The cars : Jaguar 420 development story

The final fling

The cars : Jaguar 420 development history

THE JAGUAR 420/Daimler Sovereign was the final variation of the original Utah project that evolved into the Mk1, Mk2 and S-type saloons. And if that story was not confusing enough the 420 had a similar moniker to the big 420G saloon, a revised MkX, which was announced at the same time. But to simplify matters for the layman, the Mk1 of 1955 was developed into the Mk2 of 1959, and this was further developed into the S-type of 1963 which had independent rear suspension, and was sold alongside the Mk2.

The Jaguar 420/Daimler Sovereign story seems to start in 1964. That year Jaguar produced 12,043 Mk2s, 7032 of the IRS S-type but only 2458 of the top of the range MkX saloon, an alarming drop of 62.6% over the previous year. This must have sent a warning signal to Sir William Lyons and his management team that the relatively new and sophisticated MkX was rapidly losing its sales appeal, and the model was not helped by its bulbous styling. The immediate solution to the problem was to upgun the MkX and contemporary E-type sports car with the new 4.2-litre version of the twin cam XK engine, a triple carburettor unit with a quoted 265bhp SAE, probably a more honest 220bhp.

Jaguar 420 prototype
A close look at the Jaguar 420 series prototype - the similarity with the MkX based 420G is marked.

Jaguar 420 prototype

This measure, announced on 9 October 1964, along with a new four speed Jaguar manual transmission, did boost MkX production in 1965 to 3296, but it slumped again to 2023 in 1966. Jaguar had a long term plan to replace all its saloons with the XJ4 project, but this would take some time to come to fruition, and with the MkX not selling in the volume intended and the Mk2 coming under pressure from the Rover and Triumph compact executive saloons, something else was needed to earn the company its keep until XJ4 came on stream.

Jaguar needed a car that would sell for a higher price than the S-type and retain the MkX in production to amortise its development costs. The only logical way to do this was to upgrade project Utah yet again and revise the S-type. Like the MkX, the S-type was not seen as one of Sir William Lyons. greatest styling successes and the Jaguar boss was presented with the opportunity to revise the front end of the model. One of the most pleasing aspects of the MkX had been the quad headlamp look, which Lyons was looking to apply to the forthcoming XJ4. The opportunity now presented itself to apply it to the revised S-type, now codenamed XJ16. In order to justify charging a higher price over the existing S-type, it was decided to use the new 4235cc XK engine, but using twin as opposed to the triple carburettors found on the E-type and MkX.

This reduced quoted peak power to 245bhp SAE at 5500rpm, later found to be in reality 173bhp DIN. The torque figure remained 283lb ft, but at the slightly lower 3750rpm. All the factory quoted figures are on the optimistic, not to say dubious side, and were hastily revised downwards in the 1970s as more stringent regulations came into force. Not even the fuel injected XK engines of the 1980s approached the 1960s published claims for power output. No doubt the triple carburettor 4.2-litre MkX was available for those who wanted more power, but they would have to pay extra for it. Along with the bigger engine, XJ16 was fitted with a bigger and more efficient crossflow radiator combined with a viscous coupled cooling fan in order to remedy the company’s reputation for overheating in the USA. An alternator replaced the dynamo to power all the electric gadgets that customers were now demanding in their cars.

Customers would have the choice of manual or automatic transmission. Those who liked to change gear themselves would use the new Jaguar gearbox with the option of overdrive. The torque of the new 4.2-litre-litre engine now meant that the Type 35 Borg Warner automatic transmission which was found on the S-type was now deemed not to be up to the job, so the type 8 found on the MkX was employed.

Jaguar 420 interior
Jaguar 420 interior gets a few functional changes over the S-type and Mk2.

