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The Bearing Saga.

Now it turned out that the Mk X bodyshell could resonate in sympathy with, and amplify, normal main bearing noise from the crankshaft, so rather than redesign the bodyshell it was easier to alter the bearings to detune the crank vibrations to a different frequency. Actually a very effective way of eliminating main bearing noise is to bend the crank slightly (or even deliberately machine it out of true as some manufacturers have been known to do) but this was thought impractical as a production process. The method chosen was to switch from a full oil groove main bearing to a half groove arrangement with the lower shell being plain over most of its surface. The XK always had cross drilled main bearings and therefore would still have oil feeding into one end or the other so an adequate supply to the crank pins should still be assured, one might think The trouble is that a whirling crankshaft generates centrifugal forces in the oil within its passages, and the oil has to overcome this force to enter through the main bearings before being flung out to the crankpin, any entrained air tending to remain at the centre of the crank. The centrifugally generated pressure at the crank pin rises as the square of the speed of rotation, and at high speeds will be considerable, so any excessive clearance in the bearing will allow too much leakage. In fact some early development was aimed at curbing centrifugal pressures and resulted in the right angle drillings at the crank pin but the situation had now changed for the worse.

The oil entering the crank drillings from the main bearing grooves is also subject to centrifugal force which opposes this flow and a certain critical speed will eventually be reached, hopefully well above normal operating speed, when the quantity of oil flowing into the crank will not make up for the leakage at the crankpin, with catastrophic results. Very high revving race engines use high oil pressure and have the drillings only just below the surface of the bearing journal, in any case of much smaller diameter, to alleviate this problem. Cosworth coined the term "low pressure crankshaft" when they applied the principle to the legendary DFV, but as far as I am aware, the idea originated in the BRM V8 of the early 1960s (fig 8).

At low speeds the situation is quite different because then there is negligible centrifugal pressure so gallery pressure alone feeds the oil into the crankpin bearing. The success of the entire bearing system relies on a delicate balance between gallery pressure, centrifugal pressure, oil passageways and bearing clearances and whilst most engines have a generous margin of safety, in the case of the XK changing to a half groove main bearing considerably reduced it. It is perhaps now easier to see how the oil flow into the cross drilled main bearing, far from being steady, actually must undergo constant reversals as the feed changes from end to end thereby reducing the average flow. To make sure that oil would still be able to form an adequate lubricative film on the crankpin Jaguar did something rather unusual - THEY MADE THE BIG-END BEARINGS OVAL.

The idea of this was that by providing extra clearance in the lighter loaded areas around the sides less oil pressure would be needed to fill the bearing yet the average clearance would still be close enough to prevent too much leakage. Of course we are not talking about out-of-round crank pins, the bearing shells were formed to give the required clearance (fig 9). The idea worked - most of the time - but now and again tolerance conflicts would stack up, possibly aggravated by lapses of crank grinding quality, to cause a spell of bearing trouble. Such events were often signified by a change of the drilling arrangement of the crankpins. Some were cross drilled, some had a single drilling at 90 degrees, others had a single inclined drilling, and some had a slight relief across the drilling. Each change was sufficient to shift the delicate balance of the bearings back to a safe condition to suit the circumstances. The primitive manufacturing arrangements made such changes easy. XK crankshafts were drilled on a sequence of ancient radial arm drilling machines (really a jobbing type of machine) locked in position over a jig in which the crankshaft was placed.

The bearing ovality was not great - about 0.002-3"- and continued for many years until around 1977-8 when it was decided that the bearing shells could be made more easily (cheaply?) if they were circular and the necessary side clearance achieved by machining chamfered reliefs at the mating faces (fig 10). Clearly this change upset the delicate balance on which the bearing depended for survival and small variations of clearance in the critical relieved area, in combination with normal bearing tolerances, could make the difference between insufficient oil flow into the bearing and too much leakage through it. In fact the change had gone ahead under pressure without being thoroughly tested and the end result was that for a short period almost every single engine was a potential failure and some could not even make it through the factory gate. In retrospect one has to wonder why, after the Mk X and 420G were discontinued, nobody thought about returning to a conventional bearing system. Really the problem could have been brought under control by tufftriding the crankshaft, as on the V12, to give it a hard wear resistant surface better able to tolerate marginal lubrication conditions, but it was 1983 before this happened. The delay was because of fears that the treatment would add to the problems by causing distortion of the crankshaft.

In a small way I was personally involved in these bearing problems of the XK engine during 1978. I knew then that it was a fragile engine but it was only after I left Jaguar and encountered cars not long out of warranty with serious engine problems in other ways as well that I realised how bad things really were. Most engines would survive the warranty period but after that they could be really bad news, yet no knowledge of this had seemed to penetrate the factory environment. It wasnt so much that the engine would expire without reason, it had just become very unforgiving of the slightest hint of neglect. An all too common example was that if the wrong antifreeze were to be used the radiator would lose efficiency due to deposition inside the tubes, causing the engine to overheat. Most other engines would be reasonably tolerant of such abuse for a short time but an overheating XK from around 1980 could be a total wreck in minutes and a simple hose failure could be catastrophic. Strangely, whatever damage occurred, be it loose tappet guides, gasket failure, bearing failure, block or head cracking, (often all of these together!) always seemed to start with the next to back cylinder. This would indicate that this cylinder was more thermally loaded than the others, probably because of local stagnation of coolant flow in extreme conditions, but such things happen to some extent in most engine designs.

Did nobody question the high number of exchange engines that must have been sold during those times? - or did they just rub their hands and take the money?

Ones judgment is surely coloured by circumstances and if I had been party to the XK successes of the 1950s then I am sure my attitude would be different. As it is, I experienced the XK at its worst so it is not within me to regard it with much affection. It is too much like the proverbial curates egg for my taste.

Roger Bywater.
AJ6 Engineering
4th November 1997.

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