Overdrive Gearbox

MGBs built prior to 1976 had overdrive as an optional extra and many cars were sold without this option. This is surprising since as an optional extra overdrive on a 3-synchro car cost about £60.00 whereas the second-hand value of an overdrive car was about £100.00 more than the equivalent non-overdrive example. So it made economic sense to have overdrive fitted. Today an overdrive car is seen as more desirable and so retrofitting an overdrive unit can be worthwhile.

MGB Type D OverdriveThe most difficult part about this conversion is finding the necessary components. Ideally a second-hand gearbox and overdrive should be sourced but these are quite rare, especially the early 3-synchro gearbox with the Laycock Type D overdrive. A non-overdrive 3-synchro gearbox can be modified to suit but sourcing the parts will be more difficult that finding a whole gearbox. The majority of the parts in a 3-synchro gearbox are the same except that the output shaft is different and the interlocking arm is also different. The output shaft must be replaced and the new shaft must include a cam to engage the overdrive. The interlocking arm can be modified by cutting and welded the calliper in a reverse position to enable it to fit into the overdrive extension on the rear of the box and engage the selector rods. This is not a job that can be undertaken by the average DIY restorer and is best left to a skilled gearbox technician. The parts required are;

  • overdrive output shaft complete with cam and key
  • interlocking arm for the overdrive gearbox
  • the overdrive extension
  • the overdrive unit
  • the gear-lever extension complete with new gear-lever
  • overdrive inhibitor switch
  • overdrive speedometer cable
  • overdrive wiring loom extension
  • overdrive switch and plinth (for early cars)
  • overdrive prop-shaft

Once the overdrive gearbox, reconditioned if necessary, has been assembled it is a direct replacement for the non-overdrive unit. Before fitting the gearbox/overdrive use a test light and check that the overdrive inhibitor switch is working properly. The overdrive should only operated on third and fourth gear. Extra fibre washers can be used as spacers under the inhibitor switch if required. Also, check that the overdrive solenoid is functioning properly. Connect a twelve-volt power supply to the solenoid power input wire and earth the gearbox. You should hear an audible click from the solenoid as it operates. This is particularly noticeable on the early Type D overdrive. When the gearbox has been fitted into place ensure that the wiring loom over the gearbox is secure and cannot chafe. The dash mounted overdrive switch along with the plinth should be fitted into the hole already provided in the dashboard to the right of the fuel gauge. The wiring for the switch should already be included in the wiring harness behind the dash. Consult the wiring diagram in the workshop manual for the correct wiring sequence.


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