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. 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 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 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.
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. 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.
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 alloycylinder head, hemispherical combustion chambers, inclined valves and twin side-draft SU carburetors, the dual overhead-cam 3.4 L straight-6XK 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). 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, later modified into 3.8 and 4.2 litre versions, survived until 1992.
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.
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 optional 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.
The Motor magazine road-tested an XK120 in November 1949. This pre-production car, chassis number 660001, road-registered as HKV 455, was the first prototype built. It was also the 1948 London Motor Show display model, and had been driven by Prince Bira in the 1949 Silverstone Production Car Race. When tested, it had the 8:1 compression ratio, was fitted with an undertray, and ran with hood and sidescreens in place. The magazine reported a top speed of 124.6 mph (200.5 km/h), acceleration from 0–60 mph (97 km/h) in 10.0 seconds and fuel consumption of 19.8 miles per imperial gallon (14.3 L/100 km; 16.5 mpg‑US). The car as tested cost £1263 including taxes.
Racing and rallying
XK120s were active in racing and rallying:
First race victory: In the Daily Express-sponsored One-Hour Production Car Race held on 30 August 1949 at Silverstone Circuit, England, Leslie Johnson drove the Jabbeke car to the XK120's first-ever race victory (despite an early collision with a spinning Jowett Javelin which dropped the Jaguar to fifth). The car, road-registered HKV 500, was converted to right-hand drive for Silverstone. Two other XK120s took part. One, driven by Peter Walker, finished second and the other, driven by Prince Bira, spun out of contention when a tyre punctured.
First victory in America: In January 1950 Johnson also scored the model’s first competition success in America, winning the production class in a race at Palm Beach Shores, Florida with the car that had finished second at Silverstone. The Jaguar lost its brakes but finished fourth overall. John Lea, Jaguar’s Experimental Department mechanic who attended the race, reported: "The conditions at Palm Beach were wet, windy and sandy. Water and sand gained entry into the brake drums at the front, and the mixture had the effect of accelerating the wear very considerably. Our car finished with no linings and with the steel shoes bearing on the brake drums."
Pebble Beach Cup: In May, XK120s driven by Phil Hill and Don Parkinson finished first and second in this event at the inaugural Pebble Beach Road Races in 1950.
Le Mans: Three of the allocated cars, extensively modified, entered the 1950 Le Mans 24 Hours race. Johnson, who spearheaded this factory-supported assault on the race with co-driver Bert Hadley, never ran lower than seventh place, and held second for two hours, but in the 21st hour had to retire from third place with clutch failure caused by using the gears to slow the car in the absence of brakes. (As a result, the clutch was revised to a more robust design for production models.) The Jaguar had been closing the gap to leader Louis Rosier, whose Talbot's pace was significantly slower, at a rate that would have secured victory. Haines, with co-driver Peter Clark, finished 12th, and Walker’s car, driven by Peter Whitehead and John Marshall, was 15th. The results convinced William Lyons and William Heynes Chief Engineer it was worth investing in future success at Le Mans.
Mille Miglia: Johnson took fifth place in the Mille Miglia, with John Lea as his riding mechanic, while Biondetti and co-driver Gino Bronzoni finished eighth. Fifth was an outstanding achievement for a production car, with Johnson's Jaguar beaten only by Fangio's works Alfa Romeo and the works Ferraris of Serafini, Bracco and winner Marzotto. It was Jaguar's best-ever finish in the Mille Miglia; also the best by a British car and driver combination, a feat that only Reg Parnell ever equalled, driving an Aston Martin DB3 in 1953.
Silverstone Production Car Race: Five XK120s entered the race, which Peter Walker won from Tony Rolt, with Johnson recovering to eighth after spinning on oil. Jaguar won the team prize.
Tourist Trophy: XK120s also achieved a 1–2–3 victory in the TT, held at Dundrod in heavy rain. On the eve of his 21st birthday Stirling Moss drove Tom Wisdom's car to a brilliant win ahead of Whitehead and Johnson, and Jaguar once again took the team prize.
This 1950 XK120 won a Coupe des Alpes and a Coupe d'Or
NASCAR road race: In America, an XK120 FHC was the first imported car to achieve victory in NASCAR, when Al Keller won the first Grand National road race, held at Linden Airport, New Jersey, on 13 June 1954. Foreign made cars were banned from this series after this race.
