The Jaguar 420 is a car that blurred the lines between the Jaguar S-Type and the Jaguar Mk X as much as possible , being, in fact, the result of a merger of these models.

About Jaguar Mark X

Work on the creation of the Jaguar Mk X began in April 1958. During the development stage, the prototype was nicknamed "Zenith," and it was initially planned to be built, like previous models in the series (except for the Mk 1), on a separate chassis. However, the subsequent release of the popular Jaguar Mk 2 and the acquisition of Daimler somewhat delayed the project's implementation.

The development team, led by William Heynes, continued working to bring the project to life. The company aimed to reduce the car's weight and give it a lower profile. Heynes instructed his team to build the Zenith on a 120-inch wheelbase, use 14-inch wheels (instead of 15-inch ones), and switch to a unibody design. Along with other innovations, the Mark X also became the company's first car to feature a rear suspension similar to that of the E-Type.

Initially, the model was developed with a 3.8-liter engine paired with an automatic transmission. A manual transmission was added only toward the end of the project. Production of the unibody chassis was entrusted to the familiar firm Pressed Steel Fisher Ltd. (PSF). Some speculated that this strong dependency on PSF could lead to mergers and might even pose a threat to Jaguar.

The car entered mass production on October 10, 1961, and was unveiled at the Earls Court Motor Show in the UK on October 18. While its debut wasn't a sensation, it certainly sparked interest among visitors and journalists. The car received generally positive reviews in the press. It was primarily intended for the American market, so it reached the domestic market only in March 1962. The price was around £2,200, which was quite reasonable.

In October 1961, Jaguar received an order worth £22,500,000 from North American distributors. Jaguar had exactly 12 months to build and deliver over 10,000 different cars, which required strict adherence to deadlines. However, unexpected production issues began to surface, largely due to worker protests, and these problems persisted throughout the year.

Leading automotive magazines *The Autocar* and *The Motor* were able to test the car only after 1962. While *The Autocar* published a highly favorable review, *The Motor* was more critical, using sharp language that drew the ire of the Coventry-based Jaguar group. This led to the magazine's editor issuing an apology to the company.

There is information suggesting that after acquiring Daimler in 1960, Jaguar upgraded the Daimler V8 4561 c.c. engine to a 5.0-liter version to install it in the Jaguar Mk X prototype and create a Daimler-based copy of the model. The resulting car was quite agile and maneuverable, but the project was ultimately shelved.

The official reason cited was that Daimler's production line was not suitable for mass production. However, the unofficial reason was that the resulting car was significantly better and faster than the Jaguar Mk X with its 3.8-liter engine, which could have seriously impacted Jaguar's sales.

By 1966, it was decided to give the Jaguar Mk X a minor facelift and rename it. Thus, on October 10, 1966, the revised car was unveiled at the London Motor Show under the name Jaguar 420G. The updated car differed little from its predecessor and performed even worse in terms of sales until production ceased in 1970.

By 1966, a total of 18,656 Jaguar Mark X cars had been built. Of these, the 3.8-liter versions included 9,129 right-hand drive and 3,848 left-hand drive models. For the 4.2-liter versions, there were 3,719 right-hand drive and 1,960 left-hand drive models.

Jaguar Mark X Overview

The Jaguar Mark X, launched in 1961, was Jaguar’s flagship luxury saloon, designed to compete in the high-end market, particularly in North America. It introduced a unibody construction—a first for Jaguar—giving the car a lower, sleeker profile. Built on a 120-inch wheelbase with 14-inch wheels, it combined elegance with advanced engineering.

The Mark X featured a revolutionary independent rear suspension, shared with the E-Type, providing exceptional ride comfort and handling. Initially powered by a 3.8-liter XK engine, it later offered a 4.2-liter option, paired with automatic or manual transmissions for versatile performance.

Though praised for its innovative design and driving dynamics, the Mark X faced challenges in production and sales. It was succeeded by the Jaguar 420G in 1966, marking the end of an ambitious yet imperfect era for Jaguar’s luxury saloons.

Jaguar Mark X Technical Specification

Engine Options

Jaguar Mark X 3.8 (1961–1964)

Type: 6-cylinder inline engine

Displacement: 3781 cc

Power Output: 255 bhp at 5500 rpm

Compression Ratio: 8:1 (optional 7:1 or 9:1)

Features:

"Straight Port" cylinder head

Triple SU HD8 carburetors

Cast iron block, dry liners, and lightweight aluminum pistons

Jaguar Mark X 4.2 (1964–1966)

Type: 6-cylinder inline engine

Displacement: 4235 cc

Power Output: 265 bhp at 5400 rpm

Compression Ratio: 8:1

Features:

Improved torque delivery for smoother performance

Larger bore and stroke compared to the 3.8 engine

Transmission Options

Manual:

4-speed gearbox with optional Laycock de-Normanville overdrive

Gear Ratios: 1st (12.73), 2nd (7.01), 3rd (4.84), 4th (3.77), overdrive (2.93)

Automatic:

Borg Warner 3-speed transmission

Gear Ratios: Low (8.16–17.6), Medium (5.08–10.95), High (3.54)

Rear Axle: Hypoid axle with limited-slip differential

Suspension

Front: Independent suspension with torsion bars, telescopic dampers, and anti-roll bar

Rear: Independent suspension with coil springs and hydraulic dampers

Brakes

Disc brakes on all four wheels (Dunlop system) with servo assistance

Dimensions and Weight

Length: 5100 mm

Width: 1930 mm

Height: 1370 mm

Wheelbase: 3050 mm

Ground Clearance: 165 mm

Weight: ~1778 kg (varies slightly between 3.8 and 4.2 models)

Performance

3.8-liter: Top speed ~120 mph (193 km/h)

4.2-liter: Top speed ~124 mph (200 km/h)

Both engines offered impressive acceleration for a luxury saloon, with improved low-end torque in the 4.2.

Interior Features

Luxurious leather upholstery

Polished walnut wood trim

Adjustable front and reclining rear seats

Optional air conditioning

Comprehensive dashboard with tachometer, speedometer, and auxiliary gauges

Fuel System

Dual fuel tanks (20 gallons/91 liters each)

Electric fuel pumps with a dashboard switch

Updates with the 4.2 Model

Enhanced braking and cooling systems

Refined transmission options for smoother operation

Slight updates to the interior for improved comfort

Jaguar Mark X Models

Jaguar Mark X 3.8-liter

Jaguar Mark X 4.2-liter

Jaguar Mark X Photogallery