from: "WO Bentley - engineer", written by Donald Bastow, Chapter Lagonda V12 engine, page 141

bolted on. As it is, the cylinder centre dimensions where the main bearings lie are 1in more
than the non-bearing gaps, 4.625in as compared with 3.625in so that the use of 12
cylinders has lengthened the engine, as compared with a six cylinder, by more than the
1.1in of the cylinder bank stagger, in spite of fairly narrow connecting rod bearings at 1.1in
wide, and main bearing shelis 1.25in wide for the two intermediates and for the front and
2.25in wide for the rear bearing which provides the endwise location.
	The two balance weight webs are 1.4375in thick; those adjacent to the front and rear
bearings are 0.531in thick, the associated ones forward of the front intermediate and
behind the rear intermediate bearings are 0.5in while those at each end of the crank centre
section are 0.625in. The thin webs are only acceptable because of the overlap, between the
2.5in main bearings and the 2.25in pins of 0.71in. lt is clearly the exigencies of the twelve
cylinder design which have both prompted the use of four main bearings only and the posi-
tions of the balance weights in the non-main bearing gaps, which as I peint out later,
(Chapters 13 and 14) are not ideal since they cannot give their maximum effect on any of
the bearings. However, they were adequate for the white metal lined bearings and it is only
the double weight of the connecting rods and pistons associated with each pin which
makes the bolted on weights desirable to heip the crank forgers. lt was, I believe, the desire
to reduce rotating and reciprocating weights which prompted theuse of aluminium alloy
connecting rods, which as far as I remember were still used by Standard and hence by SS
(Jaguar) at that time. W.O. had previous experience of the bearing combination,
Duralumin on nitrided steel, in the six cylinder Bentley camshaft drive. WO. told me after
the war that he no longer believed that aluminium rods were lighter than steel. General
experience has shown that there is no working stress level at which they would be
indefinitely safe from breakage. Because of the lower Youngs‘ modulus, extra thickness is
necessary round the big end to maintain its shape under operating loads. The running of
the aluminium direct on the nitrided crank is entirely satisfactory in normal service.
Because the material will not swallow dirt, traps are provided in the main bearings and
some engines after storage during the war succeeded in flushing out the contents of these
traps into the pin bearings to their considerable detriment.
	One ofthe problems ofthe aluminium connecting rod is the differential expansion of
the bolt and the rod material. The bolts are waisted between fitting diameters at head, nut
end and split, to permit easier stretch of the bolt and so reduce in particular the bearing
stress under the head which must remain under the ‘yield‘ stress ofthe aluminium at the
working temperature in order to avoid loss ofthe desired fitting bad. Associated with this
is, of course, that it is desirable to provide less initial bad on tightening so that the final
bad at working temperature is still reasonable. This requirement is met by a tightening
torque of 28 lbf ft (Motor Trader, September 13, 1939) which I would suggest produces a
cold set up bolt bad of 3,333 lbf. My estimate ofthe added bad due to a temperature rise of
105°C is 1,920 lbf so that at this temperature rise there would be a total bolt bad of
5,253lbf, a stress in the core area ofthe bolt of 69,000lbf/in2 and a bearing stress under its
head of38,600 lbf/in2 which should be below the 0.1 per cent proof stress ofthe aluminium
alloy in question.
	The presence ofthe big end bolts will somewhat reduce the expansion ofthe big end
itself with temperature, at least in the direction of the main loads along the shank. This
expansion does tend to cause increased gaps for oil flow and while this is a useful feature in
that it will tend to provide extra cooling, so long as the oil remains at a reasonable
temperature, it does tend to increase noise and also reduce oil pressure somewhat. This
makes the provision of a pump with adequate delivery all the more important. Starting
also becomes more difficult in very cold conditions, because the rod shrinks onto the pin.
	Another of the problems of the aluminium alloy direct onto the nitrided crank is that
oil must always be present to prevent local welding and scoring; the nitriding will reduce
the tendency to this but oil is still necessary for any bad carrying. To ensure a full supply
all the time the engine has two separate oiling systems; one pump supplies only the crank-
shaft main and big end bearings, and the relief valve for this is set at 75 lbf/in2 at 20mph...