Racing Green Cars

Jaguar XK 120, XK 140 and XK 150



It’s impossible to describe the impact made by the Jaguar XK120 just three short years after the end of World War 2. While Coventry was being blitzed Jaguar’s plans for post-war resurgence were already being laid, with the basis of a new engine – to be called XK – planned during fire watching duties. It would power a saloon car of unheralded luxury and performance. At that stage there was no thought of a sports car. Unexpected delays put back the production of the new saloon, leaving Jaguar with a revolutionary 3,442cc engine of six-cylinders, twin overhead camshafts set in an aluminium cylinder head but no car to be powered. Anxious to show the engine in public, a sports car was hurriedly designed and, in a matter of months, Jaguar produced a car of breathtaking beauty bringing light to those dark days. When shown to the press they reckoned the engine was more like that of a Grand Prix race car.

The Earls Court Motor Show in 1948 was to see a brave new world with the Morris Minor, Land Rover and Jaguar’s XK 120 all taking their bow. Jaguar arrived at the name by guessing the maximum speed (it proved to slightly exceed this) and then prefixing it by the engine type, hence XK 120. Confusingly the later models 140/150 are not named for their speed and, of course, there was no XK 130. The first 249 open two-seaters were made from aluminium and differ quite a bit from the later production cars. In reality manufacture really took off during 1950. Jaguar was caught out by the demand as it initially expected a limited production. The company was soon in full flow as steel bodied cars came down the line.

The XK 120 was a success straight out of the box. Its first race netted a one-two win at Silverstone and would have been three too had not a tyre burst. Be it rally or race the Jaguar XK 120 proved a winner. Many drivers had their first taste of success, including Stirling Moss and America’s first ever F1 World Champion Phil Hill. Perhaps the most successful XK 120 was ‘NUB120’ driven by Jaguar dealer and Sir William Lyons’ son-in-law Ian Appleyard (Ian married Lyons’ daughter Pat). The combination of husband and wife along with the XK proved almost unbeatable, winning many international events. Yet beneath the sublime body shape there lurked a relatively mundane car. The chassis was a modified saloon affair of ladder construction, while the live rear axle was hung on conventional half-elliptical cart springs. Up front Citroen inspired torsion bars provided the springing while the all-round drum brakes proved sufficient for the period. Steering was by recirculating ball steering box. While the engine was a masterpiece the Moss gearbox could trace its roots back pre-war.

Many motoring records were to fall to the Jaguar XK 120 in the early ’50s, not least the numerous speed/endurance records at the banked concrete circuit at Montlhery, just outside Paris. The scene of many successful attempts, the Jaguar XK 120 was to be the first production car to exceed 100 mph for 24 hours with a best average lap speed of 126.2mph. Driven by Leslie Johnson and Stirling Moss the car covered a total of 2579.16 miles. Buoyed by their success they returned and averaged 100.31 mph for 7 days and 7 nights!! An extraordinary feat when you consider the rough concrete surface of this historic banked circuit and the fact that the car covered 16,851.73 at this average speed over 7 days of hard endurance driving.

Three models were available, the open two-seater (colloquially known as the roadster), fixed-head coupe and drop-head coupe. The first car was somewhat spartan with sidescreens and a leather covered dash. Steel wheels (16-inch diameter) with full spats covered the rear wheel arches were standard wear but wire wheels became an optional extra in 1951. When the fixed head arrived in 1951 it featured a full wood-veneer dash and proper wind up windows. The drophead came in 1953, retaining the wood-veneer dash but incorporating a luxurious folding top of three-layer construction, giving virtual saloon like conditions when erected. Optional extras did exist and even a special equipment model was offered. Initially the engine produced 160 bhp, growing to 190 bhp for the SE. Should C-type specification be sought then 210 bhp was attainable.

In 1954 the Jaguar XK 140 arrived and, although very similar in appearance, only the rear wings were the same. The engine was moved further forward to give more internal space while rack and pinion steering proved a vast improvement over the steering box. The extra space also allowed an overdrive to be fitted to the gearbox. This was supplemented by an automatic gearbox in 1956, the first Jaguar ever to be offered with an automatic gearbox. All body styles were retained but all had more internal space with token seats in the coupes. The SE model featured C-type cylinder head, spot/fog lights and wire wheels.

