Six Cylinder XK Engine 1948 – 1992

3.4 Litre XK Engine
in the Trust’s XK120 – NUB 120

World War Two put a stop to private vehicle production and all manufacturers turned to war work.  SS Cars was no exception and they repaired aircraft as well as supplying sidecars and trailers to the armed forces.  William Lyons started planning a 100 mph saloon that would be available post-war which would need a new, more powerful, engine that would set a new standard in power plants and he also understood the importance of it looking attractive.  Lyons was fortunate to have three very talented engineers working for him: William Heynes, Walter Hassan and Claude Baily.  They spent the hours during fire-watching duties discussing the requirements and planning a series of engines.  In the pre-war years the small four-cylinder 1½ litre saloon sold in larger numbers than the six-cylinder 2½ litre and 3½ litre models.  Lyons foresaw a period of post-war austerity when there would be a greater demand for a smaller engine car. Consequently, both four and six cylinder versions were envisaged.

At the end of the war Jaguar Cars already had running a four-cylinder engine with a three-bearing crankshaft, a bore and stoke of 76 x 98 mm giving a capacity of 1,790 cc.  The testing on the 4 cylinder engine allowed them to develop the overall layout of: twin overhead cams, two valves per cylinder, hemispherical combustion chambers, polished cam covers, fuelled by twin SU carburettors.

A six-cylinder version of the XK engine was also developed, initially with a bore and stroke of 83 x 98 mm for a capacity of 3,181 cc.  This was run for the first time on 15 September 1947 giving a power output of 142 bhp at 5,000 rpm.  The six-cylinder layout proved better and smoother than the four cylinder and development work concentrated on this version.  The 3.2 litre six-cylinder engine offered no increase in power over the 3.5 litre pushrod engine that was already in use by Jaguar.  The stroke was lengthened to 106 mm to give a swept volume of 3,442 cc and power increased to 160 bhp at 5,000 rpm.  Jaguar now had a smooth-running, six-cylinder, double overhead camshaft (DOHC) engine and this was put into production.

XK120 Launch at the
1948 Earls Court Motor Show

The Mark VII saloon wasn’t ready for the 1948 London Motor Show so in a remarkably short time Lyons, assisted by Fred Gardner and William Heynes produced a low, two-seat, modern-looking and very stylish sports car – the XK120.  This was powered by a 3.4 litre ‘XK120’ engine with twin SU carburettors.

The XK engine would go on to power the Mark VII when it was launched at the 1950 Motor Show and many other Jaguar and Daimler cars for the next 40 years.

XK Engine production line

Development work on the six-cylinder version continued and even while the unit was in production it was being modified and improved.  Successes with the 3.4 litre XK-powered C-types at Le Mans increased Jaguar’s global sales and gave Jaguar an increase in revenue that enabled the development of a smaller saloon.

The 4 cylinder version of the XK engine was never considered smooth enough to power the smaller saloons and work commenced on shrinking the dimensions of the six cylinder XK engine.  In July 1951, tests were run on an engine with a bore and stroke of 88 x 66 mm for a capacity of 1,986 cc.  Figures record 113 bhp at 6,000 rpm, not quite what was hoped for by Jaguar.  However, by early 1954, an engine with a bore and stroke of 83 x 76 mm for a capacity of 2,483 cc was producing 155 bhp at 6,000 rpm on the test bench.  This was ideal for the small unitary construction saloon that was nearing completion and gave the options of the 2.4 and 3.4 litre engines.

D-Type engine with triple Weber carburettors in the Trust’s long nose D-Type 393 RW

This remarkable engine showed no signs of ageing and a 3.8 litre version was used by the very successful racing D-types in 1956.  This was used in the XK150S, the Mark IX saloon and for the 1961 E-type and Mark X saloon when it was fitted with triple 2” SU carburettors.

Mundy, Hassan, Heynes, and Bailey with an XK Engine with triple SU carburettors

By the mid-1960s Jaguar Cars were working on a new saloon and wanted more power from the XK, so the engine was further enlarged to 4.2 litres, again with triple SUs, and first unveiled for the E-type and 420/420G saloons in 1966.

In 1968 the new Jaguar XJ6 saloon was announced with the 4.2-litre engine and quickly established new standards across the automotive industry.

By now well over 250,000 XK engines had been manufactured and it was to remain in production to power the new generation of Jaguars throughout the 1970s and 1980s.

Sectioned 6 Cylinder XK Engine with fuel injection. On display in the Collection Centre at Gaydon.

What had begun during the long hours of fire watches during the war had progressed through several designs and re-designs to become one of the world’s truly great engines.  While Jaguar had enlarged the XK to 4.2 litres many enterprising engineers and race drivers would expand their XKs to even greater capacity.  Initially the engine used SU carburettors and also for some applications the Zenith-Stromberg was fitted, especially in the E-type for federal markets, but by the end of its life Lucas and Bosch fuel-injection systems had been developed for installation.

Other applications for the unit were considered by Jaguar and one of them was to adapt the 4.2-litre XK for military use.  Having passed stringent trials by the Army the XK was chosen for fitting to the Alvis Scorpion range of light tanks and tracked armoured vehicles.

The Scorpion, Scimitar, Samaritan and Samson army vehicles with Jaguar AJ60 (XK) engines have seen operational duty in Croatia, Bosnia, the Falklands and Iraq.

The last Daimler DS420 Limousine powered by the last production XK engine

The Jaguar XK engine may be over seventy years old and long out of production, almost 700,000 were built and specialists around the globe still rebuild, restore and fettle the adaptable engine for enthusiasts, collectors and every day owners who never cease to marvel at the power – and the distinctive sound – of a well-tuned six-cylinder XK engine.

The last production XK engine was installed in the last Daimler DS420 Limousine built at Browns Lane in 1992 – L420 YAC – now in the Jaguar Daimler Heritage Trust’s Collection.

Author: François Prins

© Text and Images – Jaguar Daimler Heritage Trust