High-speed runs and records
132.596 mph (213.393 km/h) through the flying mile: in May, Jaguar demonstrated an XK120 to the press on the high-speed autoroute between Jabbeke and Aalter in Belgium. The road was closed for the occasion. The white left-hand drive car, chassis number 670002, was the second XK120 built. Jaguar's development engineer Walter Hassan was to have driven but fell ill, so Jaguar test-driver Ron "Soapy" Sutton substituted. With hood and sidescreens erected, and the airflow under the car improved by the addition of a full-length aluminium undertray, the Jaguar was timed through the flying mile by the Royal Automobile Club of Belgium at 126.448 mph (203.498 km/h). With hood, sidescreens and windscreen removed, a metal airflow deflector fitted in front of the driver, and a tonneau cover fastened over the passenger side of the cockpit the speed improved to 132.596 mph (213.393 km/h). The Observer's Book of Automobiles said it was the fastest production car in the world. The Special Equipment 'S' series cars made in 1948-1949 had better compression ratios and were capable of top speeds of 128 mph.
107.46 mph (172.94 km/h) for 24 hours (including stops for fuel and tyres): Leslie Johnson sharing his XK120 JWK 651 with Stirling Moss at the Autodrome de Montlhéry, a steeply banked oval track near Paris. The first time a production car had averaged over 100 mph (160.93 km/h) for 24 hours. Changing drivers every three hours, the Jaguar covered 2579.16 miles, with a best lap of 126.2 mph (203.10 km/h).
131.83 mi (212.16 km) for one hour: Johnson solo in JWK 651 at Montlhéry. "No mean feat...driving at almost twice today's maximum (UK) speed limit into a steep turn, assaulted by the G-Force induced by 30 degree banking twice every minute, using Forties technology, leaf spring" (rear) "suspension and narrow cross ply tyres...Johnson remarked that the car felt so good it could have gone on for another week, an off-the-cuff comment that sowed the seed for another idea. Flat out for a week..."
This 1952 XK120 fixed-head coupė averaged 100 mph for a week
100.31 mph (161.43 km/h) for 7 days and 7 nights, again at Montlhéry: XK120 fixed-head coupé driven by Johnson, Moss, Hadley and Jack Fairman. William Lyons, mindful of the considerable kudos and advertising mileage that had already accrued from Johnson's exploits, commandeered a brand new XK120 FHC for him: bronze-colored, and fitted with wire wheels, it was Jaguar chief engineer William Heynes personal car, the second right-hand drive coupé made. The car broke a spring on the track's rough concrete surface when already well into the run. No spare was carried, and regulations stipulated that a replacement from outside would make the car ineligible for any further records beyond those already achieved before the repair. Johnson drove nine hours to save the other drivers from added risk while the speed had to be maintained on the broken spring. When he finally stopped to have it replaced, the car had taken the world and Class C 72-hour records at 105.55 mph (169.87 km/h), world and Class C four-day records at 101.17 mph (162.82 km/h), Class C 10,000-kilometer record at 107.031 mph (172.250 km/h), world and Class C 15,000-kilometer records at 101.95 mph (164.07 km/h), and world and Class C 10,000-mile (16,000 km) records at 100.65 mph (161.98 km/h). After the repair the car went on to complete the full seven days and nights, covering a total of 16,851.73 mi (27,120.23 km) at an average speed of 100.31 mph (161.43 km/h).
172.412 mph (277.470 km/h) through the flying mile, again at Jabbeke's closed-off highway. On 21 October 1953 Jaguar test driver Norman Dewis drove a modified XK120 to a two-way average flying mile 172.412 mph.
A 2-litre four-cylinder version of the twin cam XK engine was to have powered an XK100 variant of the XK120 for the UK market. Details of the model were included in an "Advance Particulars" brochure for the XK  but Jaguar's managers were dissatisfied with the engine and the project was cancelled prior to production.
^The Times, 31 May 1949
Ostend 30 May:
British Car's Speed Record
A Jaguar 3½-litre sports car . . . travelled at a timed speed of 132 mph on the Ostend-Jabbeke motorway today . . . The runs were timed by officials of the Royal Automobile Club of Belgium . . . moreover it was running on normal Belgian pump petrol and at the end of its high speed runs it demonstrated its ability to throttle down to 15 mph in top gear and to accelerate speedily without pinking.
Running with the hood up, the car averaged 126.4 mph for a mile in two runs in opposite directions. The fastest mean speed of 132.5 mph was reached with a racing windscreen in place, the best run being made at 133.2 mph. The car also covered a kilometre from a standing start at a speed of 74.1 mph and a mile at 86.4 mph.
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