A limited slip differential is one of the most worthwhile drivetrain upgrades available to the MGB owner who wants to make the most of the car's handling. With a standard open differential, drive is sent to the wheel with the least grip, so when one rear wheel loses traction, under hard acceleration, when cornering quickly, or on a slippery surface, power is lost to that spinning wheel and forward drive suffers. A limited slip differential addresses this by transferring drive across to the wheel that still has grip, putting the power down more effectively and giving noticeably more composed, predictable handling when driving with enthusiasm.
A limited slip differential was originally offered for the MGB as part of the factory competition equipment, in the form of a ZF unit listed for both axle types. Today the established choice is the Quaife automatic torque biasing differential, a gear-based unit that operates progressively and smoothly without the maintenance demands and wear characteristics of older plate-type designs.
Choosing the Correct Unit
The most important consideration when ordering is the type of rear axle fitted to your car, as the two MGB axles are not interchangeable and each requires a different differential. The earliest cars used the banjo axle, identifiable by its bolt-on differential nosepiece at the front of the casing, and this type was fitted to roadsters from the start of production in 1962 through to 1965, with intermittent fitment continuing into 1965 to 1966.
The later tube axle, recognisable by its central differential housing with a removable rear cover, was introduced on the GT from 1965 and progressively across the roadster range, with the changeover completed on all roadsters by 1967. If you are unsure which axle your car has, the front-mounted nosepiece of the banjo type versus the rear cover of the tube type is the simplest visual identifier. Quaife units are available to suit each axle, so it is essential to confirm your axle type before ordering.
Fitting Considerations
Fitting a limited slip differential is a specialist task that involves dismantling the differential and setting crown wheel and pinion backlash and pinion depth using the correct shims and distance pieces. Because of the precision required to achieve the correct mesh and bearing preload, this work is best entrusted to someone experienced in axle rebuilding. For many owners it can be more practical to have the unit fitted to a differential carrier on the bench, or to consider it alongside any planned axle overhaul rather than as a standalone job.
Lubrication
A limited slip differential has different lubrication requirements to a standard open differential. Standard EP gear oil does not contain the friction modifier additives needed for correct operation of a limited slip unit, and using it can cause noise and inconsistent behaviour. A friction-modified limited slip gear oil should always be used; the correct specification fills the axle in the normal way and ensures the differential operates smoothly and quietly throughout its working life.