This is not a technical treatise on Jaguar’s postwar inline six cylinder power plants, merely a clarification of the difference between the XK engine and the AJ6 that superseded it. All too often I’ve encountered enthusiasts who were unsure of the difference, or indeed, thought that later Jaguars were still being powered by the venerable XK. I have left out the OHV inline sixes of prewar origin as they were essentially Standard-Triumph engines and not Jaguar’s own design.


Jaguar cars are to the inline six as tea is to crumpets. Not always seen together, but inexorably linked. Sir William Lyons, Jaguar’s farsighted founder, had visions of a prestigious saloon capable of pulling “The Ton” for the postwar market. The existing range of engines, basically warmed up leftovers from Standard-Triumph, were not going to cut the mustard.

Few men had a better eye for style than Sir William. He worked with full size mockups of his cars until the body was eye-sweet and elegant. Jaguars had lavish interiors, acres of walnut and fragrant leather in the best British tradition. There were gauges for everything and Lucas switchgear sprinkled liberally on the fascia. Sir William’s attention to aesthetics extended to the engine bay, the power plant for his new saloon would also look “glamorous”. It would have to be as tough and powerful as it was handsome if Jag’s new super saloon was to succeed.

Sir William and his engineers often stayed up all night on fire watch to protect the factory from Luftwaffe bombers. All the while they brainstormed plans for the future, especially Jaguar’s new postwar engines. Various configurations were toyed with but the XK engine debuted in the XK120 sports car at Earls Court in 1948. It was a twin overhead cam inline six cylinder engine, the first one in regular series production. This was hot stuff ! In those days only real exotica had a camshaft living upstairs. The little Coventry firm had a new, powerful engine with two of them in an aluminum head on an iron block. Those cams worked bucket tappets actuating valves in hemispherical combustion chambers. Cutting edge tech. Jaguar managed to do it all on tooling Sir William acquired second hand from Standard-Triumph.

3.4 Litre XK Engine

The new engine delivered the goods, the XK120 sports car achieved 120 mph out of the box and Sir William’s super saloon, the MKVII could hit 100 mph plus with seating for five. The XK engine would power cars that were the fastest in the world in their time, fire trucks, tanks, ambulances, speed boats and even the hearse that would carry the body of Princess Diana to her final resting place. It remained in production until the early 1990’s, truly one of the greatest engines of all time.

The inline six that would supersede the XK had origins which were a bit more convoluted. It would be only the third engine developed by Jaguar. Development began in the early 70’s with single cylinder prototype engines and a stillborn four valve head for the XK. The need to amortize tooling costs had an affect on the design of the NJ6 (NJ for “New Jaguar”). The engine shares some commonalities with the iron block XK and Jag’s all alloy V12. The new engine has the XK’s main bearing dimensions and the V12’s bore center spacing. It would be the first Jaguar power unit with four valves per cylinder. Unlike ye olde long stroke XK, the AJ6 was nearly square as introduced in 3.6 litre capacity with a bore of 92mm and stroke of 91mm. There was much thought about developing a diesel version, thus the aluminum block was made very strong. Iron liners were employed instead of a “nikasil” treatment for the bores. This was probably wise and contributed to the engine’s durability. In 1980 the NJ6 began to be referred to as the AJ6 (AJ for “Advanced Jaguar”).

3.6 Litre AJ6

2.9 Litre SOHC AJ6

As in the past Jaguar chose to debut their new power plant in a sports model as preamble to a forthcoming saloon. In 1983 the XJ-S was offered with the new 3.6 litre AJ6 engine as well as the V12. The XJ40 saloon would debut in 1986, also fitted with the new engine in either 3.6 litre DOHC form, or an entry level 2.9 litre SOHC derivative with two valves per cylinder. There would eventually be 4.0 and 3.2 litre versions of the AJ6. An interesting aside, Aston-Martin used a supercharged 3.2 litre AJ6 in the DB7. The AJ6 would remain in production until 1996 when it was superseded by the smooth, powerful AJV8.

XJ40 (XJ-6) Saloon

3.6 Litre XJ-S

Leave a Reply

Your email address will not be published. Required fields are marked *