This week’s technical article follows on from the last few weeks articles on great engines. So this week we have one of the all time greats – the 6-cylinder Jaguar XK double overhead cam. Developed by Jaguar after the war, it first appeared in one of the most iconic sports cars – the XK120. This engine is a favourite of mine thanks to my family owning several Jaguars culminating in a 4.2 litre Series 1 XJ6 during the 1970s.
In 1948 the XK120 hit the market with a 3.4 litre version of the engine developing 160 horses straight from the factory. The heads were made of aluminium with a cast iron block and the engineers spent time looking at the flow of the gases to improve the output. The main engineer, Harry Weslake went on to design Formula 1 championship winning engines for Vanwall.
The engine is a Hemi with 2 large valves per cylinder with a larger than normal head to accommodate the valves. Remember the bigger the valves the more fuel/air mix goes in and therefore a bigger bang! The 3.4 had a long distinguished life in many models including the replacement to the 120 – the XK140 and also the C-type, various saloons like the Mark 1 and 2 seen in classic British cops and robbers TV shows. The baddies always had a Jag! And it finally had a home in the XJ6 until about 1986 or so.
In 1958, Jaguar brought out a 3.8 litre version in the XK150 with up to 265 hp on tap. This engine had a shorter life being fitted to the D and E types plus the Mark 9, 10 and original S type saloons. It found its way into the smaller Mark 2 saloon and even the 1970s Panther J72 that was modelled on a 1930s SS Jaguar.
Jaguar also produced a smaller variant at 2.4 litres for the Mark 1 and 240 Mark 2, cars that I grew up with! These motors produced 133 hp and were in production from 1955 to 1967.
The 4.2 litre XK engine arrived in 1965 and had re-spaced cylinders to accommodate the larger bores in the same size block. This was the ultimate version of the XK engine and produced a minimum of 170 hp although with some simple mods and tuning it could get well over 200. The Mark 10 and 420 saloons had this motor followed by the XJ6 and Daimler variants. It was also fitted to Series 1 & 2 E-types and the Daimler Limo favoured by royalty and statesmen across the world. It also found itself in the Panther J72 and De Ville models – the De Ville was modelled on a Bugatti saloon.
Early versions of the Alvis Scimitar and Scorpion military vehicles used the 4.2 litre for it’s good power to weight ratio. Alvis and Jaguar were both made in the same city. The final version that didn’t last long was a 2.8 litre variant that was the entry-level engine for the Series 1 and 2 XJ6s.
My memories of our 4.2 are quite vivid – one day coming back from town, a valve decided to get cosy with a piston head resulting in no valve and a large hole in the top of the piston. I was still at school at the time doing metal work and as the piston was made of aluminium, I took it to school, melted it down and used it in a school project. I still remember my father pulling the engine out and rebuilding it.