’ It is remarkable in many senses, because everybody else tried to copy it’
The brand new 3.8-litre XK engine block casting that you can buy directly from Jaguar Classic
need that stretch aspect in your studs.’
The 4.2 didn’ t see much use in official race action during its life – that has come later in historic competitions – but it did find a more practical home. Its durability and reliability saw it be installed in military vehicles – Scorpion light tanks and Scimitar armoured vehicles – plus the Dennis D600 fire engine. These 4.2s used a single Solex carb and a fortified distributor to make sure weather or debris didn’ t interfere with the ignition system.
By the mid-eighties, the more modern AJ6 had mostly replaced the XK, and the Series III XJ6 was the only Jaguar equipped with the old Heynes straight-six. In 1986 the production of that car ended, leaving just the Daimler DS420 XK-powered. This big limo soldiered on until 1992, meaning the XK was in service for 44 years and across six different decades.
‘ From 1948 it was a Haynes development, all the way through to at least 1974-ish.’ William confirms.‘ And then that same engine carried on throughout the whole range in main production until the late 80s. Engine-wise, it is remarkable in many senses, because everybody else tried to copy it. The Aston Martin straight-six is a direct copy, but a worse specification … a bad copy. The XK engine won Le Mans five times for a reason. And that wasn’ t necessarily down to power output; it was down to reliability.’
William is, quite clearly, extremely passionate about these engines.‘ If you cut my veins, Jaguar comes out of it, but it comes in lumps of XK engine. I really care about it, I really, really care.’ Such an attitude dictates his approach to working on these engines.‘ We brought all of our engine work in last year. I thought, as the grandson of the man who designed it, and we weren’ t building, it was absolute sacrilege.’
While big side draughts might look and sound great, for road cars, the standard SUs and air filter suit the XK well
’ It is remarkable in many senses, because everybody else tried to copy it’
Autumn / Winter 2026 The Engine Rebuilder 25