The V12 engine

The V12 is the reason a Series 3 is a different car rather than a facelifted one. It is longer, heavier at the front and wider in the track than the six cylinder cars, and it asks different things of an owner. Most of what is written about it is repeated rather than sourced, so this page names where each figure comes from.

Where the engine came from

The V12 descends from the twin overhead camshaft racing engine built for the mid engined XJ13 prototype, which first ran in July 1964. That engine was a 5.0 litre unit with hemispherical combustion chambers, and quoted at 502bhp. It is not the engine that reached the E-Type.

Walter Hassan and Harry Mundy reworked the design into a road engine with a single overhead camshaft per bank. The reasons given at the time were practical rather than romantic. Motor Sport reported that the racing unit was unduly bulky for the E-Type and costly to produce, and that a single overhead camshaft operating in line vertical valves offered savings in cost, space and weight over a twin cam layout. Harry Mundy set out the full design history in a paper to the Institution of Mechanical Engineers in 1972, which remains the definitive engineering account.

So the racing programme is why the V12 exists. It is not what the V12 is.

The flat head, and why it is not a hemi

The production V12 has a flat cylinder head. The combustion chamber is formed in the piston crown rather than in the head, an arrangement usually called the Heron layout. Period description put the chamber as the space defined by the piston crown, the bore surface between piston and head, and the area of head within the bore periphery.

The advantages are simplicity of manufacture, compact dimensions, an accurately machined flat surface, simpler valve gear and good fuel economy. The cost is heavier pistons and volumetric efficiency poorer than a conventional head. Neville Swales, who builds replica quad cam V12s, records that the racing engine's hemispherical chambers flowed well but did not allow useful charge movement within the cylinder, so combustion suffered.

Jaguar did move away from the pure flat chamber later, adopting Michael May's Fireball design on the HE of 1981. That is seven years after the last E-Type was built and has nothing to do with the Series 3.

Architecture

The aluminium block saved 116lb against a comparable cast iron one. Note that the cylinders are numbered by bank, 1A to 6A and 1B to 6B, and not one to twelve. Any firing order you see written as a run of numbers from one to twelve is using a convention Jaguar did not, and at least one widely copied version of it contradicts the period record.

The archive has not established a home market compression figure of 9.1 to 1 that one American source gives for 1971 cars, against 9.0 to 1 in the period technical press. The two are probably the same specification quoted to different precision, but the archive states both rather than choosing.

Fuelling, and the injection that was cancelled

Every production E-Type V12 ran four Zenith Stromberg carburettors, two per bank, mounted outboard of the vee and feeding steeply angled inlet ports on the inner vee through plenums and inverted U shaped manifolds. Fuel came from twin SU AVF 106 electric pumps at 1.5lb per square inch.

The carburettors were not the plan. Roger Bywater of AJ6 Engineering, who worked in Jaguar engine development, states that the V12 was designed from the outset to have electronic fuel injection and that the system was cancelled at a late stage, with the four carburettor installation improvised until Bosch D Jetronic could be introduced. Jaguar's own position in 1971 was that the engine had been designed to accept petrol injection although it was not intended to introduce the feature for the present. Hassan and Mundy also conceded that the layout was forced partly by the non availability of a satisfactory British made downdraught carburettor.

D Jetronic reached the V12 on the XJ12 and XJ-S late in 1975, after E-Type production had ended. No factory injected E-Type V12 has been established. Injected Series 3 cars are aftermarket conversions.

Points that matter when working on them:

• Metering needles differ by market and by engine number. The parts catalogue lists B1BH for European cars to engine 7S8670, B1CN for European cars from 7S8671, and B1CQ for United States and Canadian cars from 7S8179.
• Throttle spindle wear is the classic cause of an idle that will not settle, and the catalogue lists three different spindles for different positions, so they are not interchangeable.
• Diaphragms perish and are the usual reason a rebuilt carburettor transforms a car.
• Balancing is across four units rather than two, and the interconnecting rods have to be balanced as well as the carburettors themselves.
• Hot start failure caused by fuel vaporising in the carburettors and fuel lines is a recognised Series 3 complaint, addressed with heat shielding and rerouting.

Ignition, and the OPUS problem

Lucas OPUS Mk II transistorised ignition was fitted from launch, with distributor 36DE12. There is no evidence that any production Series 3 E-Type had a contact breaker distributor, so the belief that early cars had points and OPUS came later is not supported. What did change during the run is the ballast resistor variant, the distributor and vacuum unit part numbers, and above all the location of the amplifier.

Lucas published a service document headed for this exact application, and it is the strongest ignition data the archive has found. The figures below come from it.

• Dwell angle: 22 to 27 degrees
• Pick up to timing rotor gap: 0.020in to 0.022in, being 0.50mm to 0.55mm
• Coil primary resistance: 0.8 to 1.0 ohms at 20 degrees C, and Lucas states that other ballast coils are unsuitable
• Coil primary voltage running: 4 to 6 volts at the coil positive terminal
• Stall current: 5.0 to 6.5 amps
• Vacuum unit: a retard unit, not an advance unit, giving 7 to 9 distributor degrees of retard coming in between 7 and 13 inches of mercury

The vacuum unit being a retard rather than an advance catches people out, and so does the fact that the advance figures Lucas publishes are distributor degrees, which must be doubled to give crankshaft degrees.