The final incarnation was the XK 150 of 1957 and options really started to fly. All three body options were offered but significantly altered to have a higher waistline, wider radiator grille and full-width windscreen. For the first time, the open two-seater had wind-up windows. Mechanically identical to the XK 140, it initially had drum brakes and steel wheels as standard. Not long after launch, however, revolutionary disc brakes became optional, a joint development between Dunlop and Jaguar. Jaguar had initiated development of the new disc brakes on the C-Type in 1953 and in a relatively short time they were being offered for their road cars. They were a big leap forwards in technology, and almost certainly because owners were keen to display this engineering prowess wire wheels pretty much became de rigeur as well. Few 150s were sold in standard trim. The wood veneer had disappeared to be replaced by leather covering on all models, although the layout proved familiar. Heavier than the earlier models, Jaguar soon reacted to raise the performance levels and, in February 1959, it introduced the ‘S’ spec. Topped by a straight-port cylinder head with triple 2-inch SU carburettors the power was hiked from the standard 190bhp (210bhp SE) to an impressive 250bhp. But there was more than just power; a limited-slip differential and twin fuel pumps all added to the allure.

The 3.8-litre engine (accredited to Alfred Momo who developed the 3.8 for Briggs Cunningham) joined the range in October 1959 offered in both standard and ‘S’ specification. Power was 220 bhp and 265 bhp respectively. In 3.8 ‘S’ guise this engine was to prove the basis for the E-type in 1961. The XK 150 had a bewildering range of three body types, two engine sizes and two performance levels.

The Jaguar XK 140 and 150 did not share the competition success of the 120 as all manufacturers had moved to purpose-built racers, for Jaguar that meant the awesome C- and D-types. Combined they netted an impressive five Le Mans wins. Interestingly, a few styling houses attempted their own XK designs but none came close to the stunning original. Today, XKs in all formats are highly sought after.

There are many well restored XKs on the market but barn finds still keep appearing for the restorer. Simple and reliable with fabulous flowing lines the XK remains the iconic sports car of the 1950s and today still represents the best of Jaguar. Despite their age there really is no reason why these cars cannot be reliable and still represent a wonderful way to cover the miles.

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Click to view the specification for each model


XK Straight 6 cast iron block with alloy head, 2 valves per cylinder, twin overhead camshaft driven by timing chains



Max Power

160 bhp

@ 5100 rpm

Max Torque

195 lb ft

@ 2500 rpm


0 to 60 mph



Fuel economy



Max Speed




Double wishbones with torsion bars at front and live axle at rear mounted on leaf springs


Drum brakes all round


Manual recirculating ball steering box

Wheels and Tyres

Wire wheels with 6.00 x 16 tyres

Our View

The XK120 was to propel Jaguar out of the post war years and into a major manufacturer of iconic sports and saloon cars. The pre-war designs had been shelved and the future was to be based on cars with the looks and performance of this great car. However it almost never happened and it really only came about as a result of delays to the Jaguar Mk VII body so a Mk VII chassis was modified, a body built and this was used to showcase the new XK engine. The rest, as they say, is history.

The XK range, from 120 through 150 was to cover the great years of Jaguar both in engineering excellence, technological milestones and world class racing honours. As a sports car manufacturer, Jaguar was at the top of its game and to have been driving an XK in the ’50s really would have been comparable to driving a supercar today.

The competition version of the XK120, the C-Type or XK120-C was to win Le Mans at its first attempt in 1951 and was based on XK120 running gear with a lightweight chassis and aerodynamic aluminium body. C-Type specification options were to become available to purchasers of the road going XK120 and this was to cement the link between road car and racer. The XK120 was also to prove a great basis for a race car with the standard car gaining a one-two win at Silverstone on its first outing.

Whichever way you looked in the early ’50s the XK120 was winning races, rallies and Le Mans. With this pedigree the XK range was to develop through the XK140 to the 150 selling almost 30,500 cars in the process and contributing greatly to England as she tried to pull out from the shadows of the Second World War.


The first 242 of the Jaguar XK120s were effectively almost prototypes and were hand built from aluminium wrapped over an ash frame. This route was never going to meet the demand for these cars and by early 1950 the more recognisable steel bodied car had made its appearance. These still retained aluminium doors, bonnet and boot panels to keep the weight down.

Mounted on a traditional pressed steel ladder type chassis this form of car manufacture was typical of the era and was to remain with Jaguar through to the end of the XK range and the Mk VII-IX saloons.

The steel bodies were built up from pressed steel panels with a large element of hand finishing and fettling to create the final required shape. The pressed steel work was not that accurate, certainly when compared to today’s standards, and there are a number of external seams to the bodyshell that had to be filled or disguised to create the finished vehicle. Lead loading was the conventional approach to resolving a variety of bodyshell issues on a steel bodied car but it is not without its own inherent problems. Nowadays there are high specification fillers which are probably the better option but it is clearly down to the individual’s choice when repairing these cars.

