2CV ENGINE BREATHER / OIL FILLER (early and original types)
1970 ONWARDS
Although the principle is the same as the early breather the post-1970 unit is more complicated. It is in three main parts as shown (left). When you pour oil into the filler it runs down a central tube into the crankcase. This tube is surrounded by an outer chamber. When the pressure in the crankcase builds up the pressure also increases in the filler tube but, provided the filler cap is locked down, it cannot escape. So the lower part of the tube has holes drilled in it which are surrounded by a rubber sleeve with slots cut in it. The gases under pressure pass through the holes and the slots in the rubber are forced open. Now pressure has built up in the lower section of the outer chamber and this forces open a second, circular rubber valve mounted on the centre section. With this valve open the pressure passes into the upper section of the outer chamber and escapes to the atmosphere via the rubber tube to the air filter.

When the pistons commence their up stroke they try to suck air back into the crankcase but the slots in the rubber sleeve and the circular valve close so pressure in the crankcase stays below atmospheric pressure, thus reducing the likelihood of oil leaks. Any liquid which passes through the slots into the lower section of the outer chamber drains through the small tube in the base and down the dipstick tube back into the crankcase.

If your engine leaks oil and you cannot find the source then a fault in the breather may be the cause. The remedy is to buy a new one although we understand that repair kits may now be available from Burton. (The pictures are of a faulty unit which we cut apart, hence some ragged edges!). A new design of breather which uses a metal reed valve instead of the two rubber valves is also now available. It is described more fully on a separate page.
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The twin cylinder 2CV engine is arranged so that both its pistons are on their down stroke at the same time (see moving illustration). This results in the gases in the crankcase being compressed. If the pressure can't escape to the atmosphere then seals, gaskets and joints which are normally sound can start to leak oil under the pressure. The oil filler unit is designed to also act as an engine breather, allowing the pressure to escape through the air filter.
PRE 1970 CARS
Until 1970 the design of the unit was as shown in the photo (right). The rubber non-return valve, known as the beak, is forced open when crankcase pressure builds up during the pistons' downstroke and this allows the pressure to escape via the oil filler tube and a tube placed over the beak and connected to the air cleaner. When the pistons start their upstroke they try to draw air back into the crankcase but this closes the beak so pressure in the crankcase remains below atmospheric pressure, reducing the likelihood of oil leaks. Unlike the later version (below) the valve can be renewed and these are still available from the usual suppliers. This early design does not need the drain to the dipstick tube which the later design had (see below).
THE WESSEX DUCKS
THE WESSEX DUCKS
Technical pages
TESTING CRANKCASE PRESSURE
To test crankcase pressure you need a gauge as shown on the right. The plastic tube is filled with water (preferably coloured) to marks show on the gauge. The gauge is hung vertically on the bonnet catch and the long end of the tube is inserted in the dipstick tube. When the engine is started, the water should rise in the dipstick side of the tube because atmospheric pressure on the open end of the tube should be greater than the pressure in the crankcase. A scale on the instrument gives the figures involved. If the water in the tube doesn't move your engine breather may be faulty.
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