The Imp Site

couple in Imp shifting a lot of gear - Norman Thelwell 1974

Imp gear shift

The neat little engine was mated to a neat little gearbox.

When the engine was decided upon, the next thought was on the gearbox. It had to be amenable to the output of the lively engine. If the engine was to give its best, revolutions ought to be kept high - and full use should be made of the gearbox. The box that had been in the prototype sofar just was not up to the job. It was decided to have one made especially for the Imp, tailored to fit in a fairly cramped compartment.

Rootes hadn't designed a transaxle before, or a gearbox plus differential. November 1960 they hired Adrian West as Senior Transmission Engineer. He had traveled around the European continent on a scholarship studying gearbox design, visiting Renault, Fiat and Simca a.o.

Adrian West set out to tackle the problems. He wanted to set a new standard in gear shifting: speed and lightness would be remarkable !
The Triumph TR4, introduced in 1962, was one of the first English sportscars with an all-syncromesh fourbox. See also the BMW 335 (1939–1941) 

Also, in 1959 Alec Issigonis had stated that synchromesh for the first gear was impossible, and West would like to prove him wrong.

The design succeeded handsomely. All four gears have really efficient baulk-ring synchromesh, and one can change down to any gear at the maximum speed for that ratio without double declutching or synchronizing the engine speed. Few car magazines failed to sing its praise. It has a very nice, precise shift, as quick and easy as one could wish for. The choice of ratios is outstanding.
The gearbox casing is die-cast from light alloy to save weight. The transmission weighs only 65 lbs. (29.5 kg).

It is the first Hillman since 1935 to feature a full synchromesh gearbox.

John Lewis
Date: Thu, 24 Jun 2004
Subject: feedback

Stumbled upon this site by pure chance. It brought back memories of an exciting period over 40 years ago. Bill West and myself were responsible for the design of the transaxle i.e. the clutch, the gearbox, the differential and the hypoid final drive, and I was responsible for it's subsequent development.
Regards, John Lewis
gearratiomphkph |
1st 16.595 : 12540
2nd 8.905 : 14674
3rd 5.702 : 171114
4th 4.138 : 183
reverse 13.824 : 1
ratios Rallye Imp:
 1st gear16.59
 2nd gear 8.91
 3rd gear 6.19
 4th gear 4.48

The manufacturer's recommendations for the 1963 Imp were 20, 34, 52 and 70 mph in the gears. In a pinch, second gear can be taken to 50mph and third to over 70 (Imp Sport), since it will rev to 7000.
The ratio of the second gear makes it most useful on tight, uphill corners; third gear is very useful for passing on the open road.
Once you're used to the change mechanism, the short, light movements and very clearly defined H-pattern are a real pleasure.

With all the ratios indirect there is always a bit of transmission whine, even in top gear. But it is a slight, almost pleasing whine... ;-) At high speeds you can't hear it over the engine. The synchromesh is powerful, but it isn't always easy to select first gear.

"[...] a delightfully rigid lever moving in a very compact positive gate, the transition from one gear to the next becomes very smooth, particularly for quick changes as the revs rise and fall very rapidly with the light flywheel. The ratios come very near to the ideal m.p.h. steps per gear, at the same time keeping bottom gear low enough to allow a contemptuously easy start on a 1 in 3 hill. Reverse gear, protected by an impact spring loading, needs a hefty slap to get the lever in the right plain. "

Final drive is to a transaxle and the standard gearing is such that at 1,000 rpm in the highest gear the Imp does 15mph. Crownwheel/bevel pinion teeth: 34.7
In tests it was shown that maximum speeds of 27,47 and 71 mph correspond to approximately 6,500rpm. The engine could be taken up to 7,000rmp without protest. The maxima recommended by Rootes were conservative. For fourth gear the recommended maximum was at 70mph.

The drive taken to the rear wheels via Metalastik Rotoflex couplings, as on the latest racing cars in 1963. These eliminate the necessity for sliding spines while cushioning torque loadings from braking and acceleration.

A report (1963) on a Hillman Imp exported to the U.S. gave the following figures:
Gear Ratio  Over-all  Mph/1000rpm  Max. mph
Reverse2.846 13.824 -3.3 -25.2
1st 3.417 16.595 3.3 22.2
2nd 1.833 8.905 7.1 45.0
3rd 1.174 5.702 11.0 70.0
4th 0.852 4.138 15.2 80.0

Final drive ratio: 4.857 to one

Jack Knight gearbox for the Imp

The Jack Knight Imp Gearbox

Although the standard gearbox is really first-class, and one of the Imp's most endearing characteristics, there are circumstances in which one might prefer different ratios.
Knight Developments ltd (Woking, Surrey) made alternative gears for the Imp. September 1966 the majority of competition Imps were using the Jack Knight 3rd and 4th conversion. The standard bottom and second ratios, which are helical cut, are retained; the third and fourth are replaced with straight cut gears, chosen from a variety of ratios (still synchromesh).
At that time a conversion, including fitting to the box, would cost you £24. Two years later the price had not gone up.
There was another conversion available, four-speed, consisting of a complete set of straight-cut gears, constant mesh and running in needle-roller bearings. This was mainly used by those sports or G.T. cars using Imp gearboxes. There was a choice of around twenty different ratios. (1967: £97)
Of similar design is their five-speed box. (1967: £150)
In addition to these they were developing (autumn 1966) a limited-slip differential, which was expected to sell for about £55 and will be completely interchangeable with the standard unit without any mods at all. Obviously, it will be designed on similar lines to the already successful limited­slip diff for Minis, which the same firm also makes.

