The XK120 was launched in open two-seater or (US) roadster form at the 1948 London Motor Show as a testbed and show car for the new Jaguar XK engine designed by William Heynes Chief Engineer. The display car was the first prototype, chassis number 660001. It looked almost identical to the production cars except that the straight outer pillars of its windscreen would be curved on the production version. The sports car caused a sensation, which persuaded Jaguar founder and Chairman William Lyons to put it into production.

Beginning in 1948, the first 242 cars wore wood-framed open 2-seater bodies with aluminium panels. Production switched to the 1cwt or 112 lb (51 kg) heavier[5] all-steel in early 1950. The "120" in the name referred to the aluminium car's 120 mph (193 km/h) top speed (faster with the windscreen removed), which made it the world's fastest production car at the time of its launch. In 1949 the first production car, chassis number 670003, was delivered to Clark Gable.


The first production XK120, chassis number 670003 originally owned by Clark Gable, at the 2012 Pebble Beach Concours d'Elegance. The XK120 was the world's fastest production car at the time of its debut.
The XK120 was ultimately available in three versions or body styles, first as an open 2-seater described in the US market as a roadster (OTS) then as a fixed head coupé (FHC) from 1951 and finally as a drophead coupé (DHC) from 1953, all two-seaters and available with Left (LHD) or Right Hand Drive (RHD). However, certain Special Equipment roadster and fixed head coupe cars were produced between 1948 and 1949 denoted by an 'S' preceding the chassis number. These Special Equipment cars were sold as an early production build for enthusiasts.[citation needed]

A smaller-engined version with a 2-litre 4 cylinder engine, designated the XK100, intended for the UK market was cancelled prior to production.

On 30 May 1949, on the empty Ostend-Jabbeke motorway in Belgium, a prototype XK120 timed by the officials of the Royal Automobile Club of Belgium achieved an average of runs in opposing directions of 132.6 mph with the windscreen replaced by just one small aero screen and a catalogued alternative top gear ratio,[note 1] and 135 mph with a passenger-side tonneau cover in place.[7] In 1950 and 1951, at Autodrome de Linas-Montlhéry, a banked oval track in France, open XK120s averaged over 100 mph for 24 hours and over 130 mph for an hour. In 1952 a fixed-head coupé took numerous world records for speed and distance when it averaged 100 mph for a week.

XK120s were also highly successful in racing and rallying.

Construction

1950 aluminium-bodied XK120 ex-Clemente Biondetti, has competition seats and aftermarket steering wheel; positions of tachometer and speedometer have been reversed
The first 242 production XK120s, hand-built with aluminium bodies on ash framing mounted on a steel chassis mostly copied from the Jaguar Mark V chassis using many of the same parts, were constructed between late 1948 and early 1950. To meet demand, and beginning with the 1950 model year, all subsequent XK120s were mass-produced with pressed-steel bodies. Aluminium doors, bonnet, and boot lid were retained. The DHC and FHC versions, more luxuriously appointed than the constantly exposed open cars, had wind-up windows and wood veneers on the dashboard and interior door caps.

With a high-temperature, high-strength aluminum alloy cylinder head, hemispherical combustion chambers, inclined valves[8] and twin side-draft SU carburetors, the dual overhead-cam 3.4 L straight-6 XK engine was highly advanced for a mass-produced unit of the time. Using 80 octane fuel a standard 8:1 compression ratio developed 160 bhp (119 kW).[2] Most of the early cars were exported; a 7:1 low-compression version, with commensurately reduced performance, was reserved for the UK market, where the post-war austerity measures then in force restricted buyers to 70 octane "Pool petrol". The Jaguar factory's access to 80 octane fuel allowed it to provide cars with the higher compression ratio to the press, enabling journalists to test the model's optimum performance in Belgium, on a long, straight stretch of road between Jabbeke and Ostend.

The XK engine's basic design by William Heynes, later modified into 3.8 and 4.2 litre versions, survived until 1992.

All XK120s had independent Heynes designed torsion bar front suspension, semi-elliptic leaf springs at the rear, recirculating ball steering, telescopically adjustable steering column, and all-round 12-inch drum brakes which were prone to fade.[10] Some cars were fitted with Alfin (ALuminium FINned) brake drums to help overcome the fade.


1951 XK120 racing at Silverstone has a single aero screen mounted behind the removable full-width windscreen
The open two-seater's lightweight canvas top and detachable sidescreens stowed out of sight behind the seats. Its doors had no external handles. There was an interior pull-cord accessed through a flap in the sidescreens when the weather equipment was in place. The windscreen could be removed for aeroscreens to be fitted.

The drophead coupé (DHC) had a padded, lined canvas top, which folded onto the rear deck behind the seats when retracted, and roll-up windows with opening quarter lights. The flat glass two-piece windscreen was set in a steel frame that was integrated with the body and painted the same colour.


XK120 drophead coupé
Dashboards and door-caps in both the DHC and the closed coupé (FHC) were wood-veneered, whereas the open cars were leather-trimmed. All models had removable spats ("fender skirts" in America) covering the rear wheel arches, which enhanced the streamlined look. On cars fitted with optional centre-lock wire wheels (available from 1951), the spats were omitted as they gave insufficient clearance for the chromed, two-eared Rudge-Whitworth knockoff hubs. Chromium-plated wire wheels were o

ptional from 1953. Factory standard 6.00 × 16 inch cross ply tyres were fitted on 16 × 5K solid wheels (Pre–1951), with 185VR16 Pirelli Cinturato radial tyres available as a later option.

In addition to wire wheels, upgrades on the Special Equipment (SE) version (called the M version for Modified in the United States) included increased power, stiffer suspension and dual exhaust system

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