OPUS fails, and it fails in a way worth recognising. The amplifier sits in the vee between the camshaft covers, which is the hottest place on the engine, and it cooks, particularly after shut down when heat rises from the block with no airflow. The characteristic symptom of an OPUS amplifier failing is the tachometer dropping to zero while the engine is still turning. Jaguar extended the harness during production to move the amplifier to the bulkhead, and a great many surviving cars have had it moved by an owner or a specialist.

Lucas gave several warnings in that document which still apply. The oscillator adjusting screw in the amplifier must never be disturbed. The cable between distributor and amplifier must not be lengthened, shortened or replaced conductor by conductor. Amplifiers cannot be economically repaired. And when diagnosing, test the amplifier before substituting a pick up module, because an undetected amplifier fault will destroy the replacement.

Other documented weak points are the plastic centrifugal advance disc seizing, sometimes so badly that it breaks up on removal, the distributor shaft seal deteriorating and letting crankcase vapours into the distributor, and an intermittent break in one of the three trigger wires where they leave the distributor in a moulded grommet.

Cooling

Cooling is the subject that decides whether a V12 is a pleasure or a liability, and it has its own page. In short: the Series 3 uses a downflow single pass radiator with coolant from both banks entering opposite ends of the top tank, twin electric fans with no engine driven fan, and two thermostats, one per bank. Roger Bywater's assessment is that the radiator had significant pressure drop and did not really have adequate spare capacity for high demand conditions.

The failure that ends an engine is a dropped valve seat, which follows overheating and most often in the period after shut down. The full account, including why the temperature gauge can read normally while one bank overheats, is on the V12 cooling page.

Valve clearances and tappet noise

Clearances are 0.30mm to 0.35mm cold, which is 0.012in to 0.014in. Roger Bywater gives 0.012in to 0.014in as standard and 0.014in to 0.016in on emissions optimised variants, which agrees with the period figure. Adjustment is by shim under a bucket tappet, so the camshaft has to come off to change one.

This is a job that rarely needs doing. Clearances on these engines are reported holding to within a thousandth of an inch after tens of thousands of miles. What owners do hear is tappet noise, and Bywater attributes that to side clearance between the cast iron tappet and its aluminium carrier producing movement similar to piston slap, and to the two metals expanding at different rates, rather than to clearances drifting. Jaguar investigated the problem through production, including copper plating the tappets, without significant success. He also notes that inlet clearance opens by about 0.005in as the engine reaches full working temperature while exhaust clearance stays roughly constant, which is worth knowing before condemning an engine on a hot check.

Is it a long lived engine

Yes, conditionally, and the condition is thermal. The evidence for longevity is real: specialists report examples that have run 200,000 miles, valve clearances found within a thousandth of specification after 80,000 miles, and timing chain tensioner blades still flexible after 160,000 miles on engines that had never overheated. The period road test covered 2,230 miles without adding oil. The engine stayed in production in developed form until 1997.

Every catastrophic failure documented for it traces back to heat or to coolant chemistry rather than to ordinary wear. Bywater's phrase is that the engine is intolerant of neglect. An American specialist put it more bluntly to Hemmings: you have got to keep the engine cool, or it is trash.

The archive should say clearly that all of this evidence is anecdotal or trade opinion. No statistical survivorship or mileage data for these engines exists in the public domain.

The timing chain tensioner

The camshaft chain runs on a Morse tensioner with a blade of Nylatron GS, a nylon loaded with molybdenum disulphide. That material has a maximum continuous working temperature of 100 degrees C. In an engine that has overheated, the blade has been operating at or beyond its rating.

Blades embrittle from combined oil exposure and heat, groove where the chain runs, and in bad cases fracture, one documented example in three places. Longevity correlates closely with thermal history rather than with mileage. A rattle from the front of the engine is usually the timing chain, and it is not among the larger jobs on this car.

What this archive has not established

These figures are not published anywhere the archive has been able to consult, and rather than repeat numbers of unknown origin it lists them here. Most sit in the factory workshop manual E165/2 or the owner's handbook E160/2.

• Factory spark plug gap
• Factory initial ignition timing, home market and United States
• All torque figures, including cylinder head, main bearing caps, connecting rods, flywheel, damper and manifolds. Generic bolt size charts are not a substitute on an engine with an aluminium block, cast iron main caps and aluminium heads.
• Factory engine oil capacity, cooling system capacity and oil pressure specification
• Which cylinders each of the four carburettors feeds
• The factory HT lead routing and distributor cap lead order
• The identity and internal ratios of the four speed manual gearbox, and how it differs from the six cylinder unit. The often repeated claim that it is a strengthened 4.2 box could not be traced to any source and is not stated here.
• Whether axle ratios other than 3.07 to 1 were factory options on the Series 3
• Otter switch switching temperatures

If you own the factory publications and can supply any of these, the archive would be glad of them and will credit the source.

Transmission, briefly

Behind the V12 sat either an all synchromesh four speed manual, whose gearbox numbers carry a KL prefix, or a Borg Warner Model 12 three speed automatic. Automatic transmission on a Series 3 roadster was a first for the open E-Type. The Laycock de Normanville overdrive used on other Jaguars was not fitted, having proved inadequate for V12 torque, which is why owners wanting relaxed high speed cruising fit a five speed conversion or a taller axle. The car road tested by Motor Sport in January 1972 had a 3.07 to 1 axle, and a Powr-Lok limited slip differential was standard.

Common questions