Peter Gibbons runs JK.
It would appear that demand for the Imp boxes has increased dramatically in the last couple of years and they now have most of the bits and ratios available 'off the shelf'.
What they have trouble sourcing is the casings and the various original internal bits that they retain, so if anyone has these new I'm sure they would be delighted to here from you. - Mark Chater (22 Sep 1999)

For JK conversions, the early type casings are needed, ie the ones without the breather holes.
These are the boxes that can be turned over for mid-ships mounting, although new oil ways have to be drilled to accomadate... - Tim Morgan (22 Sep 1999)

a 9.7 dog gear
a 8.9 set

   advert Jack Knight
July 1977

(22 Sep 1999) The current price for a JK LSD, new without the special splined CWP, is over 500 pounds.
Malcolm Anderson lists the CWP assy at 275.
a dog box which has an LSD.

A lot of the old Imp powered single seaters used a external oil pump (usually a Jabasco electric bilge pump) to ensure proper lubrication, I don't think the Ecosse Imp did but it is 20 years since I saw one. - Andrew W. MacFadyen (23 Sep 1999)

From: Roger Gill
Date: Thu, 23 Sep 1999 09:18:01 +0100
Subject: Re: [imps] JK Ratios

I was thinking along the lines of 350 for the diff, but I have another project for the diff set up and would have to sell it to pay for the alternative, but being unsure about fitting it in I'd hate to part with one and not get the other to work properly, similar to when I fitted the Salisbury diff into the Imp.
As for my gearbox, well it's a hybrid I've made from a syncro box, but utilises JK ratios in 2nd & 3rd, no fourth fitted as yet, it has the std 1st gear set up at 16.5, it uses the std CWP, so if the box bursts which in 19 years of Autograss and Rallycross it hadn't, until the other week I tried my hand at a show sprint and split the casing, I've MIG alloy welded ( first time at home with 130 amp welder ) machine mart did the gas ( argon ) and the wire, I practised on an old casing, funnily enough it was a friends box which I had rebuilt for the same crack. it's holding together hopefully whilst the end of the season then I'll get time to rebuild it into a another syncro box.
Building the box like this gives me synco changes but JK ratios best of both worlds. also not incurring any major expense of a new CWP. as the teeth can get damaged when the casing lets go.

From a conversation on OneList - August 1999:
Mark Chater: My gearbox has a JK Straight cut conversion on 3rd and 4th. It developed a problem. When I selected 4th gear, it jumped straight out when I let the clutch up. (The linkage was made by me, cutting down a standard Imp linkage by about 6 inches, and I had to drill new holes in the floor to mount the gearstick plate as the previous owner had installed a Hewland.)
Andy Bryson replied: The problem could be the big nut on the pinion shaft coming loose, which means taking the gear box out. It could be the linkage, but if 2nd is selected ok, I suggest a quick look inside the box. You only have to take the end cover off to check the nut. You can check the detent at the same time.


A MK9 Hewland gearbox is the gearbox that was made by a firm called Hewland. It was fitted into most small engined (up to 2000cc) single seater racing cars from the early 60's up to now. It has been uprated several times over the years.

Hewland Engineering Ltd.
North Town Road, Maidenhead, Berks.


Date: Thu, 20 Jan 2000 17:06:31 +0000
From: Tim Morgan

However, there was an automatic Imp... three in fact. These were produced by experimental in the mid sixties. Apparently they weren't too clever, and surprisingly weren't adopted.

However, Richard Sozanski does own one of the fabled gearboxes and he's threatening to fit it to something one day... hmmmmm, not my cup of java:-)

Strenghtened gearbox housing

Strenghtened gearbox housing: photos from Maynard's stand, National Weekend Imp12


Design & development of the Imp transaxle / John Lewis. - Thu, 24 Jun 2004; Fri, 27 Aug 2004
"I've made a list of technical bits and pieces and events that come to mind when contemplating what I was doing in the middle of the last century."

CCC look-in on the Imp / by David Vizard. - Cars & Car Conversions 1972, April. - p.54-55,91
Gearbox and gearing ratios: specialising your Imp

    Imp gearboxes, the book

How to strip & rebuild Imp gearboxes : a step-by-step illustrative guide / Rootes ; The Imp Club. - 1998. - 64p. ; b&w photogr. : 17x25cms
ISBN 1870519531
(soft cover)
For those who wish to make an attempt at overhauling the Imp gearbox (transaxle), this book will be of assistance. It was made using original material owned by The Imp Club.

The Imp Site
   Imp Anatomy
   Bits & pieces: the transmission unit
   The Gillie prototype had a reduction gearbox designed for it by Rootes engineer Don Kitchen (hub reduction gearing)


© Franka