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MG MGB Technical - overdrive help

Hi, looking for some advice on an overdrive conversion.
I got a 1972 non overdrive b and want the overdrive put in
1. Does it drop straight in with regards to prop shaft, xmember and bell housing?
2. Is the wiring already in my 72 or will I have to wire it in
3. Where should the switch be/go
4. Is it worth doing?

Thanks in advance
C Glear

I have one in an MGA and love it! Not sure how hard to retro fit to a B. overdrive boxes are complicated compared to modern 5 speeds but they do have character and are a period piece. In years to come they will be revered like pre-selecter boxes in the MG K3 racers.
Mike Ellsmore

The gearbox casing for o/d is different from that for non o/d as is the main shaft and prop length.

Your best bet is to obtain an o/d gearbox complete. The bell housing and cross member are the same but you will need the appropriate prop. Chris Betson of Octarine Services can do this I believe. Either supply & fit or perhaps just supply. Or the usual 2nd hand suspects with the usual disclaimers. Wiring may already be there; if not it is easy to add it. Switch is generally mounted to the left of the fuel gauge but could be a gearstick mounted one. It makes cruising much quieter (and economical I guess).

Worth talking to Overdrive Repair Services (Rotherham) who (re) build o/d units
Michael Beswick

Propshaft is the same length for OD and non-OD on the 4-synch that a 72 should have, it is the 3-synch that has different lengths. But the gearbox mainshaft is different between the two as said. It's possible to convert a non-OD to an OD, but not trivial, and you need to add the lockout switch. Normally you would swap the gearbox complete and that is a direct swap.

I would expect the 72 to have the wiring for the OD. White and yellow for the manual switch behind the dash at the far right, and three wires in the gearbox harness - green and green/brown for the reversing lights, plus yellow/red for the OD, this joining to the yellow from the main harness at the mass of connectors my the fusebox. The yellow/red goes to the lockout switch, then a yellow/purple from the other side of the switch to the OD solenoid. This lockout switch is absolutely essential to ensure OD can only be engaged in 3rd and 4th. If it is engaged in lower gears it can suffer damage, almost guaranteed and major if you reverse with it engaged.
Paul Hunt

Do you have a complete transmission with OD? If so, it's a simple drop in installation. The OD switch is already mounted to your turn signal light switch. RAY
rjm RAY

Ray, US spec cars had different switch gear from UK spec. UK cars had the overdrive switch on the dashboard until 1974, then moved it to the column for 1975-6, then to the gear knob after that.

Paul is right on the money with interchangeability. Though not relevant for Chris' car, for completeness I'll add that the full synchromesh units are the same length as the non-overdrive 3 synchromesh box, and also has the mounts in the same place. Furthermore, the bellhousing bolts nearly all line up - all except those for the starter and by the slave cylinder.

Chris, the simplest way of doing the switch will be on the dashboard as the hole and wiring are already there as noted by Paul. The switch you need is BHA4513.

If you want to fit the switch at the gear lever, make sure you get finely stranded wire for the last bit which turns the corner at the base of the gear lever and goes up to the knob. Standard autoelectrical wire isn't fine enough and with the flexing that happens each time you change gear it will eventually fatigue and break.

I have seen a wiring harness turned to toast as a result of a break there, thanks to the current being fed from the unfused ignition circuit. So a further point is to include a line fuse!
Paul Walbran

This thread was discussed on 27/09/2013

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