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Jaguar MkX/420G (1961 - 1970)

Last updated 7 April 2013


Model Timeline

April 1958
Work begins on the new large Jaguar, Project Zenith

In 1958 when work began on the MkX, Jaguar Cars range consisted of the 3.4-litre XK engined Mk VIII, the 3.8-litre MkIX, the compact Mk1 saloon that ranged from 2.4 to 3.4-litres and the XK150 sports car. Codenamed Zenith, the MkX was intended at the outset to be a unitary construction chassis. The external style of Zenith was delayed by other events. Sir William Lyons was diverted by the restyling of the Mk1 compact saloon into the Mk2 and Jaguar’s purchase of Daimler in 1960. This did not prevent Jaguars Technical Director, William Heynes and his team of talented engineers from pressing ahead from developing the car’s internals. Heynes told his team to base the Zenith on a 120-inch wheelbase and the use of 14in wheels. However the track was as yet undetermined.

By abandoning the separate chassis and going over to unitary construction Jaguar hoped to save weight and make a lower profile car. The first unitary construction chassis Jaguar had been the 2.4-litre Mk1 saloon of 1955 which had its bodyshell manufactured by Pressed Steel Fisher Ltd. PSF would help in the engineering of the Zenith’s very stiff, some would say over engineered bodyshell and Jaguar would gradually become dependent on PSF for its bodyshells, which would lead to mergers that would threaten the future of the company.

Profile was resolved pretty much from the beginning.

William Lyons looks on, and seems quite happy with the results so far.

Front end treatment took some time to finalise.

…as this double-sided mock-up clearly shows.

The same model from the front three-quarters.

Note the semi-enclosed rear wheels…

Jeff Daniels in his book Jaguar: The Engineering Story was not as impressed by the Zenith’s bodyshell engineering: ‘…it must be admitted that as an exercise in unitary construction, the MkX was not a whole hearted success. The main object of the unitary approach is to save weight by eliminating the separate chassis and distributing stress throughout the entire body, making each panel take its share. Yet the unitary construction MkX weighed 182lb or 4.5 per cent more than the chassis based Mark IX, taking a like for like comparison of 3.8-litre test cars weighed by The Autocar.

Was this because the MkX was bigger? Not really. Its 120in wheelbase was the same, its front track 1 1/2 inches wider, its rear track identical. True, the new body was nearly 6 inch longer than that of the Mark IX and it was 3in wider; but it was also 8 1/2 inches lower, and probably enclosed less actual space.

‘That it was beyond dispute heavier is almost certainly because of the conservative engineering approach, adopted for two possible reasons. First, it made sure the body would accept the stresses fed into it by an all new suspension system, and even more, perhaps, that it would be stiff enough not to suffer the apparent fragility, the squeaks and rattles, which plagued the 2.4 and 3.4, Jaguars first unitary construction models. The kind of customer who would buy the MkX simply would not put up with that kind of thing… One good reason for this conservative structural approach, not to make too fine a point of it, was Jaguar’s concern that the MkX would be stiff and quiet despite the worst that Pressed Steel quality control could do, and to judge by the 2.4/3.4 that was pretty bad.’

The MkX was earmarked the same engine as the E-type, a triple carburettor 3.8-litre XK unit. This developed 265bhp (SAE) at 5500rpm, and was to give a most impressive performance. The steering was Burman power assisted and there were disc brakes on all four wheels which were independently suspended. The independent rear suspension developed by Bob Knight was similar to that found on the E-type and would feature on most Jaguar models until 1996. Bob Knight would also be responsible for the smooth ride and lack of noise and vibration in the cabin.

There was a choice of Borg Warner DG automatic transmission or a four-speed manual Moss gearbox with overdrive, although the latter was an ageing and in many ways unsatisfactory design. Originally Jaguar had intended only to offer Zenith with automatic transmission, the decision to offer the manual option was taken towards the end of development. The Zenith began the tradition of having twin petrol tanks with a fuel capacity of 20 gallons.

