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MG MGB Technical - Swapping Transimissions

I have a 72B 4 spd with a working OD.

I test drove a 77B 4 spd without OD.

1st gear syncro appears bad in 77B.

A friend has suggested I'd prefer the 77B with a solid trans w/ OD and suggested if I buy the 77B we could "simply" pull engine and trans from both cars and swap transmissions, putting the bad syncro trans from 77B into 72B.

To me this is a lot of effort, and a "simply" solution.

BUT - even if I 1) were convinced to buy 77B, and 2) felt the urge to swap tranmissions; is this as "simple" as friend implies?

Will the 72B trans w/OD bolt right up to the 77B and visa versa? Is this just a matter of exchanging or will there be all sorts of other issues, like OD switch not being on post on 77B, needing to wire in a switch on 77B for OD, etc.?

Your thoughts.

Yes, just continuing to drive my 72B is a much "simpler" and cheaper solution, but the 77B is so clean, quiet and smooth - with all that nice new paint, and new interior..... I feel drawn to conflicts !!!! :)

Thanks.

BobA
R.W Anderson

Yes, It is a straight swap. The 2 transmissions are the same length. The od switching is the only thing that is different. Your car has the control on the steering column while the '77 had it on the shift lever. But this is minor since you already have the switch built in. You could use a later shift lever with the built in switch or locate a switch wherever you desire. Good luck. RAY
RAY

Are the speedos the same for a OD and non OD car? I believe no with the early ODs but for the later ones they are? I have a couple of old OD boxes to rebuild into one good one and will need to do the swap myself.

Simon
Simon Jansen

Bob. As Ray notes, they are a straight, bolt on swap. I did this with a 79, using the older style (dip stick) four syncro, overdrive transmission. One thing you will want to ensure is that you can get to the rubber plug, behind the radio console, which you will have to remove to check the level of oil in the early type transmission. Not a big problem since it does not have to be done often.

Regarding Simon's question, yes, the speedometers had the same turns per mile rating for all of the four syncro cars. I do seem to remember that the cars with factory overdrive did use a different specification number than the cars not equipped with overdrive at the factory. But, I have not noticed any problems in using the non-OD speedos with an overdrive, nor do I know of anyone who has.

Les
Les Bengtson

BOB,

An aspect to consider: Do you like the rubber bumper car shape?

As many prefer chrome bumper model.

Jean G.
Jean Guy Catford

Bob, the speedo heads can be different and with an MGB, if they can they will be!..
You can tell the speedo revs per mile from the 4 figure number beneath the Units digit window in the odometer. The standard non O/D box is about 1280 revs per mile and 1270 for the O/D box revs per mile. R/B machines with the smaller Speedos could have either 1000 or 1270 rev speedos. It all depends on the speedo gear and pinion. These are colour coded dependant on ratio. The good news is that they are interchangeable between O/D units.

I never could understand why the speedo heads do not all have the same ratio as all the rear axles ( except auto boxes ) are the same.

Worst job on the B is changing the O/D inhibit switch in situe. Some replacement switches are too large and do not have a chamfer on the body . These just will not go in. But it is possible to take the old one apart, clean out the oil and burnish the contacts. Replace the innards and restake the body.
AG Martin

On 4-synch cars the OD and non-OD speedos are the same (it was 3-synch cars that were different), but they differ quite a bit from chrome bumper to rubber - 1280tpm on chrome and 1000tpm on rubber. These mate up to the black label and blue label ODs respectively. The wrong combination gives a very significant error in speed and odometer readings.

Up to Sept 77 the manual switch was on the column stalk, after that it was on the gear lever. If the 77 is a 'column stalk' car then on the face of it it is just a straightforward re-connection. I won't go in to details but in 76 and 77 there were a couple of changes in wiring of the OD, but if you use the 72 gearbox harness instead of the 77 you should be fine. But there can be other complications as well, some 72s had a seat-belt warning system and a switch that closed in neutral on the gearbox, and some late 76/early 77s had an extra microswitch on the gearbox that closed in 2nd and 4th!
Paul Hunt

I did notice in the Moss catalogue on the OD page there are two speedo pinion gears in the OD. One says it is white with 21 teeth (CB 67-74) and the other is red with 20 teeth (RB 74 onwards).

I haven't started pulling my two ODs apart yet so I have no idea what I have!

Simon
Simon Jansen

That is part of the difference between the CB and RB ODs, the worm gear on the output shaft is also different at 8 starts on the chrome and 6 on the rubber.

To complicate matters non-OD gearboxes have different number of teeth *and* starts, but the combination gave the same result hence the same speedo as the OD cars.

Just look at the 'Laycock' labels on the ODs and go by that.
Paul Hunt

This thread was discussed between 30/06/2009 and 03/07/2009

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