by Tom Sotomayor February, 2003
Les Bengtson's Classic MG Pages: Articles:
This article is not intended as a “how to” installation guide, but rather to assist in selecting the correct model overdrive transmission (or OD for short) for any given year MGB. Selecting the proper one from the start will likely be the most cost-effective choice in the long run!
An OD gearbox is one of the most endearing and sought after options supplied on our beloved MGBs. It allows the engine to turn fewer revs per mile, meaning less noise, heat, fuel, wear and tear. Most operate on 3rd and 4th gear. Unfortunately, it was not that common an option as installed by the factory. Time has taken its toll on MGBs; the net result is there are a lot of spare, used components available – including OD transmissions.
There exists a fair bit of confusion as to what parts to use. After all, the MGB had an effective life span of 18 years and had numerous design changes during production. What concerns us here, are the basic grouping of the four different transmissions used (two are non-synchro first gear, two are all synchro transmissions). Identification is fairly easy using the information provided. It will be simplest though if we treat the non-synchro first gear transmissions separately from its later cousin, the all synchro unit:
1962 through 1964 and 1965 through 1967 (chrome bumper)
The gearbox input shaft, flywheel and engine backing plate were changed with the advent of the 5 main bearing engine in 1965. Therefore, the transmission for a 3 main bearing engine (1962-1964) will be different than it’s later counterpart. To swap a 3 main bearing car to the later (5 main) 3-synchro transmission or vice versa, requires changing all of the above mentioned parts. The spigot bush in the crankshaft must be changed as well. The 3 main bearing used a .62 inch ID, and the 5 main used a .85 inch ID.
1968 through 1974 (chrome bumper)
1974-½ through 1980 (rubber bumper)
Along the way the factory not only changed the transmissions, they also changed the speedometers. Occasionally the change was visually dramatic (changing the gauge face and the size of the case), other times it was simply the calibration – what’s commonly referred to as “TPM” or turns per mile. This refers to the number of times the speedometer cable will turn for every mile traveled.
With the exception of the non-synchro first gear transmissions, there is no functional difference between a speedometer for an OD equipped car and a non-OD one. All USA spec MGB's from 1968 through 1974 used 1280 TPM transmissions and speedometers. From 1974-1/2 through 1980 (all “rubber bumper” cars) it was changed to 1000 TPM. A lot of confusion exists since for the non-synchro first transmissions there is a numerical difference from the OD to non-OD speedometer output. It is 1020 TPM for OD and 1040 TPM for non-OD. Overall, you’re looking at a self induced error of about 2% if you don't swap speedometers. That really wouldn't be too tough to live with.
The serial numbers are different between OD and non-OD speedometers for all synchro cars of any given year, but the TPM remain the same. A non-OD speedometer can be used for an OD application; the end result is the same. The serial number followed by the calibration number can be easily found on the gauge face at roughly the 4:00 position. Also quite a few speedometer 'heads' (where the cable attaches) have the TPM etched into it on the threaded barrel. It's usually fully visible once the works are removed from the case. Fortunately, you don't normally have to get this detailed, or swap speedometers unless you've changed the series of transmission you're using.
1. From 62-67 the OD driveshaft was different from the non-OD driveshaft and differed depending on the type of axle used. You’ll want to make sure to get this from whoever is supplying the transmission. The all synchro gearbox used the same driveshaft regardless if an OD was fitted or not. See chart below for applications:
| Driveshaft Length | Std non synchro 1st | OD non synchro 1st | Std all synchro | OD all synchro |
| 30 inches | banjo axle | |||
| 31.125 inches | tube axle | banjo axle | tube axle | tube axle |
| 32 inches | tube axle |
2. Overdrive speedometer cables are longer than the standard unit.
3. OD switch
Let’s look at some possible “mismatch” scenarios and see what can be done to fix them:
Mixing early gearboxes with later cars has been intentionally avoided in this analysis for several reasons. They are not as plentiful or robust and they can be quite costly. Best to swap it, or sell it to someone with an early car and get the proper unit for yours.
Hopefully this has answered some of your questions and will enable you to select the proper OD gearbox. Happy motoring!
This monograph may be reproduced only for non-commercial use without other permission of the author. Reproduction for commercial use only by written permission.
Copyright © 2003 by Tom Sotomayor
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