Eventually the external style was decided on by Sir William Lyons. The radiator grille sloped at the ‘reverse angle’, which. gave it a look of purpose and urgency. There were twin pairs of headlamps, the MkX was the first Jaguar to use this styling cue and the quad light look lasted into the next millennium. The car was a full five-seater and there was plenty of leg room at the back. Although it was more spacious than its predecessor the profile was considerably lower and this gave an impression of far less bulk. Certainly the MkX looked sleeker than the outgoing Mk IX, but its appearance still came as something of a shock to journalists when it was announced on 12 October 1961.

At the time it was the widest British car on sale. In fact at the time Jaguar were in no position to commence deliveries to customers as due to delays at Pressed Steel Fisher there were no cars ready. Production proper did not begin until December and deliveries did not commence until the winter of 1961/62.

October 1961
Jaguar MkX launched

In October 1961 Jaguar recieved a £22,500,000 order placed by its North American distributors. Jaguar had exactly 12 months to build and deliver over 10,000 cars and they had to strictly comply with the delivery dates specified. 40% Of this order was for MkX saloons. The trades unions pledged their support, but thirteen days after the order was placed there was trouble. About 150 men walked out in a dispute involving the companies new MkX car, and as a result 500 other workers had to be sent home. The strikers, all employed in the trim shop, complained of excessive waiting time -the lapse between jobs, when the men got a lower rate of pay.

Autocar magazine did not manage to get its hands on a MkX until November 1962 when they tested 8172 RW, an automatic. Top speed was 119.5mph, the 0-60mph time was 10.8 seconds and fuel consumption was 14.1mpg.

8172 RW had 13000 miles on the clock and Autocar paid it several compliments: ‘There was not a body creak or rattle… almost sports car stability through fast bends and over rough surfaces……..The aroma of good leather pervades the interior, and this with an abundance of timber veneer gives it almost the cosily affluent air of Edwardian library’.

A year passed before the rival Motor magazine tested a manual overdrive MkX, 1196KV. With 1196KV, the men from Motor attained a top speed of 120mph and a 0-60mph time of 10.8 seconds. Overall fuel economy was 13.6mpg. However the magazine was not so complimentary about the car as their rivals.

Meanwhile in this pre-energy crisis world, fuel was still relatively cheap and a horse power race broke out in the USA as manufacturers built more and more powerful engines with larger capacities. Jaguar was in danger of being left behind. The company had two choices: develop what it had or build an entirely new engine. When Jaguar had bought Daimler in 1960 it had inherited a failing car range, but two excellent overhead valve V8s of 1959 vintage designed by Edward Turner. The first was a 2.5-litre unit producing 140bhp and 146lb ft torque, more than the equivalent 2.4-litre XK engine. Further more, when slotted into the Mk2 bodyshell to produce the Daimler 2.5-litre V8, it produced a faster car than the smaller engined Mk2 2.4, and in the opinion of most critics, a more refined car.

The second was a 4561cc V8 which produced 220bhp at 5500rpm and an impressive torque figure of 283lb ft at 3200rpm. This V8 was fitted to the Daimler Majestic Major which was in production from 1959 to 1968. Sales of the latter were low but it could match the Jaguar MkX for top speed and beat it on acceleration quite handsomely. The logical next step was to try and fit the 4.5-litre V8 into the Zenith bodyshell to create a Daimler version of the MkX, which was duly done. Physically the V8 fitted into the engine bay quite easily, and ex-development engineer Ron Beaty recalled how it performed.

‘It lopped six seconds off the 0-100mph time, and that was with square cut exhaust manifolds and an air cleaner you wouldn’t put on a lawnmower. It buzzed around MIRA all day at 133/134mph in the hands of anyone who happened to be about’.

