Leaping Jaguar

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Article bellow is extracted from Jaguar World, written by Roger Bywater, from well known Jaguar tuning company, AJ6 Engineering.

V12 Story

It has been said that the open deck construction of the cylinder block, chosen to simplify the casting process, lacked rigidity. This is true but the multiplicity of studs to clamp the cylinder heads in place gave ample integrity to the completed structure. Main bearing shells would sometimes display witness marks indicating some movement but this never caused trouble in normal operation. On the other hand the abnormal loads generated by a bearing failure could certainly cause enough distortion to necessitate line-boring before fitting a new crank and bearings. Fortunately bearing failures were exceedingly rare and usually followed some sort of neglect. The generous spacing between cylinder bores is perhaps a matter for debate but then again the substantial structure and the volume of coolant therein may well have contributed to the refinement for which the engine was noted by absorbing vibrations. Certainly it is possible to open up the bore size from the standard 90mm to 98mm and to swing a crank throw in excess of 90mm instead of the original 70mm (98mm by 90mm gives 8.1 litres, but with a bit of work 9 litres would be feasible) but the bigger engines hardly ever seem as sweet as the original 5.3. Most observers would say that the final 6 litre (78.5mm stroke) was a bit rough but what is puzzling is that one or two experimental (84 mm stroke) 6.4s built in the 1970s ran like 5.3s. No modern engine would be designed with so much capability for stretching and ultimately this may have sealed the V12s fate. Rapid warm-up is essential for compliance with modern emissions legislation and an engine system that contains 5 or 6 gallons of coolant, as in the case of the V12 Jaguar, is struggling under a hopeless handicap.
Mechanical tribulations
By and large the V12 was a very reliable engine, as one would expect, but it was not without problems although few were really serious. The crankshaft was pretty well "bomb proof" being a substantial forging from EN16T steel and Tuftrided to create a hard wear resistant surface. Overlap of the main and crank journals was sufficient to permit straight through oil drillings (Fig. 4), carefully worked out to deposit oil at the optimum position to lubricate the crank pins under load. The dreadful sludge traps used on the XK were pointedly avoided. The rope seal at the rear main bearing was never a very happy arrangement and if it dried out through prolonged standing would either leak or worse, rub on the crank and heat it sufficiently to cause failure of the rear bearing, but such problems were rare and later engines had a proper neoprene seal anyway. Of more concern was a tendency for crank pulleys to work loose, fret and cause damage to the locating keyway. This never seemed to happen on early engines yet those made during the 1980s were susceptible. Perhaps the compression pressures of the HE induced some peculiar torsional loading which was not present before.
It has been said that the open deck construction of the cylinder block, chosen to simplify the casting process, lacked rigidity. This is true but the multiplicity of studs to clamp the cylinder heads in place gave ample integrity to the completed structure. Main bearing shells would sometimes display witness marks indicating some movement but this never caused trouble in normal operation. On the other hand the abnormal loads generated by a bearing failure could certainly cause enough distortion to necessitate line-boring before fitting a new crank and bearings. Fortunately bearing failures were exceedingly rare and usually followed some sort of neglect. The generous spacing between cylinder bores is perhaps a matter for debate but then again the substantial structure and the volume of coolant therein may well have contributed to the refinement for which the engine was noted by absorbing vibrations.
Certainly it is possible to open up the bore size from the standard 90mm to 98mm and to swing a crank throw in excess of 90mm instead of the original 70mm (98mm by 90mm gives 8.1 litres, but with a bit of work 9 litres would be feasible) but the bigger engines hardly ever seem as sweet as the original 5.3. Most observers would say that the final 6 litre (78.5mm stroke) was a bit rough but what is puzzling is that one or two experimental (84 mm stroke) 6.4s built in the 1970s ran like 5.3s. No modern engine would be designed with so much capability for stretching and ultimately this may have sealed the V12s fate. Rapid warm-up is essential for compliance with modern emissions legislation and an engine system that contains 5 or 6 gallons of coolant, as in the case of the V12 Jaguar, is struggling under a hopeless handicap.

Leaping Jaguar 2004 Copyright.

