutching when changing down to first on the move. The synchromesh components are the most common wear items within the gearbox, and worn synchromesh manifests as crunching on gear engagement, particularly when the gearbox oil is cold.
Selector Mechanism
The internal selector mechanism comprises three selector shafts (first-second, third-fourth, and reverse), their corresponding forks, selectors, springs, and balls. The arrangement that prevents two gears being engaged simultaneously relies on eight lock bolts through the selectors and forks, providing a positive interlock between the shafts. The reverse gear is engaged through a separate sub-mechanism: an idler gear with integral bush runs on its own shaft, driven by an upper and lower fork pair, operated via a link and fulcrum pin. A plunger, spring, and ball complete the detent for reverse selection.
Bearings and Seals
The mainshaft is supported by a centre bearing and a rear bearing, each with its own guard and circlip. The centre bearing also uses a spring plate for bearing retention. The rear oil seal is a felt type, the original specification for this era, simple and serviceable although prone to weeping as the felt hardens with age. The drive flange bolts to the rear of the mainshaft, retained by a washer, castellated nut, and split pin, this is the component that connects to the front yoke of the propshaft. All bearing, circlip, guard, and seal components are available as individual items for partial rebuilds.
Distance Pieces and Collars
Distance pieces, thrust washers, bearing collars, and spacers throughout the gearbox set the correct running clearances and axial positioning of the gears on their shafts. A distance piece locates the third-fourth sleeve assembly on the mainshaft, and collars position the second and third speed gears relative to their synchromesh hubs. These small-dimension components are easily damaged during disassembly, bent thrust washer plates or scored bearing collars should be renewed at overhaul rather than reused.
Ordering Considerations
Most internal gearbox components are common across all TD and TF gearbox specifications, the internal gear train, synchromesh, selectors, and bearings do not vary between the pre-9408 (7¼-inch clutch) and post-9408 (8-inch clutch) boxes, nor between the pre-16482 and post-16482 boxes with their different selector shaft support cap arrangements. When ordering for a partial rebuild, individual component-level identification is more important than gearbox variant identification, rollers, bearings, synchromesh components, selector shafts, and forks are all specified by the gearbox position they occupy rather than by the car's engine number.