Around this time Jaguar Technical Director William Heynes wrote to chairman Sir William Lyons on the subject of the larger Daimler V8: ‘There is no question that the horsepower can be brought up by redesign of the valve ports. At the same time I feel it is desirable to increase the capacity of the engine. I have therefore arranged with Daimler to revise the design to bring the capacity up to five-litres which, with revised valve ports, should produce 280/290bhp at 5000rpm with a 30% increase in maximum torque over the Jaguar 3.8 engine. The unit in this condition would give a satisfactory alternative unit for the Zenith (MkX) and also a satisfying performance in the big Daimler saloon’.

In the event the company decided not to proceed with a Daimler V8 version of the Zenith saloon. The official reason was that the the Daimler production line was not tooled up for quantity production, but it had no trouble supplying demand for the smaller 2.5-litre V8. Perhaps the real reason was one of prestige. A 5-litre Daimler Zenith would have embarrased the XK engined Jaguars in the performance stakes, Jaguar’s engine designers also had a blind prejudice against the V8 configuration, which they thought could not match six and twelve cylinder for refinement and they thought they could design a superior V12 engine. In the event it took until 1996 for Jaguar to produce a V8 engine, the superb AJ26, which in 2009 was enlarged to 5-litres.

October 1964
Jaguar MkX receives the 4.2-litre XK engine

In the meantime Jaguar decided to enlarge the XK engine from 3781 cc to 4235cc. On 6 October 1964, Jaguar announced the new 4.2-litre XK engine for use in the MkX saloon and E-type. The new engine boosted torque from the 3.8-litre’s 260lb ft at 4000rpm to 283lb ft at 4000rpm. The quoted power output remained at 265bhp (SAE), but now at the slightly lower 5400 rpm. Along with the new engine, came a new all-synchromesh four-speed gearbox designed by William GJ Watson, which when fitted to the MkX came with optional overdrive. Automatic transmission was now the Borg Warner Model 8.

Also among the revisions was a new power steering system which was called the Marles Varamatic Bendix which was further developed and manufactured by Adwest of Reading. The revised MkX was given the factory code XJ5 and was being produced at a rate of 75 per week.

Autocar magazine tested a revised 4.2-litre MkX automatic in October 1964. They managed a top speed of 121.5mph, fuel consumption of 14.5mpg and a 0-60mph time of 9.9 seconds which fulfilled Jaguar’s ambition to create a faster car than the Daimler Majestic Major.

It was not until a year later that the magazine got their hands on a manual overdrive 4.2 MkX. This time the top speed was 122.5mph, petrol consumption was 16mpg and the 0-60mph dash was completed in 10.4 seconds. Autocar commented on the new manual gearbox: ‘The latest all synchromesh gearbox is a great improvement over the superseded unit, and has an excellent set of indirect ratios, such that one is rarely short of a suitable gear for any traffic situation. Movement between gears is quite long, but the gate is well defined, and the synchromesh quite unbeatable. Second gear, with a useful maximum of 54mph, and third, good for 81mph, are both exceptionally useful for sweeping past slower vehicles.

Such is the combination of well bred power, nicely matched ratios, and an easy gearchange that even the most timid driver would find himself pushing the Mk X along very quickly on almost any sort of road, yet at city traffic speeds it is restrained and quiet.’

Autocar also compared the £2261 MkX with rival cars. The only car that was cheaper was the Vanden Plas Princess R which could manage 106mph. The £3630 Buick Riviera was 1mph slowers, and also trailing in the Jaguar’s wake was the £4086 Mercedes Benz 300SE and the £5632 Rolls Royce Silver Cloud. Around this time Jaguar announced that the MkX would now be fitted with an optional full air conditioning system. Also in 1965 Jaguar introduced a limousine version of the MkX complete with central sound-proof division. 18 cars were built.

On 11 July 1966, Sir William Lyons sold Jaguar to BMC, although he retained control, to form British Motor Holdings. By doing this Lyons secured the future of his body supply from PSF which was now owned by BMC and funding for a next generation of models.

