3 or 4 Synchro
From the launch, the MGB was fitted with a 3-synchro 4spd gearbox. Overdrive was an optional extra. Later, a 4 synchro box was fitted and again, o/d was an option. On the later 1977 onwards UK cars, o/d became standard fit. An automatic option was available for a while.
A few points worth noting is that the 3 synchro box is smaller than the 4 synchro in terms of transmission tunnel size which leads to a few problems such as differing carpet sizes, but more importantly, the ability to swap boxes between cars. It is possible to fit a later 4 synchro box into the transmission tunnel area of the earlier cars.
With regard to what oil to use, the factory recommended EP90 gear oil where no o/d was fitted or later with the 4 synchro, engine oil of the same grade as the engine at 20W50 was recommended. The reason for this is the fact the o/d utilises a cone clutch system and any fancy additive formulations designed to reduce friction will impair the performance of the cone system, such as found in EP90 oils, and other modern synthetic oils.
A number of people have fitted 5 spd gearboxes from other donor vehicles such as Datsuns, Toyotas and Fords. The Ford Type 9 is the most common here in the UK whereas in the US and Australia the Japanese boxes are more readily available.
There are various Ford kits available for the B, namely from:
For the Datsun conversion:
Looking at the various archives of the longer established BBS, there was a company in the US called Novus that offered a kit for the Toyota gearbox, but I can find no links on the internet.
Why a 5-Speed Gearbox?
This comment by Roger Parker written in 1999 is of interest and clarifies some of the potential benefits to be had from doing a conversion:
"Having done quite a number of different MG conversions I have experience with a few different gearboxes in the MGB. I do not have experience of the main 5 speeds candidates in all parts of the world, but the basics hold true anywhere.
The main factor here is the simple fact that if your car is without overdrive and as such you have a massive void. When I first bought my MGB in 1973 it was a standard 1968 roadster with just a heater and wire wheels as factory fitted options. I found that as soon as you started to travel longer distances on fast open highways the standard 4th gear was woefully short. Once I had experienced an overdrive car on the same roads there simply wasn't any contest, I had to fit an overdrive box.
This I did in early 1974 and the change was such that I still clearly remember the huge positive difference it made. The character was totally changed, becoming relaxed and calm on the open roads, yet with the addition of overdrive on 3rd gear I now had a performance option. This was when your sitting in a line of traffic waiting for an opportunity to overtake. Hold 3rd gear with overdrive engaged, wait for the gap, flick out of OD, with a far faster change than any manual operation, accelerate past and your gone! Then when cruising, drop into OD 4th and benefit from the 22% raising of the gearing to reduce noise, wear and fuel consumption.
I actually fitted a complete (post 1968 type LH OD) gearbox assembly with the OD fitted. This was a simple change and would apply to your car in exactly the same way, no other changes needed in the transmission side and just some simple wiring.
In the UK the common 5 speed box to use is one from a Ford Sierra. This involves a degree of extra modification that should be supplied with the kit of parts. There are several suppliers of this conversion. Of note is the fact that 5th gear ratio is virtually identical to the OD 4th of the standard MGB box. This in my opinion is ideal as 22 mph per 1000 rpm is high enough to give the economy and quietness, yet the engine still has enough torque to pull the ratio.
One advantage of this 5 speed conversion is the better spaced intermediate ratios. MGB gearboxes displayed their history from the 3 synchro era, where with a 'crash' (unsynchronised) 1st gear, second was always artificially lowered to enable the car to be driven easily from very low speeds. This left a short second and high third and an obvious hole in between. The Later 4 synchro MGB boxes all display this flaw. Modern 5 speeds are also quieter in use, and often more compact in dimension.
Also on the plus side for the 5 speed is the fact that it is newer, was made in huge numbers, has proven reliability and spares will be around for a very long time to come. Also the cost of one is dirt cheap since so many cars are in breakers yards. MGB boxes are no where near as easy to find with OD as they once were, and are often not cheap.
On the downside is the fact that 5 speeds are conversions and will use one off parts to complete the conversion. Clutches may be standard, but from what car? All of these oddities have to be considered in the light of what would you do in the event of something breaking. Ultimately a 5 speed is a conversion away from standard and this may well affect the cars value. This could be either way depending on the viewpoint of the prospective purchaser. However if your considering the conversion then this is for your enjoyment and use, not a future owner.
Speaking from a personal point of view I really enjoy using a 5 speed in the MGB, and have done since 1984. I have even more pleasure from driving 5 speeds in Midgets and MGAs. The MGB gives less satisfaction as it was available with OD, which gives much that the 5th gear does.
When considering any other alternative 5 speed use all the mentioned areas (and any others that crop up) to provide a complete measure for comparison before you make a decision. Ideally some comments from owners having already gone down the 5 speed route with the boxes you're considering would be of considerable use."
The Other 5 Speed Gearbox from Rover - LT77
The LT77 has become the box of choice when fitting a V8 engine into a B, but can also be considered in a few other cases, such as the O/M/T series engines. But will it fit the B?
The SD1 gearbox or it's official title of LT77, was the standard BL/Austin Rover/Rover rear drive and 4X4 transmission from early 1980's, although it first appeared in the 1976 SD1 3500. The box was replaced in 1995 by the R380 series which will interchange.
The SD1 box can be adapted to fit the B series engine by using standard production UK parts. The SD1 box became standard fit for Sherpa vans before the B series diesel was dropped. This left just over a 12 month period where you could buy a Sherpa with a 5 speed SD1 box. As the Diesel parts will bolt straight onto the petrol block here is a standard parts route to SD1 B series MGB.
The prop still has to be a one off as the box is shorter and so the prop has to be longer, usually about 6" longer. It also has to match the larger size prop flange at the gearbox end to the smaller prop flange size at the axle end. Fortunately Hardy Spicer, the original manufacturer specialises in one off shafts so can supply a purpose made shaft to this and many other specs for little money, as can various other V8 Conversion specialists
Speedo cable has to be altered at the gearbox end, but the o/d cable length is right.
Apart from the obvious difficulties in obtaining the parts from such a Sherpa van, the Ford box is smaller and probably better suited to the B Series, whereas the LT77 is better suited to more powerful engines such as the V8