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 At the front of the crank was mounted the epicyclic oil pump - an unusual choice which absorbed a fair amount of power, but apart from some noise problems in the early days it gave no trouble at all. Next to it was the timing sprocket from which the duplex timing chain had a long run round both camshaft sprockets and the central jack shaft. Some chain thrash was evident initially but with the aid of visual investigation via windows in the timing cover, damper pads were soon in place to get rid of it. In fact the timing chain of the V12 probably has an easier time than most with plenty of overlap of both the firing impulses from the crankshaft and the torsional loading from the camshafts. The design of the blade chain tensioner is more questionable. Attractively simple, it works well for most of the time with a clever one-way jamming arrangement which takes up any slack, with a simple means of disengagement if the chain needs to be relaxed for repair work. The trouble is that if it starts to slip it soon becomes useless and then a major engine strip is needed to replace it.
The slip fit cylinder liner arrangement showed strong Coventry Climax influence having a short "wet" section exposed directly to the coolant at the top located in an aluminium surround extending up out of the crankcase (Fig. 2). The dimensional changes with temperature were thereby maintained within reasonable limits so "nip" at the head gasket joint would not relax as the engine warmed up. This method had been well proven by the successful Climax V8 F1 engine of the early 1960s, after some early hiccups before the dimensions were right, so gasket sealing was never a problem on the V12.
Not surprisingly, Climax-like features are to be seen in the design of the valve gear, yet this was one area which was a source of niggling problems over many years. The complaint was excessive tappet noise, which was very puzzling because the cam profile, cam follower (tappet) and much of the valve gear differed little from the XK.
The root cause is that the cast iron followers run in an aluminium cam carrier rather than cast iron sleeves as on the XK so the running clearance varies with temperature. All engine components are manufactured within certain tolerance limits and in this case the largest permissible follower must be able to run in the smallest permissible carrier bore - BUT - this must be so down to minus 40 degrees in a severe North American winter. At the other extreme a low limit follower mated with a top limit carrier bore could be quite sloppy in a hot engine. The actions of these components are exceedingly difficult to analyse but it does seem that cam profile, tappet clearance, side movement, rock-over at peak lift, tappet rotation and valve concentricity with its seat, all play a part in the generation of what is perceived as tappet noise. The range of side clearances involved is not great, ranging from about 0.0005" to 0.002" at room temperature, so effort was concentrated on cam profiles with gentle take up which would be less likely to provoke the followers to rattle about. An acceptable cam was introduced in the early 1970s but it was always a good rule of thumb to set the inlet valve clearances to tight limit and exhausts to wide limit (being hotter the running clearance works out about the same). By 1993 the market required some further cam profile refinement although it should be noted that the fundamental valve motion hardly changed over the years.

 Manufacturing and alternative configurations
Throughout its life the XK engine was made by what might be called knife and fork methods on out-of-date machinery. A complex engine like the V12 could not have been viable without using modern automated processes (Fig. 5). The cost for such a facility, reputed to have been about £3,000,000 in the late 1960s, seems like peanuts today yet the need to raise such funding prompted the sale of Jaguar into the BMC group although this also opened the way to much needed dealerships around the world. The need to replace the aging XK was an issue of obvious importance but surely it was not thought that the V12 would be the only engine for the future. Indeed, had it been known that the various alternative engine concepts based on using segments of the V12 would all prove to be unusable the V12 might never have got off the ground. These included a 60 degree 3.5 litre V8, rejected for lack of refinement, and a 2.65 litre slant six using just one cylinder bank which was too small. Increasing the stroke to 90mm would have resolved that problem but then the extra block height could not be accommodated on the V12 machinery. However a number of slant sixes using cut and welded up V12 crankcases were found very useful for testing 4 valve and May cylinder heads. An advantage of the original flat cylinder heads had been ease of manufacture but the May combustion chamber for the HE required some further operations which cost about £500,000 to implement. In these days of Formula 1 racing engines reputed to cost £1,000,000 apiece, the V12 facility looks like a bargain. It seems sad that it is now shut down for ever, but at least the V12 did not linger on like the XK waiting desperately for a successor. The V12s successor, the AJ-V8, is already here and carving its own place in Jaguar history, but the V12, despite being a little paunchy and lacking a real punch, was a class act and will always remain just that little bit special.
Roger Bywater.
AJ6 Engineering
June 1997.

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