October 1966
Jaguar MkX facelifted into the 420G

On 13 October, Jaguar announced another revision to the Zenith project when the MkX was renamed as the 420G. Rather confusingly at the same time the company announced the 420, a facelifted S-Type with the quad headlight look. The 420G recieved a new grille, chrome beading along the flanks, a padded dash rail and revised wheel trims. A limousine version continued to be available and another 24 examples were manufactured. Although not actually specified, the ‘G’ is believed to have stood for Grand. Mechanically the car was unchanged.

While all this was going on Jaguar were working on the car that would replace the MkX/420G and all the other Jaguar saloons, the project codenamed XJ4 and eventually released to the public as the XJ6 and later still as the XJ12. The MkX/420G would become the testbed for various XJ components including the V12 engine. The V12 MkX/420Gs were codenamed XJ10 and were used to help Jaguar decide on whether to opt for a quad cam or twin cam V12 engine. The four cam XJ10 gave smooth but unspectacular acceleration up to 3500 rpm but beyond that point it really took off and had a maximum speed of nearly 150mph, but in the end Jaguar opted for the ultimately slower twin cam V12 as it offered more low down acceleration where it mattered in real world driving.

In 1968 the Zenith/XJ5 platform spawned the XDM3 Daimler DS420 Limousine using a stretched floorpan and 4.2-litre XK engine. This long lived vehicle survived until 1992 and was the last to use the XK engine, ceasing production after 5043 examples. Also in 1968 the ill fated British Leyland Motor Corporation was formed with Sir William Lyons as deputy chairman and Jaguar at the pinnacle of the car range. With Rover and Triumph in the fold with their 2000 saloons, which were in the process of receiving larger engines, Jaguar did not need to replace the now ageing Mk2 based compact saloons and could concentrate on the one model XJ4 project.

In September that year, Jaguar announced the XJ4 project as the XJ6 saloon. In terms of ride, handling, refinement, stability and comfort it was a quantum leap. Journalists fell over themselves to award superlatives to this car and a huge waiting list soon built up of customers wanting to buy this new automotive sensation. The XJ6 made all Jaguar’s existing cars obsolete and the company got down to the task of rationalising its range. By axing its existing models, the company freed-up Browns Lane to satisfy demand for the new car. First to go was the Jaguar 420 in August 1968; that was quickly followed by the Mk2 240 in April 1969; the Daimler Sovereign in July 1969; and the V8 250 in August 1969.

Only the Mk X/420G survived the cull – for the time being. It remained the companies range topper. When the XJ6 was launched, the top of the range 4.2-litre interestingly used the twin carburettor XK engine rated at 220bhp SAE, in reality a more honest 173 bhp DIN, while the 420G continued to use the more powerful triple carburettor unit, perhaps as a sales incentive.

June 1970
Jaguar 420G production ceased

By October 1969, the 420G range topper was listed at £2671, compared to £2475 for the XJ6 4.2. A difference of £196, which equates to £2500 in 2009 terms. It was sold as a luxury car powered by the more powerful 265bhp XK engine, but demand wasn’t strong and Browns Lane probably built no more than around 30 cars per week. The demand for the XJ6 was such, that the big car was phased out in June 1970, leaving the XJ as the sole saloon representative in the range. The 420G’s place on the assembly line was taken by yet more XJ saloons. And the void created by the demise of Zenith/XJ5 was filled by long wheelbase XJ Jaguar and Daimler variants, being joined by the XJ12 in July 1972.

Only 25,211 Zenith/XJ5s were produced in nine years, Jaguar would later produce more XJ saloons in a year. At its peak the Mark 10 was produced at a rate of 110 a week, but by the time the 420G came along only 30 were being produced weekly. Of course the other thing that needs to be asked with such a low production figure, did the big Jaguar pay its way?

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