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MG MGB Technical - Overdrive

What is the difference between the early black label, and the later, blue label overdrives?

Are they interchangable?

Thanks

Colin
Colin Parkinson

They are interchangeable, but the speedometer drives differ in that the early ones had 1280 tpm while the later had 1000 tpm. Also, the later Blue Label OD units only allowed OD to be engaged in 4th gear. There were some minor internal differences, the most important being a large snap ring that held the one way clutch in place inside the Black Label units. Why this was deleted in the Blue Label units is anybody's guess. RAY
rjm RAY

On UK models, overdrive was always on 3rd and 4th, except for later V8s, I believe.
Dave O'Neill2

As far as which gear they operate in surely that is down to the inhibitor switches and the wiring?

I am not too bothered about speedo accuracy, but is 1 better / more desirable than the other?

Put it this way if I had a choice which would I go for and why?

Colin
Colin Parkinson

North American ODs changed from being available in 3rd and 4th to 4th only, when they got the Transmission Controlled Spark Advance system that worked in 4th only, and they used the same switch for both.

UK 4-cylinders had OD in 3rd and 4th for the whole of production as Dave says.

V8s were initially 3rd and 4th but they suffered damage from high torque reversals in 3rd, and there was a change to 4th only. It seems the change was made retrospectively, as hardly any have been found with it in 3rd, even when they were supplied before the recorded change date.

The difference between 1000 tpm and 1280 tpm is very significant, personally I wouldn't want that amount of error regardless of what direction it was in. If you have the choice you should go for the one that matches your speedo, for that reason. For operational purposes there is no other significant difference. In the UK the speedos reduced in size at the same time, with the start of rubber bumper production, so fitting a rubber bumper speedo to a chrome bumper car or vice-versa is not really an option. However Mk1 cars had 1020 and 1040 speedos and these *are* an option for a CB car with a blue-label OD as the difference is minimal. For an RB car with a black label you might need to obtain a North American speedo, as they were smaller from 67 to 76, ironically then going back to larger. Or get your speedo recalibrated.
PaulH Solihull

Thanks Paul, very explicit! As the car is a chrome bumper car, a black label overdrive would be the first choice.

Luckily I have 1 of those!!

Colin
Colin Parkinson

This thread was discussed between 01/03/2013 and 02/03/2013

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