Jaguar XK120/140/150, Frequently Asked Questions

Buying

Which XK model should I buy as my first Jaguar sports car?

The XK140 FHC (Fixed Head Coupe) is the best entry point. It has the improved rack-and-pinion steering that makes it significantly easier to drive than the XK120, disc wheels that require less maintenance than wires, a fixed roof for year-round usability, and prices that are more accessible than the roadsters. It’s also the most practical body style, a proper boot and wind-up windows make a real difference. Once you understand the car, you can trade up to a roadster or an XK150S if the bug bites.

How much should I budget for purchase and first-year costs?

Beyond the purchase price, budget 15-25% of the car’s value for the first year. A $120,000 XK140 FHC might need $15,000-30,000 in deferred maintenance, sorting, and unexpected repairs in year one. This is not unusual, most XKs change hands because the previous owner deferred expensive work. Get a thorough pre-purchase inspection and factor the findings into your budget, not just the purchase price.

What’s the difference between an alloy-bodied and a steel-bodied XK120?

The first approximately 240 XK120s (1948-1950) had hand-formed aluminium body panels over an ash (wood) frame. From 1950, production switched to pressed steel bodies on the same chassis. The alloy cars are lighter (by about 100kg), rarer, and significantly more valuable. They can be identified by their body construction, the bonnet, doors, and boot lid are aluminium on all XK120s, but on alloy cars the wings, sills, and main body panels are also aluminium. A magnet will stick to steel but not to aluminium, this is the simplest test.

Are XK parts readily available?

Yes, remarkably so for a car this old. The major Jaguar parts suppliers, SNG Barratt, Martin Robey, David Manners, carry a comprehensive range. Mechanical parts (engine components, gearbox internals, suspension parts, brakes) are well served. Body panels are available in reproduction form but quality varies, trial-fit before committing. Interior trim (leather, carpet, hood material) is available from specialists. Rare items like original instruments, chrome trim pieces, and body-specific fittings can be harder to source and expensive.

Should I buy a restored car or a project?

Buy a restored car unless you have experience, a workshop, specialist contacts, and a large contingency budget. An XK restoration is a multi-year, $80,000-150,000+ undertaking. The economics rarely make sense, you’ll spend more on the restoration than the finished car is worth, unless it’s a rare variant (alloy XK120, XK150S 3.8). A good, sorted, driving XK will give you immediate enjoyment and, paradoxically, cost you less in the long run.

Driving and Ownership

Can I use an XK as a regular weekend car?

Yes, absolutely. A well-maintained XK is a genuinely usable car. The FHC body style is the most practical for regular use, but even a roadster can be driven regularly if you accept its limitations (no wind-up windows, less weather protection). Allow 15 minutes warm-up time, check oil and water before every drive, and keep the fuel tank full to reduce condensation. The main limitation is the drum brakes on XK120/140s, you need to adjust your driving style and leave more braking distance than you would in a modern car.

How do I drive a car with the Moss gearbox?

The Moss gearbox has no synchromesh on first gear, you must double-declutch to engage first while moving, or come to a complete stop before selecting first. In practice, first gear is only needed from standstill. The technique: clutch down, move to neutral, clutch up, blip the throttle to match revs, clutch down, engage first. Second, third, and fourth have synchromesh and can be used normally, though they benefit from deliberate, unhurried shifts. Rushing the Moss gearbox will damage the synchromesh rings.

What oil should I use in the XK engine?

20W-50 mineral or semi-synthetic oil. Do not use fully synthetic oil in an engine with original-type seals, the thin oil will find every gap. Castrol Classic XL 20W-50, Penrite Classic, or Valvoline VR1 20W-50 are all suitable. The XK engine holds approximately 8.5 litres with filter change. Change every 5,000 km or annually, whichever comes first.

How often does an XK need servicing?

More often than a modern car. Oil and filter every 5,000 km. Valve clearance check every 10,000 km (the XK uses tappet adjustment rather than hydraulic lifters). Points and condenser every 10,000 km (or fit electronic ignition). Carburettor synchronisation every 10,000 km. Greasing of all suspension and steering nipples every 5,000 km. Timing chain inspection every 30,000 km. Cooling system drain and refill every two years.

Can I fit electronic ignition?

Yes, and it’s one of the best modifications you can make. A Pertronix Ignitor or Lumenition unit replaces the points and condenser inside the existing distributor. It’s invisible from outside (preserving originality for show purposes), completely reversible, and it provides more consistent timing and stronger spark. Cost: $200-400. Fitting is straightforward.

What fuel should I use?

98 RON premium unleaded. The XK engine has a high compression ratio (8:1 standard, higher with SE or S-type heads) and requires high-octane fuel. Modern unleaded fuel is safe for the XK’s valve seats, the hardened exhaust valve seats fitted from the factory are compatible with unleaded fuel. However, if the head has been skimmed or rebuilt, confirm that hardened valve seat inserts have been fitted. Add a fuel system stabiliser if the car will sit for more than a month to prevent ethanol damage.

