The Jaguar XK engine

Every XK120, XK140 and XK150 used the same engine: a 3,442 cc twin-overhead-camshaft straight six, cast iron block, aluminium head, seven main bearings. It was the first mass-produced twin-cam engine of its kind, it won Le Mans five times, and it was still being built in the 1980s.

This page covers it as fitted to the three XK sports cars, with enough of the wider story to make sense of them.

Where it came from

The engine was the work of a team under William Heynes, Jaguar's chief engineer, with Walter Hassan and Claude Baily, and with Harry Weslake consulting on cylinder head gas flow. William Lyons set the requirement rather than designing anything.

The familiar story is that they worked it out on fire-watching duty on the factory roof during air raids — at Swallow Road, Coventry, in the Foleshill district, which is one place and not two despite different sources naming it differently. Lyons' brief is recorded as: smooth torque across a wide range, a minimum of 160 bhp, refinement fit for a luxury car, shared dimensions between capacities for manufacturing economy — and an engine that looked impressive.

Two things complicate the romantic version, and this archive would rather record them than smooth them over.

The first is what the twin-cam layout was actually for. The usual account has Heynes' engineering vision overcoming conservative colleagues. Against that, Walter Hassan — one of those colleagues, and usually credited as a co-designer — is reported from a recorded interview as saying they could have got the power required from a pushrod engine, and that Lyons specifically wanted it to look like a Grand Prix engine. If that is accurately reported, the twin cams were at least partly a styling decision. It is forum-reported testimony about an interview rather than the interview itself, and it should be weighed as such — but it is first-person testimony from a named participant, and it fits Lyons' documented criterion that the engine should look impressive. Hassan's autobiography, Climax in Coventry, is the source to check.

The second is the fire-watching itself. Hassan joined SS Cars in 1938, but The Autocar's account of his retirement has him working on aero engines for the Bristol Engine Company during the Second World War, returning to Coventry at the end of it. If that is right he was not in Coventry for much of the period the story describes. Hassan himself confirms the fire-watching sessions happened. No source we could find reconciles the two. It is a real crack in a much-repeated story.

What "XK" stands for

Nothing. X for experimental, and K for the eleventh design in the sequence. The convention is documented and it is not acronymic. This is the single most useful thing to know about the name.

The series ran alphabetically from XA. The early letters appear to have been paper studies only. Two were built and tested: XG, a four-cylinder pushrod engine with crossover pushrods after the BMW 328, and XF, a twin-cam four. XG was rejected on gas flow and noise, and the twin-cam route was taken. XJ was the immediate six-cylinder predecessor, and then XK.

The individual capacities are contested and this page does not give a set. Two accounts disagree materially about XF's size, and only one of them mentions XJ at all.

How it is built

On 3,441 against 3,442. The arithmetic — π/4 × 83² × 106 × 6 — gives 3,441.15 cc. Jaguar published 3,442, rounding up by a fraction of a cubic centimetre. Both figures are in circulation and they are the same engine, not two claims about it. Exactly computed, the capacity is 209.99 cubic inches, which is why "210 cu in" is so clean a number in American sources.

For completeness, the same calculation across the family, checked against the published figures — the 3.4 is the only version where Jaguar's published figure is a visible rounding:

Cylinder heads

This is the most confused area of the whole record, and the naming is part of the problem. The A, B and C head letters have nothing to do with the C-type and D-type racing cars — though the C-type head is named after the C-type car.

The C-type head

The best account comes from Roger Payne on jag-lovers, and it contains one fact that kills a common workshop claim: the casting itself is different, so an A-type head cannot be modified into a C-type head.

What differs: revised inlet ports cast in as an integral part of the casting, not machined in afterwards; and a revised valve guide and valve area that allows machining for bigger valves. Note that wording carefully — the casting permits bigger valves, the valves are a separate fitment. Beyond the casting, the gains came from bolt-on parts: valves, camshafts, and a revised inlet manifold.

Valve sizes and cam timing for either head are not established. The figures that circulate are unattributed. We could not obtain inlet or exhaust valve diameters for the standard or the C-type head from any acceptable source, and this page says so rather than repeating them.

Casting number C7707, superseded by C7707/1 in December 1955 (Service Bulletin 174).

How the factory said to identify one, from Service Bulletin 163 of February 1955: the C-type head is painted red between the valve covers and has a "Jaguar Type C" badge on each cover. That is contemporaneous and authoritative — and paint and badges are trivially faked. Early C-type heads also lack the cast-in "C" in the plug valley, so no single test is sufficient.

A fourth head almost nobody mentions

Service Bulletin 174 of December 1955 lists three heads modified together for reduced tapped-hole depth:

There was a "D type" head in the factory parts system, referred to under "XK.120 'D' All engines". It is not the same thing as the straight-port head. What it was, which cars received it, and how it related to the D-type racing car, we could not establish — but the bulletin is solid and the part number is recorded here in the hope that somebody knows.

The XK150 head question, unresolved

For the XK120 and XK140 the position is clear: standard cars had the A-type head, the C-type was an option. For the XK150 three sources say three incompatible things:

• The B-type head was on the XK150 3.4 and 3.8 generally, with straight-port on the S.
• The earliest XK150 3.4s carried the standard A-type head and the B-type came in during the run — this from jag-lovers' own head-swapping chart.
• The B-type was on the 3.4 SE only, from launch in May 1957.