The interior of the XJ16 was to be broadly similar to the S-type, but with the addition of a padded dash rail as safety issues started to come to the fore. Also part of the XJ16 programme was a Daimler variant, codenamed XDM16. Jaguar had purchased Daimler in 1960 and had successfully rejuvenated the marque by installing the company’s excellent 2.5-litre Daimler V8 in the Mk2 body to create the acclaimed Daimler V8 2.5-litre saloon. This created a market for a car with the refinement and performance of a Jaguar, but with more luxurious trappings, with items that were options on the Jaguar being standard fittings on the Daimler, and all for a higher price. The next step was to Daimler-ise the larger MkX saloon, using the 4.5-litre Daimler V8 enlarged to 5-litres, but this project came to naught. For XDM16 it was decided to dispense with the idea of using a bespoke Daimler engine, and simply employ badge engineering BMC-style, as was the case with all subsequent Daimler’s.

The XDM16 received the obligatory fluted Daimler grille and other discrete embellishments. A sign that Sir William Lyons was in a hurry to get both XJ16 and XJ4 onto the market place as soon as possible is evident in a memo he sent to Technical Director William Heynes on 8 December 1964.

‘I have given considerable thought to the very unsatisfactory manner in which our organisation is operating… the almost chaotic state is entirely due to our inability to introduce a new model into production within many months of the target dates….It is vital to our future that steps are taken to correct this critical state of affairs.’

By 8 February 1966, Lyons was once again railing against Heynes about delays to the XJ4 programme. He wrote: ‘We have failed because we have believed that we have no suitable people within our Engineering Division to whom responsibility can be entrusted… these people will have to be found if Jaguar Cars is to continue to be a successful company… Initial design and development must come under the control of two different teams, one concentrating on design and the other on development. They must be given responsibility and not held up by waiting for decisions from above… In this way projects can be stepped up, and although it may mean some of them will have to be discarded, the full time of the design team will be employed on producing something new… a tremendous amount of brain power and time is wasted as we are operating today.’

Lyons also advocated the creation of a forward programme to assist in, ‘The commonisation of components, and generally in the rationalisation of specifications, thus dealing with the problem of multiplicity of components which is such a handicap to our production… We are quite obviously now going to have little time for XJ4 development. I am sure you must agree that it is imperative we should once again have a new look at our design organisation. It must be obvious to you, as it is to me, that the lack of regard to the time factor cannot do other than result in calamity at some time in the future.’

If anything, this indicates the kind of pressure Lyons must have been under with stagnating production, inadequate funds available to put XJ4 into production, and all this combined with wildcat strikes. XJ16 was needed to tide the company over as a stopgap until XJ4 arrived in the showrooms. By 27 April 1966 it was recorded that XJ4 was running three months late.

On 11 July 1966 Lyons announced the merger of Jaguar and BMC to solve his company’s long term problems and he could press ahead with his new model programme. On 23 August 1966 the Jaguar plant at Browns Lane produced the first XJ16, now christened the Jaguar 420. This was followed two days later by the first XDM16, now called the Daimler Sovereign. The two models were announced on 13 October 1966. Confusingly at the same time the revised Jaguar MkX was announced as the 420G, many pundits assume the ‘G’ stood for grand. It wasn’t helped that the XJ16 420 was in effect a miniaturized MkX/420G in appearance.

Jaguar 420
Autocar's road test 420 achieved an impressive 123mph.

Autocar magazine was the first to get their hands on a Jaguar 420 in March 1967. GKV 67D was a manual overdrive car fitted with the optional Marles Varamatic power steering and the magazine was impressed with what they found. It commented: ‘The ideal cruising speed seems to be just over 100mph… Engine silence is impressive in the lower speed ranges… Directional stability is very good indeed and cross winds have little effect; the car runs dead straight, calling for minimum steering correction, right up to maximum speed.’

The magazine concluded by saying: ‘Eagerly awaited since its announcement, the 420 in every way lives up to high expectations. It has an extraordinary dual character in that it can at one moment provide stately, luxurious travel for an elderly party, and behave like a high performance sports car the next.’

