In March 1961 Jaguar stunned the world with its iconic E-type, however few people remember that the Mk X also debuted that year, in the October of 1961. Built with the American market firmly in mind, the Mk X – codenamed Zenith - was bigger and heavier than anything Jaguar had produced before. In fact its scale dwarfed even the Rolls-Royce Silver Cloud with a width of 6ft 4in and length of 16ft 10in, weighing in at a massive 37-cwt. Luxuriously appointed with disc brakes all-round, power steering and a sports car engine, it had an impressive specification.

Behind this behemoth of a car was a monster of modern technology. Following the advances made in unitary construction pioneered by Jaguar with the Mk 1 and Mk 2 saloons, the same system was adopted for the company’s flagship saloon. The entire power plant and transmission is fully isolated from the body to give a sublime ride. Already some 13-years old by the time of the Mk X’s launch, the XK engine continued to be a market leader and was lifted directly from the E-type, the triple carburettor engine was said to give 250bhp on 8:1 compression ratio (the E-type was 265 with 9:1). Perhaps the most advanced feature though was the IRS (independent rear suspension), which it also shared in principle with the E-type. This assembly was developed by Jaguar engineer Bob Knight and proved so robust that it continued in production until the Aston Martin DB7; it also remains the set-up of choice by many people who build hot rods and custom cars. Curiously the road wheel diameter was 14-inches, the only Jaguar ever to use this size, apparently favoured in America.

Perhaps the only downside was the transmission. Most models had the three-speed DG automatic gearbox, although the standard Moss four-speed (with or without overdrive) was also offered. With disc brakes all-round, the 3.8 was boosted by the curious Kelsey-Hayes bellows servo, again a version as used on the E-type. Little in the way of body revisions was done with the Mk X. In 1964, the 4.2-litre engine replaced the 3.8. Power rose to 265bhp while torque increased from 260 ft/lb to 283 ft/lbs to improve driving quality. The all-synchromesh gearbox with overdrive became an option with the automatic gearbox updated to the Borg Warner model 8. Brakes were similarly improved as the bellows servo was replaced by a conventional vacuum in-line servo. The steering box now had the much-improved Bendix Varamatic, proving a delight to use.

It was the interior that set the Mk X apart and continues to impress today with wood-veneer measured by the acre and leather by the herd. Passengers have so much space that they might as well occupy a different postcode! In the rear the pull-down picnic tables live up to their names with enough space for Jeeves to bring on the silver service. The boot was suitably voluminous ready to accept ball gown and dress suit along with enough to do the Grand Tour and still have space for a set of golf clubs. Two big fuel tanks flank each side. The Mk X was no boulevard cruiser. In fact once on the move the car drives extremely well and handles far better than its size might suggest. Drivers need be brave only once to appreciate the high cornering levels. It’s brisk too. But in truth the Mk X is more at home conducting its occupants in total luxury, giving little or nothing away to the best in the world. Fuel consumption was from another age though and mid-teens appears the norm.

Air-conditioning (Delany Galley) and, a Jaguar first, electric windows, became options. There was even a special limousine version fitted out with a centre partition, cocktail cabinet and massive picnic tables. In 1966 the Mk X went bling to become the 420G, gaining dashing chrome waistline strips and slightly modified interior featuring a padded dash top. Strictly speaking there was no Daimler version but in 1968, the 420G wheelbase was used as the platform for the Daimler DS420 limousine, effectively keeping the Mk X genes in the Jaguar pool until 1992. Announced in the same year as the DS420 was the world-conquering XJ6 and although such an advance model should have signalled the demise of the 420G, such was the demand for the traditional Jaguar saloon that it remained in production until June 1970.

Jaguar always kept an open mind in its development programme and while developing a possible sixties assault on Le Mans, Jaguar developed a mid-engined V12 racer with a single car being built. But it did make seven engines with a possibility of serious production in a refined saloon. A version was fitted to the Mk X with considerable testing done at the MIRA proving ground in Warwickshire and on the road. Legendary motor journalist Denis Jenkinson managed a drive quoting that it was, “leaving my E-type so far behind it was ridiculous.” Of course that was the quad cam version of the V12, quite dissimilar to the production version announced in 1971. In May 1960 Jaguar bought Daimler and with it came the brand’s two V8 engines, one of 2.5 the other of 4.5-litres. The larger engine was tried in the Mk X giving a useful gain in performance, achieving some 133mph at MIRA. However, although many reckon pride was the reason for it not being adopted in the production car, true to Jaguar’s rationale it was the cost that made it uneconomic for series production.

There is a strong following for the biggest of Jaguars and with so many consigned to the scrapyards or trashed on the banger racing circuits, survivors are now sought after. One interesting conversion in more modern times was when a handful of cars were made into full convertibles, transforming them into extremely pleasant open-top cruisers.