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MG MGB Technical - D-type Overdrive Drains

While replacing the starter on the '63 B, I was checking out the drain plug situation on the transmission.

The tranmission housing has a square lug that appears to be a drain plug, and the overdrive unit has its own drain plug.

Is the transmission drained at the O/D plug or at both? Are they both filled through the port behind the speaker panel? What is the total capacity of both?

Thanks again,
Greg Van Hook

The gearbox plug is used to drain both. It won't completely drain the OD but that is what you do for an oil change. The plugs under the OD hide things like the relief valve and pump, they are not drain plugs, but it does have a sump which is usually removed as part of an oil change to clean or replace the filter screen and the magnets. You will always get a little more out of the OD when removing the sump and other plugs even after draining the gearbox. They are both refilled from the same point - top-fill inside the cabin on chrome bumper and side fill on rubber. 3-synch OD *dry* capacity is 5 1/3rd Imperial pints, FWIW 4-synch OD *dry* capacity is 6 Imperial Pints. You won't need quite this much if just doing a drain and refill.
Paul Hunt

Thanks a lot, Paul. So helpful.

Greg Van Hook


You are right and Paul is wrong - the 3 synch D type OD DOES have a drain plug and it should be drained during an oil change as well as the gearbox drain - both refill through the gearbox.

The LH type OD fitted to 4 synch cars doesn't have a drain plug in the OD.
Chris at Octarine Services

Yes, I did subsequently notice that the D-type does have a drain, I sit corrected.
Paul Hunt

Thanks to you both, very much.

Greg Van Hook

It is best to use a 6 sided socket when undoing the drain plug on the D type OD as they are sometimes very tight. It the one that is not lock wired. Get a new gasket before removing the filter cover.
On the 4 syncro cars you are supposed to remove the filter plate & clean the filter when changing the oil.
Garth Bagnall

This thread was discussed between 26/04/2009 and 01/05/2009

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