Many owners are surprised to know that there are three leather washers in the oil pump for 2.75hp models from 1912 (including EW), (Note. The earlier models have a smaller bore pump that only uses two leather washers and incorporates a rotary valve. This is operated by the plunger to select oil tank or engine. There is no spring as the pump is manually operated).
However, back to the more common three leather washer spring assisted pump. Most people find two washers when they unscrew the plunger from the top of the pump but then ask where to put the third. So, perhaps it is time for a look into the pump and to explain, for those who always wondered, just how it works.
The pump has to perform three functions. Firstly it has to be able to draw up a quantity of oil from the oil tank. Secondly it needs to be able to deliver that oil to the engine and thirdly, prevent the oil delivery from going back into the oil tank.
Mounted on the base of the plunger below the large spring is a threaded brass spacer fitted so that the larger diameter is uppermost supporting the spring. Two leather washers are fitted back to back under the spacer supported by a steel washer and brass nut at the base of the plunger. The upper leather washer is the one that creates a partial vacuum below when the plunger is pulled up permitting oil to be drawn from the base of the oil tank. The lower leather washer then provides the seal to push the oil into the engine via the drip feed regulator as the plunger descends under spring pressure. The third leather washer is fitted on the top of the base of the oil pump body, which must be unscrewed, having disconnected the oil pipe to the drip feed lubricator. The purpose of the third washer is to cover the half ring of small holes around the front of the base of the pump body. These holes provide the means for the oil to enter the pump from the oil tank. The leather washer faces upwards so that the oil flows past its skirt as the oil is drawn into the pump and then seals the holes when the plunger pressurises the oil under spring pressure. It should be noted that this lower washer needs a larger hole in its centre to fit over the hollow threaded stub on the ‘Lid’ of the lower valve chamber.
Yes! There is more! Under the ‘Lid’, hidden in the very base of the pump, is another non-return valve using a disc and spring housed in a brass insert. The disc and spring fitted here are exactly the same as those in the non-return valve screwed into the crankcase through which the oil passes before entering the engine. The ‘Lid’ is actually a threaded disc screwed into the top of the pump base. This can be quite difficult to remove, as the slot for a screwdriver, on top of the threaded stub, is small in relation to the diameter of the threaded disc. One dodge is to hold the hexagon body in a vice and then to use a steel rule across the slot rather than a screwdriver. This gives additional leverage and is less likely to slip off but if the disc is really tight, you may shear off the top of the thread. It might be safer to drill a small indent near its periphery to enable it to be tapped round using a small punch.
If further proof were needed that Douglas had thought this through, a neat additional touch is a small hole at the rear near the top of the pump body within the oil tank. This allows air to go back into the oil tank as the plunger is pulled up and also permits oil to drain back if the lower leather washer should leak when the oil is pressurised. This helps to reduce oil coming out around the plunger shaft and running down the petrol tank when the plunger is pulled up.
To ensure perfect functioning of the lubrication system it is vital that all three leather washers are in good condition and that both non-return valves are functioning correctly.
Good luck with your lubes.
