The Imp Site

Design & development of the Imp transaxle

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

   Bill West
Bill West at the time of the Imp's introduction
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. [see: press release 11th May 1963, car no. 13]
Regards, John Lewis

From: John Lewis
Date: Fri, 27 Aug 2004

I'm back from holiday and as promised 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.
In reading through them I feel a bit like an anorak. Maybe only I find them interesting.
Anyway use them, or any of them as you see fit.

This is the file that Mr. Lewis sent me:


   clutch; early and standard
A standard change made quite early on in the Imp's life was to fit a larger clutch. The original 5½ in. dia. became 6¼ in. and the thrust race was improved, too.

From design and through prototypes the clutch was 5 ½" diameter and was completely adequate. However, once in production and in the hands of the general public an unforeseen factor necessitated an increase in size to 6 ¼".

This unforeseen factor being the fact that the Imp was the first high volume family saloon with synchro on 1st gear. Prior to this, the crash 1st gear, your driving instructor taught you, was an emergency low gear, only used for starting on hills, i.e. in those days you pulled away in 2nd gear. The Imp gearbox with its 1st gear synchro and all indirect gears was designed with gear ratios ranging from a usable 1st gear to an overdrive 4th. Unfortunately too many owners took the attitude that they had always started in 2nd and 2nd they were going to start in. This resulted in clutch slip and burn out of the 5 ½" clutch.


When the Imp entered production the gearbox was unvented, not because we thought it was a good idea, but because, in the limited time between the start of the gearbox design and production, we had been unable to find a conventional type of breather that did not leak oil profusely. Every type of breather used in other gearboxes and axles was tried and all leaked. The problem with not having a breather was that at running temperature the gearbox became pressurised and an unimportant weep at an oil seal became a serious leak.


We in Engineering had developed a workable breather that was used in all the Competition Imps and was completely satisfactory. It comprised a 1/8" diameter hole drilled at an angle of 45 through the gearbox input shaft, in line with the input shaft gearbox oil seal i.e. this hole bridged either side of the oil seal connecting the inside of the gearbox with the void of the clutch housing which was open to atmosphere. The input shaft was above the gearbox oil level and when rotating centrifugal force prevented oil flowing through the breather hole into the clutch housing. (see attached sketch).

Unfortunately Rootes Management vetoed a breather venting into the clutch housing. Not being practical engineers they were haunted by memories of failed clutches caused by oil entering the clutch housing from leaking engine crankshafts.

Eventually a breather utilising the centrifugal principle that oil would not flow through a hole drilled radially into a rotating shaft and then venting through the gearbox end cover was developed and went into production.

Another memory is when, just prior to it's unveiling, the Imp was being introduced, over 3 or 4 days, to European dealers and journalists and somebody boasted that the synchro-mesh was so powerful you could engage 1st gear at 70 mph. Nobody could foresee that not only would one Italian journalist put this to the test but that he would then re-engage the clutch. The engine was a write-off!

Final drive

The final drive to the differential unit was a hypoid crown wheel and pinion, the choice of which produces a compact design with tremendous load carrying capacity and the potential for quiet running. It does however result in high axial as well as radial loads in the supporting bearings. The only practical choice of bearings to carry these loads is taper roller bearings.

Unlike front wheel hubs which, when carried on taper bearings, need a small amount of free play (without free play the effect of the differential expansion of the hot hub and cool wheel spindle during breaking would cause excessive axial loads in the bearings) it is essential, if the gears are to run quietly that no free play occurs in the bearings carrying the crown wheel pinion at all times. This is achieved by pre-loading the bearings during assembly by a small amount (typically 004 to 005 of an inch).

In the case of the Imp gearbox the differential expansion of the aluminium casing and the steel gears and shafts had to be carefully considered. Linear expansion of the casing would tend to increase the pre-loading of the bearings to unacceptable levels. This effect was offset by insuring a heavy interference fit of the bearing outer races into the housing. The effect of differential expansion at running temperature was to reduce this interference fit allowing the bearing outer races to expand which had the effect of reducing the bearing pre-load.


I can't remember the exact year, it might have been 1966, but I well remember a meeting in the Chief Engineer's office which included myself and Des O'Dell, the Competition Manager, it was a post-mortem into why 3 Imps, which were in winning positions well into the R.A.C. Rally, all went out with broken differential cages.

It transpired that the foundry supplying the cages was having difficulty keeping up with production demands using Pearlitic Malleable cast iron as specified on the engineering drawing and had persuaded the Purchase Department that Blackheart Malleable cast iron would do the same job but was easier to produce.

Needless to say the 3 transaxles carefully prepared with new innards for the R.A.C. all contained these inferior diff cages.

It's amazing how many people in Management, Purchasing, and Production treat engineering drawings as a rough guide to what the finished component should look like.

This cavalier attitude to quality control, which is almost exclusively a management responsibility, was behind all the Imp's woes. Those who blame the inexperienced work force, mainly ex-shipbuilders, for this lack of quality control should have a word with Nissan on Tyneside and Weir, another ex-shipbuilding area. Nissan's Sunderland plant is up there with the best in the world when it comes to quality control and productivity.

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