Jaguar showed the XK120 at the Earls Court Motor Show in October 1948, intending a limited run to carry the new twin-cam engine while the saloon it was really built for was finished. It became the fastest production car in the world, and Jaguar built 12,056 of them.
This page is here because the XK140 cannot be understood without it. Where a figure below differs from the XK140's, the difference is the point.
The aluminium cars, and why everyone gives a different number
The first XK120s had wood-framed bodies panelled in aluminium, built at Holbrook Lane, Coventry, by methods closer to coachbuilding than to production. When the car proved far more popular than intended, Jaguar re-tooled in pressed steel.
Ask how many aluminium cars were built and you will be told 242, or 240, or about 200.
240 cars were built, from 242 allocated chassis numbers. John Elmgreen, working from the original factory chassis records, gives the explanation and it is exact: two numbers in the alloy block were not alloy cars. 660010 went to the experimental department as a bare chassis. 670172 carried the prototype steel body.
| Block | Chassis block | Numbers | Alloy cars |
|---|---|---|---|
| Roadster, right-hand drive | 660001–660058 | 58 | 57 — less 660010, chassis only |
| Roadster, left-hand drive | 670001–670184 | 184 | 183 — less 670172, the steel prototype |
| Total | 242 | 240 |
This is the same distinction the archive established for the XK140: chassis numbers allocated against cars actually built. Chris Harvey's 242 is right about chassis numbers and wrong about cars. Elmgreen's 240 is the cars-built figure. The two are not in conflict; they answer different questions.
The "about 200" is the odd one out, and it is on the Jaguar Daimler Heritage Trust's own website. It appears to be the planned production figure leaking into the built figure. See the note on the Trust below.
When the change happened is better defined by chassis number than by date, and even then two good sources differ by one car. Elmgreen has alloy ending at 660058 and 670184, steel beginning at 660059 and 670185. Rob Reilly has alloy running to 660059 and 670184, steel from 660060 and 670185. They agree exactly on the left-hand-drive boundary and differ by one on the right. We could not establish which is correct.
Dates given range from "early 1950" through April 1950 and April–May to May 1950. No Service Bulletin announcing the body material change exists in the 1948–54 bulletin list — consistent with a manufacturing change that had no service consequence, but it means no factory document stands behind any of those dates.
Chassis numbers
Six-digit blocks by body style, right- and left-hand drive sequenced separately, left-hand drive offset by ten thousand. Note the blocks are not in body-style order: the drophead sits numerically below the fixed head coupe, and both below the left-hand-drive roadster block. The closed-car blocks were evidently reserved early.
| Body style | Drive | Chassis range | Standard | SE | Cars |
|---|---|---|---|---|---|
| Roadster, aluminium | RHD | 660001–660058 | 57 | — | 57 |
| Roadster, steel | RHD | 660059–661176 | 1,069 | 47 | 1,116 |
| Roadster, aluminium | LHD | 670001–670184 | 183 | — | 183 |
| Roadster, steel | LHD | 670185–676438 | 4,049 | 2,206 | 6,255 |
| Fixed Head Coupe | RHD | 669001–669195 | 137 | 58 | 195 |
| Fixed Head Coupe | LHD | 679001–681485 | 1,730 | 753 | 2,483 |
| Drophead Coupe | RHD | 667001–667295 | 261 | 34 | 295 |
| Drophead Coupe | LHD | 677001–678472 | 763 | 709 | 1,472 |
Elmgreen's figures, from the factory chassis records. Four chassis are excluded as chassis-only rather than cars: 660010 to the experimental department, 670172 the steel body prototype, and 660750 and 660751, sent as bare chassis to distributors in New Zealand and Belgium.
The arithmetic checks, which is why this archive trusts the source. The right-hand-drive roadster block spans 1,176 numbers; less the three chassis-only cars gives 1,173, which is exactly 57 + 1,069 + 47. The left-hand-drive block spans 6,438, and 183 + 4,049 + 2,206 is 6,438. The body-style totals sum to 12,056 precisely.
The block start numbers are unanimous across every source: roadster 660001 and 670001, fixed head 669001 and 679001, drophead 667001 and 677001. Unlike the XK140, we found no club error in the XK120 chassis blocks.
Engine numbers, and the trap
| Prefix | From | Notes |
|---|---|---|
| W | W1001 | The original series |
| F | F1001, November 1953 | Factory — Service Bulletin 136 |
The W-to-F change is factory-documented, which makes it one of the firmest dates on this page. The highest F number is given as F4251 by one source and F4249 by another; neither is corroborated and we could not establish the true final engine number.
