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MG MGB Technical - Early Gearbox differences

As listed on another thread, the non overdrive gearbox on my early 64B is on its way out.
Was thinking originally of fitting a Ford 5 speed , but now maybe i should keep it original and go overdrive.
QUESTION is the 3 speed syncro gearbox different on a 5 Bearing engine to a 3 Bearing engine. As my engine is a 18G do i have to find a really early box to fit it, or can i fit a 5 bearing 3 syncro overdrive box onto an early engine.
And will i need an overdrive prop as well, is that different .

Thanks Roy
R Mcknight

Roy

The 5 bearing will fit onto your engine but you will have to change the first motion shaft as yours is smaller than the 5 bearing version.
The prop is just an overdrive later shaft from a 4 synchro car.
yuours is the rare one as it was only made for that model so dont throw it.

Ste
Ste Brown

A friend of mine, who also owns a '64B, converted his car to a 4 synchro/od unit. The later 3 synchro transmissions have the same input shaft diameter as the 4 synchro boxes. When he installed the trans in his car, he drilled out the crankshaft, so that he could use the larger input shaft. As mentioned, a longer driveshaft is required when doing the conversion on a 3 synchro trans with OD from a 3 synchro trans without OD. RAY
rjm RAY

If the first motion shaft and gear in you present gearbox is in good condition, it could probably be swapped into the e syncro gearbox with OD. The first motion shaft is the only difference between the very early 3 syncro boxes for the 3 main bearing MGBs and the later boxes for the 5 man bearing cars through 67. Cheers - Dave
David DuBois

When I first put a 3 synchro gearbox with OD in my car (1963) with the 18GA engine (3 bearing) I removed the first motion shaft and had it machined down to the same OD as the original gearbox.
It is not hard to remove the first motion shaft and replace it without dismantling the gearbox completely if you keep the gearbox vertical when you do it.
It was a very long time ago that I did this, but I did not have a problem as I recall.
Ian Buckley

If you can locate a '65 to '67 3 synchro/od trans, it would be wise to purchase it. Besides the input shaft being larger, the layshaft was increased in diameter due to early failures associated with the early layshafts having too much friction. The layshaft bearings were also increased in size accordingly. The laygear, itself, was also bored out to accept these upgrades. RAY
rjm RAY

This thread was discussed between 07/03/2011 and 08/03/2011

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