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The XK Joins the Army.

Anyone wandering around certain areas of Jaguar in the 1970s or 80s would from time to time see a rather odd looking XK painted all over in a bland pale green, with a huge black carburetter on top of the inlet manifold. This would be the military version used to power the Scorpion light tank and its derivatives. Aside from the enormous down-draft Solex carburetter, which would have looked more at home on a wartime aero engine, another external difference would have been the massive distributor, completely screened and water tight, with heavy screwed-in braided leads trailing away and screwing onto the bodies of unusual looking spark plugs. Understandably, military folk dont like ignition systems which cause interference or cant stand a bit (perhaps a lot) of damp.

A revised breather system with flame traps was another demand of the military mind which not surprisingly has a serious aversion to engines fires.

Internally these engines had low compression (7.75:1) pistons so were not dependent on having a supply of high octane fuel. Also the tappets were larger than those used in normal production engines so that they could accommodate a collapsing spring and washer mechanism to ensure positive rotation of the valves, with the aim of making valve seat wear less likely in arduous conditions using doubtful fuel.

More Mechanical Changes.

Coming back to earth with production changes, one which served no purpose at all was that from two to four bolt fixing for attachment of the vernier sprockets to the camshaft flanges. Two bolts had given no trouble whatsoever so why the change? Believe it or not, it was the result of a cost cutting exercise which went wrong. The serrated vernier plate is a cumbersome thing to produce and the plan was to replace it with a simple pressing rather like a pastry cutter. This would not work with two bolts so four were tried before the futility of the scheme became obvious, but the four bolt fixing remained from that day on, thought by some for no better reason than to protect the reputation of the perpetrator.

Actually there is something of an anomaly regarding the vernier sprockets. They permitted very precise adjustment of cam timing, yet the accuracy of phase angles between individual cams along the shafts was not always as good as it should have been and as far as the 5/16" lift cams were concerned might best be described as "vague". The 3/8" lift cams were much better in this respect.

The 87 mm bore 3.8 litre was the maximum practical stretch within the original cylinder block and was a good example of a speculative modification proven under racing pressure feeding back into production. The 2.4 (83 x 76.5mm) was really rather a disappointment in terms of specific performance because the ports and valves were just too big for that size of engine. In any event the twin cam / hemispherical combustion chamber layout never really worked so well with short stroke engines partly because of the piston shape needed to achieve a reasonable compression ratio. The later 240 engine was encumbered with the straight port head and only retention of the mild 5/16" lift cam enabled it to produce acceptable mid-range torque. At least the weight of these small capacity engines, including the later 2.8 litre, was reduced by use of a shortened cylinder block accompanied by shorter conrods.

Prior to the 4.2 all XK cylinder blocks were cast with space for coolant to circulate all the way around each cylinder (fig 1). The 92.07 mm bore of the 4.2 left little room for such a luxury and also required the bore centres to be moved apart so the inner and outer pairs became offset from the combustion chambers in the cylinder head which remained unchanged. Within the casting the cylinders were now joined together in two groups of three, a so-called siamesed block. Coolant flow between cylinders was still thought desirable and was achieved by the rather cumbersome means of machining a hole through from one bore into the next just below the top deck. Of course this meant that another change had to be introduced - linered bores to seal off what now became narrow coolant passages. It was to be many years before the slotted block was introduced to permit coolant flow between bores without the need for liners. In this respect the 4.2 was a bit of a bodge driven by the need for extra capacity.

When the 3.4 engine size was resurrected in the mid-seventies it was really just a small bore version of the 4.2 with siamesed bores, liners and straight port head and was some way removed from its illustrious ancestor. This 3.4 always had S.U. carbs like its namesake but was cursed with the appalling AED which the 4.2 was fortunate to escape from for its final years. Nothing highlights the superficial nature of some aspects of Jaguars much hyped "Quality Drive" of the early 1980s more than the fact that the AED was never junked from the 3.4.

The first obvious signs that the XK was becoming fragile must have been when 2.8 XJ6s began burning pistons. Strangely, a hard driven 2.8 would run for ever, and the Jaguar test fleet in those days would have been hard driven in the belief that it was the sure way to find a weakness, yet customers found that after a few weeks of gentle motoring in town, a full throttle burst to get up to motorway speed could "smoke" a piston in very short order. The reasons for it are somewhat obscure and there are varying opinions on the matter but my understanding is that extra piston dwell around TDC on account of the shorter stroke might have played a part allied to a humped piston crown that came into close proximity with the hot exhaust valve. In essence, if the geometry is such that the piston is unduly slow in accelerating away from TDC then pre-ignition can be more easily promoted by soft deposits laid down in the combustion chamber during light load operation. The official name is "Deposit Induced Pre-ignition" and there can be a narrow margin between survival and failure for the brief time it takes for the deposits to burn off. Unfortunately, in the case of the 2.8 the condition could, on occasions, last long enough to melt a piston.

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