The MGB uses a Borg & Beck single dry plate diaphragm spring clutch, 8 inches in diameter on all four-cylinder models, with hydraulic actuation via a Lockheed master cylinder and slave cylinder. While the fundamental layout remained unchanged from 1962 to 1980, there are component differences across the production run that affect parts selection, particularly in the hydraulic system and release mechanism.
Clutch Kits & Release Bearing
The clutch kit, comprising clutch plate, cover, and release bearing, fits all MGB models and is the standard starting point for a clutch replacement. A performance version, manufactured by Borg & Beck, the original factory supplier, is available for road and track use and features a higher clamp load and uprated friction material to handle increased torque loads.
The original carbon block release bearing is prone to wearing out, and occasionally breaking, before the clutch itself, which is a frustrating result given the gearbox removal required to replace it. The roller release bearing kit is a direct-fit improvement that offers superior reliability and is strongly recommended regardless of whether the car is used for road or competition.
When replacing the clutch, the release bearing should always be renewed at the same time.
Release Arm & Gaiter
The release arm differs between three-synchromesh and four-synchromesh gearbox cars. The release arm gaiter also changed, chrome bumper cars use an oval section gaiter, while rubber bumper cars use a rectangular section type. The release arm pivot bush and bolt should be inspected during any clutch replacement and renewed where worn, as play in the pivot produces a spongy clutch feel and uneven release bearing wear.
Clutch Hydraulics
The clutch master cylinder is mounted on the pedal box alongside the brake master cylinder, with its own integral fluid reservoir. The master cylinder changed during production, the original type was revised at the start of Mark II production in late 1967 and revised again around 1973.
The later type can be identified by two concentric rings around the cylinder barrel and is interchangeable with the earlier type. The slave cylinder is mounted on the right-hand side of the clutch housing, connected to the master cylinder by a copper pipe and flexible hose. Stainless steel braided hoses are available as an upgrade over the standard rubber type. Seal kits are available for both master and slave cylinders.
Alignment Tools
Two clutch alignment tools are available, one for 18G and 18GA engines (1962 to 1964) and one for 18GB, GD, GG, and 18V engines from 1965 onwards, reflecting the different first motion shaft spigot diameters between three-main-bearing and five-main-bearing engines. The correct tool must be used to match the gearbox fitted to the car, not simply the engine, as components may have been changed during a previous rebuild. The MGOC Spares range covers clutch kits in original and performance specifications, roller release bearing kits, release arms, gaiters, pivot bushes, master and slave cylinders, seal kits, copper clutch pipes, flexible hoses, alignment tools, and all associated hydraulic fittings for every MGB application across the full production run.