The XK engine was one of Jaguar’s greatest assets. It re-launched the company in the immediate post-war era with the introduction in 1948 of the breathtaking XK120 . Interestingly. The XK120 was only designed to have a production run of just 200 cars to give Jaguars some sporting kudos prior to the introduction of the Mark VII saloon, for which the new engine had been designed.The engine exclusively provided power for all Jaguar sports and saloon cars until 1971 (when the V12 engine ran in parallel with it). The engine produced many Le Mans winners and powered one of the greatest luxury performance cars of all time - the XJ6.

A gamble? Well, on paper there were safer ways of obtaining the 150 bhp odd needed to give William Lyons the 100 mph luxury saloon he aspired to. SS Cars Ltd (as it was in the 30s and 40s) could have continued to use engines from other manufacturers, or a good solid overhead valve design could have been used. But instead Lyons chose the twin overhead camshaft route, encouraged by his chief engineer William Heynes. Work commenced , despite some understandable misgivings from the other two key engineering players at SS Jaguar, Walter Hassan and Claude Baily.
The seeds for the new engine were sown around 1943 when, with an eventual allied victory apparently assured, Lyons used the time spent fire-watching at the Coventry factory to spell out to his tiny engineering team his postwar ambitions for car production. Bill Heynes, like his boss a keen follower of motorcycle racing, chose the twin ohc engine because these dispensed with long pushrods, allowing higher engine speeds while also suiting the hemispherical combustion chambers he also favoured.

In the 1940s this was radical stuff. Previously nearly all twin ohc engines had been seen only in expensive, low-volume or pure racing cars. Yet here was William Lyons keen to use this exotic format in a big, soft, refined touring car which would be produced in relatively large numbers, and be driven everyday by mechanically inexpert owners who would expect to have their cars serviced by the local town garage. At first glance the risks seemed unjustifiable; no wonder Hassan and Baily initially argued that a good conventional pushrod design could produce the required horsepower with less fuss, less cost and less risk of big warranty claims.

But this is an instance of how the normally cautious William Lyons could, when he felt the occasion demanded it, depart markedly from the conventional. He knew the need to elevate the Jaguar car out of the "re-engineered Standard" bracket to which many still relegated it; he wanted a truly modern design that could be further developed to meet future requirements; and he wanted a power unit that would look visually impressive when the bonnet was opened.

From the drawing board came a succession of experimental engines, each tested and evaluated in a disciplined and highly systematic manner. The first recorded running example was a little 1,360 cc four cylinder codenamed XF; with its twin overhead cams running in an aluminium hemihead, cast iron block and twin SU carburettors, it carried all the essential features of the engine to be. It seemingly first ran on November 30 1944, when it gave 53.6 bhp at 5,000 rpm on 70-74 octane Pool petrol. XG (an opposed pushrod design like the BMW 328’s) and XJ followed. XJ was described by Heynes as "the true forerunner" of the eventual production engine. It was built in two main forms, an 80.5 x 98 mm, 1,996 cc "four" and an 83 x 98 mm, 3.2 litre "six", but with many variations on valve gear, camshaft drive and other features. Valuable lessons were learned, including (for quietness) a change from single to twin chain drive for the camshafts.

A 2 litre XJ had the honour of being the first of the new engines to appear in public. Loaned to Major Goldie Gardner, it propelled his streamlined MG special at 176.694 mph average over the flying kilometre, taking an international 2.0 litre category record. Not bad for first time out of the box! The unit was not super-tuned but did have a 12:1 compression ratio to help it produce 146 bhp at 6,000 rpm.

Further refined, the XJ evolved into the XK - and nearly, but not quite, the production engine. There was yet to be another even more dramatic last-minute change. At first, the original capacities of 2.0 litres and 3.2 litres remained, the former intended to replace the existing 1.7 litre pushrod engine and the latter both the 2.5 and 3.5 litre pushrod units. But it was found that low-speed was poorer than the old 3.5 litre, and so the stroke was changed from 98 mm to 106 mm, giving with the 83 mm bore a capacity of 3,442 cc. The XK engine had arrived.

This change appeared to have taken place quite late in 1948, and perhaps even after the XK120 - hurriedly built for the Motor Show that year - had been made. Harry Weslake, who for many years played a key role in helping Jaguar develop cylinder heads, claimed credit for the extra capacity.

The engine produced a fairly genuine 150-160 bhp at 5,000 rpm and in 1950 finally made it into the car it had really been designed for, the Jaguar MK VII. From then on there was no looking back, and this magnificent engine was at the heart of every Jaguar for decades afterwards.

In all its many forms and diverse applications, over 1,000,000 of these wonderful engines were made by Jaguar Cars Ltd.