Does your green oil pressure warning light flicker reassuringly when in a traffic queue with a hot engine or has it been constantly brightly lit since you first hit the restricted speed limit? Perhaps it doesn’t come on at all when the engine has been stopped whilst still cold, and illuminates only reluctantly after a long wait with a still-warm engine. The conditions under which individual lights come on seem to vary and it may not be down to differing efficiencies of oil pumps or inconsistent clearances of the oil-supplied bearing surfaces. It came as a surprise to me during the ‘minor adjustments’ phase of getting the Running Beam back on the road that the oil-pressure switch is, in fact, adjustable.

I had not seen any documentation on this adjustment and wondered if other Fellows would be interested in a little write-up.

During one of my trawls for bits when building Manfred von Richthofen (the red S8) from spare parts, I came across a cache of bits in Essex. Included were four or five oil pressure switches. So, when the warning light switch which came with the Runner Beam refused to play (no light came on under any circumstances) I just tried the others until I found one which would switch on. This one was very good at being switched on and would stay on until the pressure was well up to 15 or 16 p.s.i. – a pressure which is not maintained by my engine when hot under about 35m.p.h. in top gear. Other switches I tried exhibited similar unsatisfactory characteristics most not wanting to switch on in the first place. In desperation a few were taken apart and a variety of springs emerged – different lengths of springs and only some with a secondary thin weak spring as well as the main one.

Combinations of switches and springs were tried until a reasonably satisfactory compromise was reached. But then, and at last, it was noticed that from one of the recesses into which the bottom of the spring fits in one instance (one which came with one of the longer springs) something was missing. Investigation showed that what was missing was a small screw which, not only locates the bottom of the spring, but also screws in and out of the part with the recess, the large bit to which the wire is connected – the screw provides adjustment! My ‘eureka moment’. The positioning of this screw will press the spring harder or softer against the diaphragm which makes, or breaks, contact with the switch housing depending on the oil pressure from the other side counteracting the effect of the spring. All very obvious once the penny had dropped and probably known to everyone else in the Fellowship, except me.

The Bedside Book mentions that the switch would usually be expected to operate at 8p.s.i., but I now have warning light with which I am happy – one which switches on/off at a fairly low pressure (about 6 p.s.i.) – and I can watch the pressure fluctuating on the pressure gauge without the additional worry of the warning light overstating the point. Getting to the point when I was satisfied was a little more trying as the adjustment is fairly fine. Take the switch off (catch the oil which dribbles out), dismantle it, tweak the screw a little, reconstruct and re-fit, try the switch-off pressure by kicking the bike over, repeat the above steps many times making small adjustments until it seems to be alright, start the bike up and warm up the oil to check the setting in working conditions, and then repeat all the above as many times as necessary.

The (to me) not-very-obvious, hidden-away, adjusting screw can be seen on the accompanying exploded-parts picture.

Jem