Fitting An Overdrive Gearbox

I noticed from the workshop manual exploded drawings that the Magnette gearbox and the early MGB 3-synchro gearbox are essentailly the same internally. It is only the bell housing casting (and therefore the starter and clutch release positions) that makes them different.

Further examination of the drawings showed that at the rear end, where the Magnette remote control bolts on the joint face is identical to the MGB. This revealed an opportunity. I could remove the Magnette remote control, substitute an MGB overdrive converter section and fit the overdrive itself. This would also necessitate fitting the gearbox with the longer MGB final drive shaft that extends back into the overdrive unit, as well as the oil cam that the overdrive needs to create the oil pressure that enables it to work.

My gearbox was well overdue for a rebuild and sounded like a tin of loose nuts & bolts, so I decided that if it had to come out, it may as well go back in with the overdrive attached! I acquired the appropriate MGB gearbox second-hand so as to have all the relevant parts I needed but I believe these are now getting scarce and hence expensive. A conversation with Chris Betson of Octarine Services confirmed that there should be no problem tackling the gearbox conversion, so I asked him to carry out the work. He has a wealth of experience with engine and gearbox rebuilds, is very reasonable, cost-wise, and consulted me along the way once he had the boxes apart. The idea was to use the best parts from either box, but inevitably there are parts like baulk rings and bearings where it is a false economy to re-use old parts. It is the shafts and gearsets that can be costly if you put in brand new parts, so Chris explained that some of my parts were serviceable, although not perfect and gave me the budgetary option of whether to replace them. As my car  does not generally do more than 5000 miles a year, I felt safe in letting some of the parts soldier on for a bit. It helped keep the over all cost down.

I knew from reading in the Register Modifications Manual the experience of other members that had fitted overdrives that the job brought with it some additional complications. Firstly, the MGB unit has a remote comntrol linkage that extends above the overdrive unit, so it needs more space at the top of the transmission tunnel. Metal has to be cut away. The MGB propellor shaft has a universal joint at both ends, rather than the slide joint at the gearbox end, and this, together with the extra length of the unit meant that a new prop shaft had to be made up. I managed to get a brand new MGB prop shaft at second-hand price in the Silverstone Traders Row, so once the gearbox/overdrive had been installed, a measure between the flanges gave the dimension for the shaft. The MGB shaft can be cut down to fit by a competent engineering shop, but make sure it is balanced afterwards or you will get unwanted vibration.

The new unit also requires a different support cross-member and I followed the Modifications Manual in cutting and welding an MGB part to fit the Magnette mounting holes.The modification to the bracket is illustrated in the following diagrams and is carried out as follows:

1. Cut both ends off down lines A - B, removing and discarding pieces 1.

2. Cut down at line C - D leaving the bottom surface of the bracket intact.

3. Bend piece K flat.

4. On the side opposite piece K, cut along line E - F, removing and discarding      piece J.

5. Bend at line F - G taking the bottom plate and piece K joined to it down to horizontal, leaving a gap 1/4" deep at the line C - D.

6. Fill the gap on both sides with weld.

7. Drill holes on the horizontal piece to match the mounting holes on the Magnette bracket. Check carefully their relationship to eachother and to the position of the "cradle" on which the gearbox rests.

8. Repeat the operation at the other end, ensuring both pieces K are on the same side, not diagonally opposed.


To connect the speedometer cable I had a slightly longer cable made up and I connected it using the right-angle converter as fitted to the MGB. This gives you a nice smooth cable run without tight bends. The speedometer will not read correctly but the degree of error depends on your rear axle ratio. With a satnav it is now easy to establish true speed and compare it to speed as displayed. Using these figures, you can work out the ratio and obtain a step-up or step-down box to insert in the cable. Because the Magnette speedometer is unique, I am given to understand that it cannot be recalibrated owing to lack of interchangeable parts, so a stepping box avoids the problem by adjusting the number of times the cable spins per wheel revolution. In my case, I found I needed to increase the spins at the speedo head by a ratio of 5:4. For all things speedo and cable related, go to Speedograph Richfield.

