The Laycock LH-type overdrive was introduced with the all-synchromesh four-speed gearbox at the start of Mark II production in late 1967 and continued in use through to the end of MGB production in 1980. It was also fitted to the MGC and the MGB GT V8, making it a genuinely shared unit across all three cars. The LH-type superseded the earlier D-type unit and shares no parts with it, being a completely different design, though both are epicyclic overdrives operating on the same hydraulic principle.
Operation & Ratios
The LH-type has a reduction ratio of 0.82:1, reducing propshaft speed by approximately 22% relative to engine speed and effectively providing a higher cruising ratio that significantly reduces engine rpm at speed. Engagement is via an electrical solenoid that operates a hydraulic valve, directing pressure generated by an internal oil pump to engage a cone clutch that locks the planetary gear set into overdrive. On four-cylinder cars it operates on third and top gears. It was optional on the MGC, operating on third and top.
On the MGB GT V8 it was fitted as standard and originally operated on both third and top, though from gearbox 1404 this was revised to top gear only for the majority of V8 production, while RHD home-market cars continued to operate on both third and top to the end of production. The driver controls engagement via a column-stalk switch, with inhibitor switches in the gearbox preventing engagement in first, second, or reverse, and faulty inhibitor switches are a common cause of non-engagement and should be the first item checked.
Chrome vs Rubber Bumper Variants
Two versions of the LH-type unit are distinguished by their Laycock identification plate colour, reflecting a change made at the start of rubber bumper production in 1974. Chrome bumper cars from 1968 to 1974 use the unit identifiable by a black Laycock label and an 8-tooth (blue) speedometer drive gear.
Rubber bumper cars from 1975 onwards use the unit with a blue Laycock label and a 6-tooth (red) speedometer drive gear. These differ internally in their speedo drive gearing as a direct result of the different speedo cable calibration used on rubber bumper cars, and the reconditioned units are listed separately, for chrome bumper (1280 TPM) and for rubber bumper (1000 TPM), both available on exchange. There is also a distinction on the earliest four-synchro applications, where very early gearboxes used a Woodruff key to drive the oil pump cam on the mainshaft while later specification gearboxes used a circlip-retained cam without a key, both available individually, and the correct one must be identified before ordering cam-related components.
Components & MGC Differences
A late-specification ball-located oil pump cam is available, along with oil pump assemblies, non-return valve balls, O-rings, oil pressure control valve, solenoid valve assembly, solenoid seal kit, solenoid cover gasket, planet carrier and bearing, unidirectional clutch spring ring, oil thrower, thrust washer, sun wheel and bearing, clutch release and engagement springs, sliding member and circlip, annulus and bearings, rear oil seal, sump, sump filter and gasket, overdrive-to-gearbox gasket and studs, bridge bars, pistons and O-rings, shims, fit kit, selector shaft O-ring, and all associated fixings. Complete gasket and seal sets and reconditioned sliding members are available on exchange. The only MGC-specific internal differences are the clutch engagement spring and the relief valve, which differ to handle the higher torque output of the C-series engine, and as neither is available as a new part, serviceable originals should be retained during any overhaul. Speedo drive gears for both chrome bumper and rubber bumper variants, along with the speedo pinion housing, O-ring, lock plate, and oil seal, are all available.