The suffix digit is the compression ratio — 7, 8, and 9 according to some sources though not all. Punctuation is inconsistent in practice: surviving cars carry W-1058-7, W1161-8 and F-1682-8S. Hyphenation is not diagnostic.
Now the trap, and it is the single most important thing on this page for anyone reading numbers across the two models.
On an XK120, a trailing S means Special Equipment. On an XK140, a trailing S means the C-type cylinder head.
Service Bulletin 109 of September 1952 introduced the Special Equipment XK120, identified by an S prefix to the chassis number and an S suffix to the engine number. An XK120 stamped -8S is a 180 bhp Special Equipment car, not a 210 bhp C-type car. That difference is worth money, and the mistake is easy to make.
The C-type head on an XK120 was unmarked. Service Bulletin 95A of April 1953 introduced it as an extra-cost option — revised porting, 1⅝ in exhaust valves, enlarged throats, supplied as complete assembly SD 1025 at £150 retail, including high-lift cams and rocker covers. No prefix or suffix records it.
| Marking | XK120 | XK140 |
|---|---|---|
| S chassis prefix | Special Equipment | SE with C-type head |
| A chassis prefix | does not exist | SE with standard head |
| S engine suffix | Special Equipment, 180 bhp | C-type cylinder head |
| C-type head | Optional from April 1953, unmarked | Marked by S |
| DN / BW suffixes | do not exist | Overdrive / Borg-Warner automatic |
"M" is the American name for Special Equipment, exactly as on the XK140 — an M-badged car and an S-prefixed car are the same thing. One source glosses M as "Modified". We could not verify that against any factory document, and this archive does not adopt it. See the myths section.
What the Special Equipment package contained: high-lift camshafts of 3/8 in against the standard 5/16 in, improved valves, revised distributor mapping, heavier torsion bars, an extra leaf in the rear springs, wire wheels and a dual exhaust. Bulletin 109 says eighteen items were incorporated; the transcription does not enumerate them, and we could not obtain the full list.
The three body styles
The XK120 launched as a roadster only. The Fixed Head Coupe arrived in 1951 and the Drophead Coupe in 1953. That is worth noting against the XK140, which offered all three from its announcement in October 1954.
Neither the XK120 coupe nor the drophead had rear seats. The occasional rear seating in the closed XK140s was a first for the model line.
How many were built
12,056 cars, from 12,059 allocated chassis numbers.
| Body style | Cars |
|---|---|
| Open Two Seater | 7,611 |
| Fixed Head Coupe | 2,678 |
| Drophead Coupe | 1,767 |
| Total | 12,056 |
By drive side: right-hand drive 1,663, left-hand drive 10,393. Of the whole run, 3,807 were Special Equipment — roughly 32 per cent. That Special Equipment breakdown appears to be unique to Elmgreen; no other source publishes it.
Five totals are in print: 12,045, 12,055, 12,056, 12,059 and 12,061. The allocated-against-built distinction explains 12,059 and 12,056. It does not explain the other three.
- 12,055 is one less than the built figure, and its components differ in both directions rather than systematically. It looks like transcription drift from a shared source we could not identify.
- 12,045 comes from a Wikipedia table that carries no citation at all and is driven almost entirely by a fixed-head figure of 2,477 against everyone else's 2,483. Unlike its XK140 table, it names no source, and it contradicts Wikipedia's own infobox. It should not be cited as Heritage Trust data.
- 12,061 is on the Heritage Trust's own website. It exceeds the total number of chassis numbers in the six blocks, and we cannot reconcile it with anything.
Specification
| Item | Figure |
|---|---|
| Engine | Twin overhead camshaft straight six, iron block, alloy head, hemispherical chambers |
| Bore and stroke | 83 × 106 mm (3.2677 × 4.1732 in) |
| Capacity | 3,442 cc — see the note on the XK engine page |
| Firing order | 1–5–3–6–2–4 |
| Normal oil pressure | 40 psi |
| Carburettors | Two SU H6, standard and Special Equipment alike |
| Specification | Compression | Power | Torque |
|---|---|---|---|
| Standard | 8:1 | 160 bhp at 5,000 rpm | 195 lb ft at 2,500 rpm |
| Low compression option | 7:1 | — | — |
| Special Equipment / M | 9.0:1 | 180 bhp at 5,300 rpm | 203 lb ft at 4,000 rpm |
| With C-type head, from April 1953 | — | 210 bhp at 5,750 rpm | 213 lb ft at 4,000 rpm |
All gross figures, by the convention of the day. Note that the C-type head figure is identical on the XK120 and the XK140 — as it should be, since it is the same head.