When the bare steel bodyshell was jigged and welded at the Jaguar factory all the relevant external seams were lead loaded which effectively means they were soldered over. This process requires the use of an acidic flux to clean the steel and the relevant area is then “tinned” whereby a layer of solder is attached to the steel surface. This process requires heating the steel to the melting point of the solder or tin and then building up the tin to the required thickness to effect the repair. Where the “tinning” process is correctly achieved all is well but elsewhere acidic flux can remain under or within the repair. Once the lead loading was complete the required shape was formed by wiping the hot solder or by filing and sanding the cooled material with obvious health implications given the constituents of the materials involved.

What this process means in practice is that there are complex, relatively poorly formed steel joints that have been intensively heated and coated with an acid based flux which is not necessarily fully cleaned. All in all a perfect place for rust to start and get a grip of such a beautiful bodyshell.

There are lead loaded seams to the scuttle panel, boot surround, front wings and grille aperture and anywhere where the shape of the base panels was not the required shape of the finished car. In reality the glorious shapes of all the XK range were sculpted by hand by the engineers at Jaguar as that was really the only way in the ’50s. XK bodyshells were not given any level of rust protection apart from the external painting process so all the internal surfaces really were just waiting to rust away and only needed a damp climate to start this process.

Repairing an XK is a relatively straight forward process and all the panels are readily available with the exception, perhaps, of the bulkhead itself and some roof panels. Specialists have recreated all the panels that make up the bulk of the structure which is good news for the owners of cars where the dreaded rust worm has taken hold.

When looking at an XK then typical areas for corrosion are floor pans, sills, door skins, boot floor, A-post but there are projects available where virtually nothing is left of the body. The cars can rust badly as can the chassis but generally not to the point where the car cannot be rebuilt. Rain can get into these cars relatively easily so do check for damp carpets etc.

The Mighty XK Engines

The famed XK engine, in 3.4 litre and 3.8 litre variants, was a reliable unit and had been designed specifically for the new Jaguar Mk VII. However the XK120 had beaten the Mk VII to market and claimed this world class power unit.

With the design commenced during the war years the 3.4 litre XK engine was to prove to be a great engine and to remain in production with Jaguar until the early 1990s. In the early ’50s this was the engine to have in your sports car as it had just won at Le Mans which was about as high an accolade as was attainable. Imagine a similar situation today with a sports car being offered by a major manufacturer with a Le Mans winning engine under the bonnet!

The XK engine comprised of a cast iron block with alloy cylinder head with twin overhead camshafts operating 2 valves per cylinder in hemispherical combustion chambers. The crankshaft of EN16 steel was fitted with a Metalastic harmonic damper to remove damaging harmonic vibrations from the crankshaft. This design changed remarkably little over the next 40 plus years which really goes to show how good the original design and development process had been.

These engines are still regularly raced and can be totally reliable if well looked after or rebuilt correctly. Really all that should be needed is regular servicing but careful attention should be taken of the antifreeze or coolant.

Overheating the XK engine can lead to a number of problems typically blown head gasket or more seriously movement of the bucket guides in the head. This can be detected by a knocking noise from the camshaft area and generally on the exhaust side. This will require a cylinder head off rebuild and some machining operations to resolve. Low oil pressures tend to indicate a worn bottom end and will almost certainly lead to a full rebuild of the engine. Parts are all readily available.

Loss of coolant and subsequent low concentrations of antifreeze can lead to a cumulation of problems associated with the cooling system due to corrosion and build up of sludges and corrosion by-products. All XK cylinder heads were cast from RR50, an alloy developed by Rolls Royce during the war specifically for pistons for aircraft. This alloy was developed for high strength at high temperatures so is probably over specified for a cylinder head but one of its downsides is that it can suffer from significant corrosion to the water jacket. This can most easily be spotted at the interface with the cylinder head gasket where the waterways can corrode to the outside world in bad examples.

Internal corrosion to the blocks on Jaguars leads to underlying tendencies to overheat whenever the external temperature rise unduly, this is often put down to radiators, cooling fans etc but can remain as an unsolvable problem due to the fundamental nature of the corrosion.

Radiators, whilst they can become blocked and inefficient, are generally ok but again the problems always show themselves on hot days when the cooling system is pushed to its limits and struggling to keep core temperatures down.

There is no reason for XK engines to run hot providing they have been built correctly and have fully cleaned waterways.

PLEASE keep all Jaguars running on the correct levels of coolant. This is by far the best way to maintain the cooling systems and engines generally!!


The XK120 was fitted with the Moss gearbox and given the engine was such a modern masterpiece for the period, the Moss gearbox harked back to previous times as it was effectively pre-war. Originally manufactured by The Moss Gear Company, one of Jaguar’s many suppliers, manufacture was taken over by Jaguar and Moss seems to have disappeared by the late 1950s. The gearbox is a relatively simple 4 speed with no synchromesh on first or reverse. Reasonably strong in practise if a little agricultural it was to remain the standard equipment on the XK range. The XK140 was offered with overdrive and the option of an automatic gearbox, the first Jaguar to feature this option.

