The Laycock de Normanville D-type overdrive was the unit fitted to the MGB three-synchromesh gearbox from early 1963 through to the end of three-synchro production in late 1967. It covers cars with 18G, 18GA, and 18GB engine series and gives a reduction ratio of 0.802:1, bringing the overall top gear ratio down from 3.909:1 to 3.135:1 against the standard MGB final drive. The D-type operates on both third and top gears and is electrically activated, on these early cars via a switch originally mounted at the end of the driver’s-side facia, later changed to the distinctive shepherd’s crook stalk from April 1965. Cars that were originally factory-fitted with the D-type overdrive can be identified by the letter R in their engine number prefix, for example 18GRU-H rather than 18G-U-H.
The overdrive housing occupies the rear half of the gearbox rear extension, and the gear lever on overdrive cars has a single bend at the bottom in contrast to the double-bend lever of non-overdrive three-synchro cars.
How the D-Type Works
The D-type is an epicyclic unit driven hydraulically, with an integral oil pump driven off the gearbox mainshaft via a cam. The pump supplies oil pressure to operate the pistons that engage the sliding member, which locks the annulus and allows direct drive through the unit, or, when the solenoid is energised, releases to engage overdrive through the sun wheel and planet carrier assembly. A unidirectional clutch within the unit provides a freewheel function in direct drive. The solenoid is the component most likely to require attention in service and is available as an individual item.
An important detail when sourcing cam and Woodruff key parts concerns the very earliest D-type overdrive gearbox mainshafts: these used a Woodruff key to drive the oil pump cam, while later units used a different cam design without a key. This distinction is relevant when ordering cam-related components, as the key-driven and later designs are not interchangeable. The MGOC Spares range includes reconditioned D-type overdrive assemblies on a one-for-one exchange basis, as well as a reconditioned sliding member available on an exchange basis with surcharge. Individual components available include the solenoid assembly, oil pump assembly, non-return valve, relief valve, drain plug, filter, filter cover gasket, magnet set, operating valve, cam, Woodruff key, bridge bars, pistons and O-rings, sun wheel bearing and spring ring, sliding member circlip, thrust washer, annulus and annulus bearings, annulus spacer, rear oil seal, speedometer pinion and housing, right-angle speedo drive adaptor, overdrive-to-gearbox gasket and studs, and propshaft bolts.