Background
Thank goodness William Lyons wasn’t happy just building motorcycle sidecars, or you wouldn’t be reading these words now. More importantly, the Jag XK120 would never have come into being, and that would certainly make the world a less interesting place.
Luckily, the Swallow Sidecar Company did so well that Lyons decided to branch out into making coach-built sporting bodies on Austin Seven chassis, until in 1931 he built his own complete car. By the beginning of WW2 Lyons was already laying the plans for an exciting new double overhead cam engine which, when hostilities finally ceased, became the XK six cylinder.
1948 was a pretty grim time in world history, with nations struggling to rebuild in the aftermath of World War 2. So the backdrop for the XK120 wasn’t billionaires trying to become astronauts, it was bombed out streets, food rationing and 70 octane pool petrol. Then Jaguar’s chief engineer, William Haynes, lit up the greyscape with a stunning sports car featuring an exciting new engine that would go on to make history itself.
The XK120 was launched at the London Motorshow and was designed to showcase the new 3.4-litre six cylinder XK engine. With twin overhead camshafts, hemispherical combustion chambers and a lightweight alloy cylinder head, it had been designed to provide the Jaguar company with powerplants for many years to come, although it’s unlikely Sir William Lyons imagined it would continue in production until 1992 (it was last used in the Jaguar XJ6 Series 3 in 4.2-litre fuel-injected form).
Even in the early 1990s it was still an effective design, but in 1948 it was genuinely ground breaking. Motorshow reaction was overwhelmingly positive and Jaguar decided to launch the car immediately, with the bulk of production in left-hand drive form destined for the USA – where the first customer was former fighter pilot and Gone With The Wind star, Clark Gable.
The first 242 cars had ash framed bodies clad in aluminium panels but the practicalities of producing enough cars to satisfy ever-increasing demand meant that Jaguar swapped to an all steel body construction in 1950, adding a little more than 50Kg to the car’s overall weight. The chassis design was borrowed from the MkV saloon, and as such provided an immensely strong base for the lightweight sports car.
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, 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 100mph for 24 hours and over 130mph for an hour. In 1952 a fixed-head coupé took numerous world records for speed and distance, when it averaged 100mph for a whole week.
Independent front suspension was provided by Haynes designed torsion bars (a set up that would continue on to the E Type 11 years later), while at the rear, semi elliptic leaf springs supported the live axle. A steering box and 12 inch drum brakes also featured, the latter being prone to fade under race conditions.
The 120 refers to the top speed, specifically that of the first alloy bodied examples, as the extra weight of the steel cars did dull the performance very slightly. But not enough for anyone to really care – the XK120 was still the sports car to have as it was quicker, prettier and better than the competition. Talking of which…
It won its first race in August 1949, a one hour production car event at Silverstone. A year later it clinched its first production class victory in America at Florida’s Palm Beach Shores.
A year later Jaguar entered three cars in that year’s Le Mans 24 Hour, and driver Leslie Johnson with co-driver Bert Hadley never ran lower than seventh place, and held second for two hours. But in the 21st hour they 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. It was the XK120 and this race that convinced Jaguar to return to Le Mans (they won it the following year).