How do I prevent my XK from overheating in Australian conditions?

The original cooling system was marginal for British conditions and inadequate for Australian summers. Essential upgrades: recore the radiator with an uprated four-row core, fit an auxiliary electric fan (Kenlowe or Davies Craig) as a supplement to the mechanical fan, ensure the thermostat opens at 82 degrees C (replace if in doubt), use a quality coolant mix (50/50 with distilled water), and check that all hoses are in good condition with no internal collapse. With these measures, the XK engine will run happily in Australian conditions.

Technical

What’s the difference between the B-type, C-type, and straight-port heads?

These are different cylinder head designs used on the XK engine:

• B-type head: The standard production head with small valves and modest ports. Fitted to the XK120, XK140, and standard XK150. Produces 160-190bhp depending on tune.
• C-type head: Larger valves, improved ports, and a higher compression ratio. Originally developed for the C-Type racing car. Fitted to SE-spec XK140s and SE XK150s. Produces 210bhp.
• Straight-port head: The ultimate XK head, with straight intake ports (rather than the siamesed ports of the B and C heads), larger valves, and ports designed for triple carburettors. Fitted to XK150S models. Produces 250bhp (3.4) or 265bhp (3.8).

Can I fit disc brakes to an XK120 or XK140?

Yes, this is a common and well-documented upgrade. Several kits are available that use Jaguar components (typically from the XK150 or Mk2) to replace the front drum brakes with discs. The rear drums are generally adequate if properly maintained, most of the braking effort is at the front. Cost: $2,000-4,000 for a front disc brake conversion kit installed. This modification is reversible and widely accepted in the XK community, though concours judges may deduct points.

What’s the correct tyre size?

XK120 and XK140: 6.00 x 16 crossply (original) or 185VR16 radial (modern equivalent). XK150: 6.00 x 16 crossply or 185VR16 (early) and some late cars used 6.40 x 15. Radial tyres significantly improve handling and grip compared to crossply, and most owners fit radials unless preparing for concours judging. Avon Turbospeed, Pirelli Cinturato, and Michelin XAS are all appropriate choices.

How do I verify authenticity and matching numbers?

Jaguar Heritage Trust (JDHT) can issue a heritage certificate for any Jaguar by chassis number, confirming the original specification, engine number, body colour, trim, and equipment as delivered from the factory. This costs approximately $150 and is essential if you’re paying a premium for a matching-numbers car. The chassis number is stamped on the chassis frame (near the front suspension on XK120/140) and on the commission plate. The engine number is stamped on the top of the engine block, front right.

What’s the XK150S and why is it special?

The “S” designation (Special) was available on the XK150 from 1958. It featured the straight-port cylinder head, triple SU HD8 carburettors, a lightened flywheel, and revised cam profiles. In 3.4-litre form it produced 250bhp; in 3.8-litre form (from 1959) it produced 265bhp. The XK150S was the fastest road-going Jaguar until the E-Type arrived. The 3.8S is the most desirable and valuable of all standard-production XK sports cars. Only a few hundred 3.8S roadsters were built, making them extremely rare and correspondingly expensive.

Value and Investment

Are XK values still rising?

The XK sports cars are established blue-chip classics. Values rose dramatically during the 2005-2015 period, stabilised, and have resumed gradual appreciation. The most dramatic increases have been in the XK120 roadster (particularly alloy-bodied cars) and the XK150S 3.8. FHCs and DHCs have appreciated more slowly but from a lower base. The long-term trajectory is upward, these are historically significant cars with finite supply.

Which XK is the best investment?

From a pure investment perspective: the XK120 alloy-bodied roadster and the XK150S 3.8 roadster. Both are rare, historically important, and in strong demand from serious collectors. However, if you’re buying an XK primarily as an investment, you’re doing it wrong. Buy the car you want to drive, maintain it properly, and enjoy it. The investment return is a pleasant side effect, not the primary purpose.

Does originality matter?

Yes, enormously, at the top of the market. A matching-numbers, original-specification car in excellent condition will always be worth significantly more than a restored car with non-matching components. However, for a driving car in the middle of the market, sympathetic modifications (disc brake conversion, electronic ignition, alternator conversion) are widely accepted and can actually make the car more enjoyable and usable without significantly affecting value.

What documentation should an XK come with?

Ideally: Jaguar Heritage certificate (confirming original specification), chassis plate, any surviving original paperwork (log book, handbook), comprehensive service and repair receipts, photographs of any restoration work, and provenance documentation (previous owners, competition history if any). For cars valued above $200,000, provenance and documentation become critically important.

How do XK prices compare internationally?

XK values are broadly similar across major markets (UK, USA, Europe, Australia), but exchange rate fluctuations create opportunities. Australian-delivered cars sometimes carry a slight premium domestically due to provenance, but an identical UK or US car is just as desirable. The main price differentiator is condition, specification, and documentation, not country of origin.