The compression options make sense in their fuel context: 8:1 was for 80 octane, and the 7:1 option existed for the home market where 70 octane was what there was. These are pool-petrol-era ratings.
One source gives the Special Equipment as 190 bhp. Six others give 180, and 190 is the XK140's standard figure — this looks like contamination between the two models.
Transmission and running gear. Moss four-speed. Rear axle standardised from 3.77 to 3.54 in April 1953; earlier cars ran an ENV axle, later cars Salisbury. Suspension is independent at the front on torsion bars, live axle on semi-elliptic leaf springs at the rear. Steering is by Burman. Drums on all four wheels.
What changed, 1948 to 1954
From the factory Service Bulletins. A late XK120 is a materially different and better car from an alloy car of 1949.
| Date | Bulletin | Change | From |
|---|---|---|---|
| Nov 1949 | 57 | Front track rod bearings: split pins to plain nuts with tab washers | 660015 / 670025 |
| Apr 1950 | 65 | Valve timing now requires a special gauge instead of keyway alignment | engine W.1124 |
| Apr 1950 | 66 | Running-in throttle restrictor; front brake air scoops with blanking plates | — |
| Apr–May 1950 | none found | Aluminium body to pressed steel | 660059 or 660060 / 670185 |
| Aug 1950 | 71 | Air cleaners standard, offered free to earlier owners | — |
| Jan 1951 | 89 | Brake linings Mintex M.15 to M.14 | 660551 / 671097 |
| Mar 1951 | 90 | ENV rear axle casing revised | JHS.1603 |
| 1951 | — | Fixed Head Coupe introduced; wire wheels optional | 669001 / 679001 |
| Jun 1951 | 95 | Competition Modifications bulletin — engine, exhaust, clutch, suspension, brake and axle options for racing | — |
| Feb 1952 | 104 | New short-mainshaft gearbox, SL/JL series. Not interchangeable with SH/JH | 660935 / 671797 |
| Feb 1952 | 101 | Salisbury rear axle (3.77) replaces ENV on some cars | after 660935 / 671797 |
| Apr 1952 | 106 | Self-adjusting front brakes and tandem master cylinder | 660980 / 672049 |
| May 1952 | 107 | Rims 5 in to 5.5 in; track up half an inch front and rear | — |
| Jun 1952 | 108 | Newton front dampers revised; Girling rear dampers | 660980 / 672049 |
| Sep 1952 | 109 | Special Equipment introduced. S chassis prefix, S engine suffix, eighteen items, high-lift cams, rear track 4ft 2in to 4ft 3⅜in | S-prefixed cars |
| Dec 1952 | 114 | Synthetic enamel paint process begins | body F 5272 / J 2375 |
| Dec 1952 | 115 | Stiffer rear springs (C.5721) | 661040 / 673320 |
| 1953 | — | Drophead Coupe introduced; chrome wire wheels optional | 667001 / 677001 |
| Apr 1953 | 95A | C-type cylinder head option — revised porting, 1⅝in exhaust valves, assembly SD 1025, £150 | option |
| Apr 1953 | 95B | Close-ratio gearbox option; intermediates 1.982/1.367 to 1.74/1.21 | option |
| Apr 1953 | 122 | Rear axle standardised 3.77 to 3.54 | 661054 / 673695 |
| May 1953 | 123 | Crankshaft damper cast iron to malleable iron | engine W.8381 |
| May 1953 | 124 | Lightened flywheel standard | engine W.8275 |
| Jun 1953 | 127 | Brake linings Mintex M20 on late cars | 661087 / 674076 |
| Nov 1953 | 136 | Engine prefix W to F, from F.1001. Champion N8B plugs for 8:1 engines from F.1050 | engine F.1001 |
| Apr 1954 | 146 | Modified inlet valve with depression in head | engine F.2365 |
The shape of the run. Two clusters dominate. Mid-1952 is effectively one co-ordinated chassis update — new gearbox, self-adjusting brakes with tandem master cylinder, wider rims and track, revised dampers front and rear, and the Special Equipment option. April to June 1953 is the performance update — C-type head, close-ratio box, higher final drive, lightened flywheel, better crank damper, better linings.
