That really wouldn't be too tough to live with. Overall, you're looking at a self induced error of about 2% if you don't swap speedometers. It is 1020 TPM for OD and 1040 TPM for non-OD. 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. From 1974-1/2 through 1980 (all 'rubber bumper' cars) it was changed to 1000 TPM. All USA spec MGB's from 1968 through 1974 used 1280 TPM transmissions and speedometers. 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. This refers to the number of times the speedometer cable will turn for every mile traveled. Occasionally the change was visually dramatic (changing the gauge face and the size of the case), other times it was simply the calibration вЂ“ what's commonly referred to as 'TPM' or turns per mile.

OD operates in 4th gear only in units made from February 1977 onwardДЄlong the way the factory not only changed the transmissions, they also changed the speedometers.

Speedometer driven gear (on removable drive housing) is red with 20 teeth.

Speedometer drive gear (on the mainshaft) is red.

Blue label on the OD solenoid cover stamped '22/62005'.

Speedometer driven gear (on removable drive housing) is white with 21 teeth.

Speedometer drive gear (on the mainshaft) is blue.

Black label on the OD solenoid cover stamped '22/61972'.

The spigot bush in the crankshaft must be changed as well. 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. Therefore, the transmission for a 3 main bearing engine (1962-1964) will be different than it's later counterpart. The gearbox input shaft, flywheel and engine backing plate were changed with the advent of the 5 main bearing engine in 1965.

1020 TPM for OD and 1040 TPM for non-OD.

A hole in the bell housing where the starter nose pokes through.

Laycock Type D OD (note external solenoid).

It will be simplest though if we treat the non-synchro first gear transmissions separately from its later cousin, the all synchro unit: Non-Synchronized 1st Gear TransmissionДЎ962 through 19 through 1967 (chrome bumper) Identification is fairly easy using the information provided. 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).

After all, the MGB had an effective life span of 18 years and had numerous design changes during production. There exists a fair bit of confusion as to what parts to use. Time has taken its toll on MGBs the net result is there are a lot of spare, used components available вЂ“ including OD transmissions.

Unfortunately, it was not that common an option as installed by the factory. It allows the engine to turn fewer revs per mile, meaning less noise, heat, fuel, wear and tear.

Selecting the proper one from the start will likely be the most cost-effective choice in the long run!ДЄn OD gearbox is one of the most endearing and sought after options supplied on our beloved MGBs. 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. Jan 11 Finally, Off The Rotisserie An.Jan 14 Gearbox & Overdrive Rebuild (7.Jan 23 Voltage Regulator With Alternator.Jan 26 Saving An OD From The Scrap Heap.

Laycock overdrive clutch sticks