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The oil filler also acts as a breather for the crank case. As the engine has two opposing pistons this means that the air in the crank case is alternately compressed and expanded. The positive pressure will tend to push oil out of any less than perfect joints. |
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| To stop this the engine breather contains a non-return valve. You can't see it as it is in the double wall section of the breather. I believe that there is a rubber sleeve in there over the holes that acts like a "flap valve". Air from the crankcase can pass out but not return. This causes a depression in the crank case pressure. So if there are any potential leaks air is sucked in rather than the oil being pushed out. |  |
| You can't see if it is working or not but you can test it using a simple manometer. A length of polythene tube clipped to a board and filled with water will do. Put the end in the dipstick hole and seal it with some tape. With the engine on tick over there should be at least 6cm of depression. Here I have about 12cm. Make sure you have plenty of tube so that the water does not get sucked into the engine! |  |