Roger Payne states that the earliest B-type head, casting C12500, was introduced with the standard 3.4 XK150, the Mark VIII and the 3.4 Mark 1 in 1957, and was superseded after a couple of years by C14956. That supports the first account and cuts against the other two. Payne is on the whole the most reliable of the three sources; the chart that contradicts him is on the same site. We record the disagreement rather than settle it.

The straight-port head, and what it was worth

Named for ports that run straight rather than curved, and attributed to Harry Weslake. Fitted to the XK150S with triple SU HD8s.

The gain is smaller than usually claimed, and it is rev-dependent. On jag-lovers, one contributor cites factory data giving a straight-port 3.4 at 225 bhp against a standard 3.4 at 210 — a 15 bhp difference. Another independently estimates about 15 bhp while calling it speculation. More usefully, the extra torque arrives higher up: about 4,500 rpm for the 3.4 XK150S against 3,000 rpm for the B-type. The published torque peaks corroborate that — 216 lb ft at 3,000 rpm for the 3.4 SE against 240 lb ft at 4,500 rpm for the 3.4S.

One hard interchangeability rule: an induction pipe for a straight-port head will not fit an A, B or C head, or the reverse. Induction and water pipes also differ between the sports cars and the saloons of the same period, even where the engine is the same.

Compression ratios

The 9:1 question for the XK140 is genuinely open. Jaguar's launch bulletin lists 8:1 and 7:1 and mentions no 9:1. Later secondary sources add it, some specifically for the C-type head car. Against that, a C-type-head drophead sold at Bonhams carries engine number G8354-8S — C-type head, 8:1 compression. And the needle bulletin of June 1955 covers 7:1 and 8:1 and, again, does not mention 9:1.

The XK150 is the only one of the three for which the suffix scheme is documented as running to 9.

Engine numbers

Lead with the trap. On an XK120, a trailing S means Special Equipment. On an XK140, a trailing S means the C-type cylinder head. Same letter, different meaning, adjacent models. Anyone reading an XK120 "-8S" as a C-type-head car has it wrong — and Roger Payne is explicit that an XK120 "-8S" denotes the 180 bhp Special Equipment engine.

He adds something useful for owners: age for age, a standard XK120 block is the same as a Special Equipment block. The SE package is about the head.

What distinguishes a W-prefix XK120 engine from an F-prefix one, we could not establish. Both are listed and no source we reached explains the difference. It is a real gap. One single-sourced claim, recorded as unverified, is that some of the earliest XK140s used remaining XK120 F-prefix engine stock.

Do not confuse engine and chassis marks. On the XK140 the specification lives in the chassis prefix — A for Special Equipment with the standard head, S for Special Equipment with the C-type head — with DN for overdrive and BW for the Borg-Warner automatic as chassis suffixes. The engine number carries compression and head type only. This is set out in full on Jaguar XK140 chassis and engine numbers.

Where the numbers are. The Heritage Trust's own vehicle numbers locator gives the XK140 engine number as being on the right-hand side of the block, on the horizontal flange above the oil filter, and on the cylinder head, on the vertical surface at the front of the central valley. The head stamping matters: it is how a head is matched to its original engine, and it is the basis of C-type identification — surviving examples are stamped in the plug valley in the form "G1499 8S".

Carburation by model

Note. The XK120's Special Equipment package kept the H6 carburettors; the H8s came with the C-type head as a racing modification. Whether the XK140's Special Equipment package included H8s is disputed, and the factory's own wording points against it.

The HD is not simply a bigger H, and this matters if you own more than one XK. In the HD type the jet glands are replaced by a flexible synthetic rubber diaphragm. So an XK150 has a perishable rubber diaphragm where an XK140 has cork glands — a different service item and a different failure mode.

Running changes in the XK140

The factory Service Bulletins record a number of changes with engine-number breakpoints. Two are worth singling out.

The chain tensioner changed at engine G4431. Engines before that number carry the earlier arrangement. This archive's own subject car, WPK38, has engine G2909 — before the change — and its restoration record notes a broken bottom chain tensioning spring found on dismantling. The general knowledge predicted the specific fault.

The thermostat changed at engine G2841 in March 1955, from part 43570 opening at 60°C to C3731/1 opening at 73°C.

The full bulletin record is transcribed at jag-lovers. Note that these are notes on the factory documents rather than the documents themselves, so the transcription may be fallible — but the breakpoints are specific enough to check against an individual car.

What became of it

Five Le Mans wins: 1951 and 1953 with the C-type, 1955, 1956 and 1957 with the D-type.

After the sports cars the engine went into the Mark VII, VIII and IX, the Mark 1 and Mark 2 saloons, the S-type, the Mark X and the E-type, growing to 3.8 and then to 4.2 litres, and it was still in production long after the cars it was designed for had gone. Few engines designed in the 1940s were still being made forty years later.

What this page corrects

• "XK stands for something." It does not. X for experimental, K for the eleventh design.
• "You can modify an A-type head into a C-type head." You cannot. The casting is different.
• "The C-type head was worth 210 bhp." It was worth 210 bhp on the XK140 inlet manifold. On the XK120 manifold the same head gives about 200. That is the most plausible explanation of the 200-versus-210 argument that runs through the literature: two real figures for two induction specifications, not two guesses at one engine.
• "A trailing S means a C-type head." On an XK140, yes. On an XK120 it means Special Equipment. The letter was reused.
• "3,441 and 3,442 cc are different claims." They are the same engine. The geometry gives 3,441.15.
• "The straight-port head was worth a lot." About 15 bhp, and only higher up the rev range.