Kent Auto Developments Ltd - Mini 5 Speed Gearbox

KAD 5 speed Racing Gearbox   

When KAD decided to produce their own 5 speed race gearbox there were two main criteria which had to be met by the design. Firstly it should fit a standard gear casing without expensive modifications. Race engines can have blow ups and replacing an expensive casing just adds to the misery. Secondly it should be direct drive in fifth, we figured that this would offer the maximum torque output at the point where aerodynamic drag was at its greatest: high speed. The other 5 speed boxes on offer were all of the over driven fifth type which meant torque would be lost when engaging fifth. The end result is a no compromise race gearbox that is easy to use with gear ratios ideally suited to today’s race tracks with their frequent slow to medium speed corners.



The compact gear train fits into a standard A+ rod change gear casing and has straight cut gears with face dogs, typical of conventional racing gearboxes. The gears are manufactured from a vacuum arc remelted steel (VAR), this is a very high specification material which has higher strength and greater purity than commercially produced steels giving stronger gears and allowing a narrower gear width and lower frictional losses. The selector forks are made from aluminium bronze which has good wearing properties and high bending resistance. The selector mechanism has been designed to offer a fast reliable change and the shift pattern has been arranged with first on a dog leg, to the left and back.

This puts 2nd, 3rd, 4th and 5th in an H pattern, a great benefit when making high speed down changes from fifth to fourth. Quickshift gear levers are not required as the forwards-backwards change is faster than standard whilst the side to side movement is the same. This makes gear selection easy, precise and fast. Input and output shaft splines are the same as standard Mini allowing the same transfer and final drive gears to be used.

The KAD pinion bearing support housing is used with the 5 speed to reinforce the free end of the mainshaft, this gives better pinion gear life as it prevents the pinion from trying to climb the crown wheel under hard acceleration (available separately for four speed boxes).

A centre oil pick up pipe is also supplied as the laygear layout requires a different path for the pipe. Gear ratios have been chosen with competition in mind.

Gear ratios are 2.398:1 first, 1.824:1 second, 1.532:1 third, 1.186:1 fourth and 1:1 in fifth.
The KAD 5 speed race gearbox is available as a self assembly kit or built into a gear casing for the addition of your existing gear train components.

• Face dog type, straight cut gears.
• Compact gear train fits into a standard rod change casing
• 1:1 direct top gear, lower torque loss in fifth gear compared to an overdriven 5 speed unit.
• Dog leg first; 2nd, 3rd, 4th and 5th gears in 'H' pattern.
• Gears manufactured from S156, an Aerospace VAR steel, selector forks from AB2 aluminium bronze.
• Ratios: 1st 2.398 2nd 1.824:1, 3rd 1.532:1, 4th 1.186:1, 5th 1:1
• Input and output spline as standard Mini
• Supplied with extra bearing support for pinion end of main shaft and centre oil pickup pipe

1011300 5 speed KAD gear kit

KAD LSD Crown and Pinion

With our rally development work we have found that the crown wheels and pinions previously available have inherent faults in the gear cutting, materials, design and heat treatment. We have taken a clean sheet of paper and worked from first principles to design a series of final drive gears made to a no-compromise standard of quality. It looks pretty much the same as any other LSD crown wheel and pinion but we have gone that much further in manufacture to ensure that adequate strength and reliability are built in and to ensure longevity in use. We have bought expensive steel, machined and gear cut to close tolerances, hard rolled the gears and carefully heat treated the steel. The careful control of backlash We have succeeded: early testing has shown remarkably little wear and no failures. The following photograph shows two 4.3 13 tooth pinions, the competitors pitted gear on the left has covered less stage miles than the KAD gear on the right. Both have completed tarmac stage miles in a 180bhp rally Mini with 126lbft of torque.

Ratios available:

1018267 4.1:1 ratio (13/54 teeth) Crown wheel and pinion set for A series

1018268 4.3:1 ratio (13/56 teeth)