MGB GT Gearbox
MGB GT Gearbox
All MGB’s from 1962 to 1967 used a four-speed manual gearbox with a non synchromesh, straight cut first gear, an optional overdrive was available. This gearbox was based on that used in the MGA with some minor upgrades to cope with the additional output of the larger MGB engine. In 1968 the early gearbox was replaced by a full synchromesh unit based on the MGC gearbox.
This unit was designed to handle the 150 bhp of the 3 litre engine of the MGC and was therefore over engineered when mated with the standard MGB B Series engine. In fact, the same transmission was even used in the 3.5 litre V8 version of the MGB GT V8. An automatic three speed transmission was also offered as a factory option but proved to be fairly unpopular.
Electrically engaged overdrive gearboxes were an available option on all MGB’s. The overdrive unit was operational in third and fourth gears until 1977, when overdrive was only operational in fourth, but the overall ratio in third gear overdrive was roughly the same as fourth gear direct. The overdrive unit was engaged by a toggle switch located on the dashboard. The switch was moved to the top of the gear lever in 1977. Overdrives were fitted to less than 20% of all MGB’s, making it a very desirable feature.
There were three different types of overdrive transmissions fitted to the MGB.
1962 to 1964, 1965 to 1967
The Laycock Type D OD with external solenoid had a hole in the bell housing where the starter nose pokes through, a “Shield” shaped access cover, 1020 TPM for OD and 1040 TPM for non OD, the gearbox input shaft, flywheel and engine backing plate were changed with the advent of the 5 main bearing engine in 1965. Therefore, the transmission for a 3 main bearing engine (1962-1964) differs from its later counterpart.
1968 to 1974/5
The Laycock Type LH OD with rectangular shaped access cover had an oval clutch fork boot, a dipstick for checking oil level, a black label on the OD solenoid cover stamped “22/61972”, 1280 TPM for OD and non OD, the speedometer drive gear on the mainshaft is blue and the speedometer driven gear on the removable drive housing is white with 21 teeth.
1974/5 to 1980
The Laycock Type LH OD with rectangular shaped access cover had a square clutch fork boot, side fill plug without a dipstick, a blue label on the OD solenoid cover stamped “22/62005”, 1000 TPM for OD and non OD, the speedometer drive gear on the mainshaft was red, the speedometer driven gear on removable drive housing was red with 20 teeth and the
overdrive operates in 4th gear only in units made from February 1977 onward.