Replacing a 3-speed Gearbox with a 4-speed

My RM saloon had a three speed, crash gearbox which was also ‘back to front’ in that the lowest gears were nearest the driver and the higher gears nearest the passenger. This is a little confusing when you first get to grips with it but soon becomes second nature except for the ‘odd moment’ when noise emanates from the box when trying to select first instead of top. The other problem, if that isn’t too harsh a description, is that on steep hills the passenger has to be prepared to leap out to reduce the weight. Bottom gear on a 3-speed box is not as low as bottom on a 4-speed box!

I thought I would if I could fit one of my spare 4-speed boxes without making major irrevocable changes to the car. Traditionally, when a 4-speed box replaces a 3-speed the starter motor has to be replaced by a ‘Bacon Slicer’ starter but as the engine has to be moved 3/4 “ further forward in the chassis to accommodate the slightly longer 4-speed box the bulkhead has to be modified to allow the ‘Bacon Slicer’ starter to be fitted.



The first task was to see if I could retain the existing starter motor, which is inside the car by the passenger’s feet, by making up a replacement flywheel cover to which the starter is bolted. The first step was to obtain a spare flywheel cover/starter bracket and see how this fitted onto my spare engine and 4-speed box.

Clearly the top of the gearbox where the gear lever fits was well and truly in the way. I did find however that I could position the starter motor, without the flywheel cover fitted, at a position on the extreme left of the gearbox where the starter dog would engage in the flywheel and the body of the starter would be relatively clear of the top of the gearbox.




I made up a steel plate, 1/4” thick, with a cut-out, to bolt over the flywheel in the same manner as the original flywheel cover normally fitted. I also made a steel plate of the same thickness to fit the front of the starter motor. These two plates were offered up together and so positioned that the starter motor would fit and operate correctly. The joint between the two plates was welded up as strongly as I could, strength is more important that neatness ! Having checked it all out I then created a cover using a steel flywheel cover and fibreglass.












I could now set about sorting out the clutch operating mechanism the 3-speed box uses a different thrust system, however, you can blend together the two systems using the pedal shaft and arms of the 4-speed with the thrust bearing assembly of the 3-speed. To achieve this I had to grind a flat on the pedal shaft and use the pedal from a 3-speed system, using a cotter rather than a pinch bolt. In doing this I made sure the pedal arm was slightly more upright to prevent it hitting the car floor too early as the whole assembly would be 3/4” further forward in the car.

I also had to make a stop inside the clutch housing so that the pedal shaft operating arms would not go back so far towards the gearbox, as is the case in the 3-speed box. I did this by replacing the locking washers under two of the centre bearing housing bolts and using these bolts to hold a plate in place which formed the stop. The edges of the plate were used as tab washers to prevent the bolts from undoing.

Engine mounting holes had to be re drilled 3/4” further forward. Do this with care. The best method is to drop the engine into the chassis and move it as far forward as possible, the clutch arm and the oil pressure relief valve cover will stop the forward progress of the crankcase. Now put a drill down the mounting holes and mark the chassis. Remove the engine and drill the holes.

I replaced the fan assembly, which had a long nose between the fan and the pulley, with a Ruby type which is appreciably shorter. This means that little or no change needs to be made to the position of the radiator.

The ignition timing control rod from the bottom of the steering column to the distributor will need the be lengthened by 3/4” either by cutting and joining it with a sleeve or by making a new rod.

gearbox7A notch will have to be cut in the floor-pan on the nearside of the gearbox opening, adjacent to the body mounting point to accommodate the oil filler plug. This is the only irrevocable change to the bodywork. A new removable floor plate above the gearbox will have to be made up out of 20 gauge steel rather than modify the existing one.

The only other change required is the speedo drive. You will need to obtain the brass driving cog from a 3-speed box and have the collar removed so that it is of the same thickness as the 4-speed type. The housing at the back of the gearbox will also have to be modified, the small speedo drive thrust bearing will have to be set further into the casting so that extension on the end of the 3-speed driven cog fits.

The end result?  The car is a lot easier to drive, particularly for the less experienced driver.

The hill climbing ability is far superior especially where most cars would be in second gear, a 3-speed box would most likely be in first.


The car is quieter, 3-speed boxes have straight cut gears which generate noise (except for top gear).


Only a careful examination of the inside of the car reveals that the gearbox is 4-speed rather than 3.


This article, written by Malcolm Watts, originally appeared in CA7C Seven Focus in Oct 2002 pp14-17.

See also:

Camshaft Bush Modification

Crankshaft Conversion

Every Piston Tells a Story

Front Main Bearing Retaining Lip

Gearbox Oil Seals

Good Sump Joints

Little End Bolt Failure

Overhauling the Gearbox

Synchromesh Set-up