What the XK140 changed
| Change | The detail |
|---|---|
| Steering | Burman box to rack and pinion by Alford & Adler. The largest single improvement, and the one owners name first |
| Cockpit space | The engine, bulkhead and dash all moved forward, gaining three inches of legroom. The wheelbase is unchanged at 102 in — the room came from repackaging, not a longer chassis |
| Power | 160 bhp standard to 190 bhp standard. The C-type head figure, 210 bhp, is the same on both |
| Rear dampers | Lever-arm to telescopic — at the rear only. The XK120's front dampers were already telescopic Newtons |
| Bumpers | Full-width bumpers with overriders, replacing the XK120's small separate bumperettes |
| Overdrive | Laycock de Normanville, chassis suffix DN. The XK120 had none |
| Automatic | Borg-Warner from 1956, chassis suffix BW — the first on any Jaguar sports car. The XK120 had none |
| Rear seats | Occasional seating in the closed cars, from a lengthened roofline. No XK120 had any |
| Body styles | All three from launch, against the XK120's roadster-then-coupe-then-drophead |
| Size | Longer, 176 in against 173 in, and wider, 64.5 in against 61.5 in |
| Doors | Genuinely different construction — see below |
| Brakes | Unchanged in principle. Drums on both. Discs did not arrive until the XK150 |
What this page corrects
"The XK120 had aluminium doors." True — and this is the one place where the XK120 and the XK140 invert.
XK120 roadster doors were aluminium throughout the entire run, on alloy and steel-bodied cars alike: aluminium skins on an aluminium hinge face, with steel only in the detachable hinge mount plates and fasteners. The alloy-bodied cars additionally had timber framing inside the doors; the steel-bodied cars did not. The bonnet and boot lid also stayed aluminium on steel-bodied cars.
This archive's buying guide records that aluminium bonnets and boot lids are "XK120 lore" that does not apply to the XK140. That conclusion is now confirmed: it is XK120 fact and XK140 fiction. The XK140's coupe and drophead doors had steel outer skins over timber frames. Roger Payne makes the connecting observation — that late XK120 coupe and drophead doors already had steel skins, which suggests there was never an all-aluminium XK140 closed-car door.
The practical consequence is the opposite of the XK140's. On an XK120 roadster, a magnet test on a door, bonnet or boot lid is meaningful; those panels should be non-magnetic on any XK120 of either body era. On an XK120 coupe or drophead it tells you little, the same trap as on the XK140. We could not establish whether XK120 closed-car doors were steel-skinned from the start of coupe production — Payne's wording implies a change during it that we could not date or find a bulletin for. That question is open.
Why aluminium at all? Two explanations circulate, neither documented: post-war steel shortage and export quotas, or surplus wartime aircraft stock. The first is at least historically plausible. Both are unsourced tradition and this archive does not assert either.
"S on an XK120 engine number means a C-type head." False. It means Special Equipment. See above.
"Special Equipment cars had a different block." False. Age for age a standard block is the same as a Special Equipment block. The package is about the head.
"The XK120 did 132.6 mph in standard trim." Heavily qualified. At Jabbeke on 30 May 1949 the car recorded 126.448 mph with screen and hood erect and 132.596 mph with the screen removed, driven by R. N. V. Sutton and timed by the Royal Automobile Club of Belgium. Later writers add further modifications the period report does not mention. The honest figure for a road-equipped XK120 is 126.448 mph — which was still the highest speed ever attained by a production car, and nearly 22 mph faster than the Healey record it beat.
"The rear dampers were Armstrong." They were Girling.
"The XK140 fixed the XK120's brakes." It did not. Both cars had drums throughout. What did improve the XK120's brakes was April 1952, with self-adjusting fronts and a tandem master cylinder — so a late XK120 has better brakes than an early one, which is more useful to an owner than a blanket condemnation of the model.
"M stood for Modified." Unverified, like "Mechanically Complete" and "Means Competitive" for the XK140's MC. M demonstrably denoted the Special Equipment specification in the American market on both cars. This archive states that and leaves the letter unexpanded.
On the track
Jabbeke, 30 May 1949. 126.448 mph over the flying mile with screen and hood erect, 132.596 mph with the screen removed, 132.362 mph over the flying kilometre.
Silverstone, 30 August 1949 — the XK120's first race win, in the one-hour production car race.
Le Mans, the Montlhéry endurance runs, Stirling Moss's Tourist Trophy win of 1950, and Ian and Pat Appleyard's Alpine Rally record in NUB 120 followed. The XK120's competition record is well documented elsewhere and this archive does not attempt to duplicate it.