Oil inlet / breather valve

The oil inlet on the 2CV motor is more than just a funnel to fill in the oil. The unit controls the pressure inside the crankcase. Inside the cylinder the breather valve found its home.
It is quite easy to explain the necessity of this valve. Imagine the pistons are just at the upper dead center. This means one cylinder just fired. The explosion drives the piston down. Via the crankshaft the other piston is also pulled down. As long as both pistons were at the upper dead center we had a certain amount of air inside the crankcase with atmospheric pressure. With the pistons coming down the space inside will be reduced by about 600 cc. This would increase the inside pressure what would make the pistons to fight against this increasing pressure. The result would be counterproductive. To avoid this effect we have the breather valve. It is located inside the cylinder of the oil inlet. When the higher pressure wants to build up, the valve opens and lets the motor breathe out. Once the pistons reach the lower dead center the valve closes again. The pistons now continue upwards what gives us a little under pressure inside the crankcase.

When the motor breathes out it will also take some fine oil out of the motor. The crankshaft splashes into the oil sump in every cycle. So the oil sprays everywhere. The air inside the crankcase contains a fog of microscopic drops of oil. When the motor breathes out this oil fog goes with the air. If now this air would be released into the engine compartment, pretty soon a fair film of oil would cover anything (including the front brakes).
To avoid this the air is led into the air cleaner. For this we have the hose between the oil inlet and the air filter housing. The carburetor will now suck the oil drops in and the motor burns it.

The breather valve is designed very simple. It is just a flat layer of rubber what lies on top of a flat panel covering holes. If now the pressure on the underside rises over the pressure on top the rubber will just be lifted up. The over pressure will escape. When the pressure on the underside drops down the rubber is pulled down sealing off the holes.

Open oil inlet with breather valve

Over the years and miles the rubber turns hard or some dirt settles between the rubber and the panel. Therefore the under pressure cannot build up. The result is that the motor pumps out about 600 cc of air in every cycle. So far no problem, but remember the fog of oil. When a lot of air is pumped out, the amount of oil going with it also rises. In the air cleaner housing the oil condensates.
In a good motor the "basement" under the air filter is oily and dirty. If the breather valve doesn't work proper you will find a lot of oil in the air cleaner. As long as the motor runs it pumps oil into the air cleaner. When the motor stops the oil will run back into the crankcase.

Anyhow - if you find a lot of oil in the air cleaner it doesn't necessarily mean that the breather valve is defective. It can also be an indicator that the compression isn't good anymore, i.e. if the piston rings are broken or worn some pressure from the explosion will find its way into the crankcase. This overpressure will be pumped out via the breather valve transporting the oil into the air cleaner.

If the breather valve is defective the whole oil inlet assembly has to be replaced. A repair is normally not possible. My attempts to wash out the oil inlet cylinder with solvent fluids did not work.
The assembly is attached to the crankcase with two 7mm bolts. The hose to the air cleaner is simply to be pulled off. Before you disassemble the oil inlet it is recommended to loosen up the holder for the alternator.
When the bolts were removed you can simply pull off the oil inlet. Mostly the seal between the crankcase and the foot of the oil inlet rips apart. To avoid anything from falling through the hole into the crankcase you should push some clean tissue paper into the hole.
Before the new oil inlet can be installed the rest of the old seal has to be removed from the crankcase. Use a sharp tool to scratch it off but don't hurt the aluminum surface.
To install the new oil inlet you have to use a new seal. I always use some liquid seal to compensate little scratches on the surfaces. This stuff also holds the seal on the foot of the oil inlet. Quite often the little rubber hose between the foot of the oil inlet and the dipstick turns hard. This makes it impossible to seal off again. To avoid an oil leak I put some liquid seal on the outside of the little pipe on the oil inlet. The hose pushes back the excessive sealant material when you push it in. This sealed off every motor where I replaced the oil inlet. It is much easier to use liquid seal then clamps on the ends of the hose. It would be very hard to reach the clamps to tighten them when the oil inlet is installed.

When you remount the new oil inlet (Don't forget to take out the tissue paper from the hole) put the foot in the right place and insert the bolts (Don't forget the holder for the alternator). It is very tricky to get the bolts in the right position. Due to the shape of the oil inlet it is hard to get the bolts in straight. If you now use a tool to force the bolts in, this will destroy the threads in the crankcase. So it is recommendable to put the bolts in just by using a 11mm socket and extension. When the bolt is almost all the way in you can use a ratchet.

When the bolts are tight and the hose to the air cleaner is connected you are done.