Clutches are again reliable but original coil spring type clutch cover is a definite candidate for replacement to a diaphragm type as fitted to later Jaguars.

Front Suspension and Brakes

Given that the XK120 was originally a showcase for the new Jaguar XK engine and was based on a modified Jaguar Mk VII chassis it should be no surprise to anyone that the suspension is virtually identical to a Mk VII saloon.

The front suspension on the all the XKs is virtually the same and is well designed and robust. The car is supported on torsion bars running parallel to the main chassis rails. Anchored onto the chassis under the footwells the torsion bars run forwards to the lower wishbones and effectively form the lower wishbone pivot. The chassis anchorage is fully adjustable and provides the facility to load the suspension, effectively setting the front ride height of the car.

The suspension arrangement is twin wishbone with an upright mounted on upper and lower balljoints. This design of front suspension was to remain with Jaguar for many years. All the components in the front suspension are easy and straight forwards to maintain and simply need a regular grease to maintain in good condition as are all from the era when grease nipples were fitted to everything.

The XK120 was slightly different to the later XKs insofar as it relied on a steering box and lever arm type shock absorbers but the XK140 moved on to a more conventional rack and pinion steering rack and cylindrical shock absorbers.

The steering boxes as fitted to the XK120 are reasonably well engineered but suffer from a number of faults. Racing Green offers an upgrade to the steering box replacing the inadequate ball bearings with tapered rollers and improving the feel tremendously in the process.

The XK range spanned great changes to the braking systems as fitted to Jaguars. The XK120 was supplied with drum brakes all round as was the mighty Jaguar Mk VII saloon but by 1953 Jaguar had developed brakes discs for the racing variant of the XK120; the C-type. Whilst the XK120 and XK140 were fitted with drums and it was possible to have drums on the early XK150s, shortly after its launch in 1957 Jaguar moved to disc brakes all round on this road car. This was a major technological advance and whilst the Dunlop/Girling individual caliper piston units are dated by today’s standards they were a monumental leap forwards on the roads of the ’50s.

On todays roads consideration should be given to more conventional calipers however XKs are still racing on the drum brakes and seem to be able to slow down! The drums are prone to brake fade and a period upgrade to the standard brake drums was the fitting of the aluminium finned Alfin drums which allows for more cooling and hence a reduction in brake fade.

Rear Suspension and Brakes

The rear suspension is again relatively conventional and is a simple live axle located on leaf springs running front to rear. Location is not great and there are modifications such anti tramp bars to provide for a bit more positive positioning of the live axle.

Rear brake information really is the same as that given above for the front brakes.

Exhausts, Electrics and Fuel

Exhausts are relatively simple and should fit reasonably well as they literally run down the centre of the car. The only problem is likely to be in the region of the downpipes and their proximity to the “chassis” rails.

The fuel systems are generally fine as long as the filters are regularly replaced and tanks kept clean. The fuel tank is external to the boot so it is not unknown for the debris of years to mount up and corrode its way into the tank.

Electrics are all relatively straight forward and period but there is no real reason why it cannot be reliable if well maintained.

In General

Unfortunately water always seems to find a way into older cars especially given the UK climate! Over time this leads to great damage on a number of fronts and avoiding this degradation is very beneficial to the longevity of any car. Water tends to pool in the lower areas of the car ie footwells and carpets but as the temperature rises on warmer days this moisture evaporates and fills the car with high humidity air. As the day cools in the evening this moisture condenses on the colder horizontal surfaces of the inside of the vehicle such as the inside of the roof and boot panels etc. This condensate then runs off and into sections of the car where water could not normally access and can cause significant problems over time. Bodyshell, electrics, trim and interior finishes can all suffer with this continual degradation over time.

Ideally try and keep all classics in a dry environment with windows slightly open to allow the air to circulate and reduce the moisture levels in the car. Dehumidifiers are also very beneficial over the longer term.

Interior is relatively straight forward but water entry, as described above, to the cabin can lead to seat stitching rotting and to headlining and carpets suffering. Recently we received an E-Type coupe which had been unused for a year or so but moths had taken up residence and had done a surprising amount of damage. All those natural fibres had kept them well sustained!

The Jaguar XK120 burst onto the scene in 1948 and quite literally took the world by storm. The range of XKs through the ’50s was the sports car to have and they are still sought after to this day. Prices have been steadily climbing and they are beautiful, iconic and fast and complement any occasion or garage. Some minor upgrades to the earlier brakes, ignition systems and starter motors should prove sensible and add a battery conditioner/charger to ensure it starts on those glorious days when the only place for an XK is on the open road.

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