The overdrive is switched in electrically by a solenoid fitted to it. This requires a circuit controlled by the ignition switch and a separate switch to engage the overdrive. Just use the early MGB wiring diagram as an example. However, note that the early Bs also included a vacuum operated switch to ensure that the overdrive only engaged at appropriate engine speeds and loads. This was dropped in later models, so I concluded that it was not necessary. However, my local garage owner, a bit of a sage in this sort of thing,  recommended that the electrical power feed for the overdrive be taken from an ignition coil terminal. This means that when overdrive is engaged, there is a momentary drain of power from the coil that interrupts the ignition circuit and makes the engine hesitate long enough for the overdrive to engage without load. Cunning, but it seems to be working fine!

Initially, to switch in the overdrive I used a simple toggle switch mounted below the dash and it worked fine but the long-term plan was always to fit a switch on the gear-lever knob. There are advantages in being able to change gear and engage or disengage the overdrive as part of a single operation.Switched (decorative) knobs can be obtained as generic after-market parts but at a considerable cost. I therefore decided to look at using the set-up fitted in later MGBs (early Bs also used a dash-mounted curved toggle-switch but mounted conveniently close to one’s hands holding the steering wheel). Again at Silverstone, I acquired the appropriate B gear-lever complete with knob, integral switch and vinyl cuff. The complication is that the lever is not the same at the point where it sits in the remote control and engages the change rods. Whilst both levers have the same shape of lower shaft and the pivot ball is the same size, the early lever has a flat on the pivot ball, while the later one does not. I took both levers to a small machine shop and asked them to make the later one look like the early one. They did it for a reasonable price but said it was not easy because of the hardness of the metal and the difficulty of gripping the work in a lathe. Anyway, now the later lever fits and it is an easy job to connect to the concealed wires on the lever and run the wiring up behind the dash to substitute for the temporary switch. Now, particularly when changing down, say, on the approach to a rounabout, one can disengage overdrive top then change down to third in a slick operation. Similarly, accelerating up to cruising speed, one can go from third to overdrive third then overdrive top without letting go of the gear lever.

One problem that I had not foreseen was the fact that the new cross-member interferes with the normal run of the exhaust pipe, so to get the car back on the road in time for my annual holiday, I just had a section of the existing pipe cut out and a piece of flexible pipe put in. This was only a temporary measure until I arranged for Maniflow in Salisbury to make up a new two-box larger bore system mated to a standard MGA manifold. This, together with the overdrive, has transformed the car’s performance.Overdrive

  One downside to this modification is the fact that the transmission tunnels has to be cut to accommodate the MGB gearbox remote control and lever. Rather than attempt to fabricate a tunnel modification, I  acquired a tunnel top cut from an early overdrive MGB (from Andy Jennings, MG breaker in Southampton)  Initially I had it cut well oversize and then did some detailed measuring and fitting up until it is now quite a neat job. The new top is screwed to the old tunnel and sealed with mastic to help keep noise and fumes out.  The B tunnel includes a removable panel giving access to the remote control when dropping the gearbox.  The ability to unscrew the top of the tunnel to gain access to the remote control and the electrical switch fitted to it is an undoubted advantage, particularly if the switch fails in the future.

The switch itself is actuated when one engages reverse. When wiring up the overdrive one wires in this switch so that overdrive cannot be engaged accidentally when in reverse. (See early MGB wiring diagram). It would cause untold damage. However, with the use of a dual relay (one make, one break), you can use the switch not only to cut out the overdrive circuit but also to switch on the reversing light, thereby preserving an important original feature of the Magnette.

I have covered several long continental trips now as well as many on home turf and I can confirm that the benefits of the overdrive make the fitting work worth the effort. The big advantage over just fitting a higher ratio differential is that one preserves the tractability of the lower gears whilst adding two ratios at the top. Overdrive top is some 20% higher than the standard top ratio, so when cruising, the revs drop by this percentage and fuel is saved accordingly. The car feels much more relaxed too and I have no problem cruising for long distances at 65 – 70 mph.

An alternative to this modification is the fitting of the 5-Speed Ford gearbox kit supplied by Hi-Gear Engineering. when I did my conversion, the overdrive was the cheaper option but with scarcity of parts, that situation may have changed.

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