Top speed of GKV 67D was an impressive 123mph, 0-60mph was reached in 9.9 seconds and overall fuel consumption was 15.7mpg. Rival organ, Motor, had its say in May 1967 when they tested GKV 66D, a 420 automatic. The men from Motor extracted a top speed of 115mph out of the car and an overall fuel consumption of 15.4mpg. The 0 to 60mph acceleration was recorded in two different settings of the Borg Warner type 8 auto box. In D1 it was a mere 9.4 seconds, in D2 a longer 11.8 seconds. It commented: ‘As a Grand Tourer – far more grandiose, in fact, than many so called GT cars – the 420 is superb and at its impressive best on a long distance journey when it will carry four adults of medium height in great comfort, with room to stow all their luggage in a big boot.’

The magazine perhaps hit the nail on the head as to the point of the 420 when it said: ‘What it does – and does very effectively – is to provide 420G (ex MkX) appearance, prestige and performance in a smaller, cheaper car, a package we ourselves think more appealing and practical if you are prepared to sacrifice a little accommodation. The 420 gives virtually nothing else away.’

In November 1967 Motor tested the Daimler Sovereign variant, JWK 811E, in the rare manual overdrive version. Top speed was 117mph with a 0-60mph time of 9.2 seconds, while overall fuel consumption was 15.6mpg. The positive reviews certainly had their effect. In 1967 Jaguar produced 7722 420/Sovereign models to make it the company’s most popular saloon that year, some 46% of saloon production.

Daimler Sovereign
Daimler Sovereign's main styling alteration was the fluted grille.

The move upmarket was badly needed as demand for Jaguar saloons seemed to have slumped somewhat since 1964/65, with the top of the range 420G now declining further to 1640. In 1968 420/Sovereign production dipped to 5525, but the waiting was nearly over as XJ4 was standing by in the wings. In order to clear the decks at Browns Lane for XJ4, 420 production ceased on 6 September 1968 after 10,236 examples. The XJ4 was launched on 26 September 1968 as the Jaguar XJ6 and 639 were built before the year was out. Because as yet there was no Daimler version available, production of the XDM16 Sovereign continued into 1969.

If Sir William Lyons had gambled the long term survival of his company on the XJ4 project, then it paid off handsomely in 1969. In February that year Jaguar Cars began its first ever night shift, indicating that prior to this whatever writers may have thought of the companies products through rose tinted spectacles, there clearly was not a huge demand for the firms cars until the XJ4/6 came on stream. XJ4/6 raised the game in the luxury car sector, it was that good. XJ4/6 production in 1969 was 13769 and was ramped up further in 1970 to 21,833.

Production of the XDM16 Daimler Sovereign ceased on 9 July 1969 after 5824 cars pending the introduction of an XJ4/6 based Daimler saloon. So the final version of the compact Jaguar saloons faded into automotive history. Intended as a stopgap, it was probably the best Jaguar saloon until the XJ6.

Daimler Sovereign

Keith Adams


  1. good day,
    im writting to you from zambia, central africa, i have found a Daimler Sovereign that i would like to buy. i was wondering if anyone would be in the position to give me information on buying spares for the same vehicle. preferably original parts. any information would be of great help. thank you.

  2. @2 sounds a little dodgy! Next you will be asking if someone can transfer £5000 into your bank account so you can get treatment for your gammy leg (prior to flying over to pick up the Jag spares of course).

  3. A little over 173bhp, which was the figure for XJ6 AED and HS8 equipped. 1st XJ6 series with aux carb and HD8 was rated at 245 SAE gross like the 420 or 186 DIN which more or less is 186 bhp net.
    Triple carb variant never available, no room under bonnet.

  4. I recall the launch of both the 420 & 420G. Strange that they used the same 420 moniker on two different bodied cars. I was previously familiar with the MK X and regarded that as the most luxurious car I had ever seen when I was of junior school age.

    All these years later I would think the smaller 420 the better choice with its offering of luxury and performance

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