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I've noticed, when changing the clutch, that i can turn the flywheel almost by hand. This leads me to the conclusion that my compression is very bad. It became so in the last 20K km. I always monitor the oil level, so the engine never lacked oil, but now I don't know what could be the cause of this problem.

As said the engine is Simca G1A, it has like 600K km, so it could be normal wear, but why would it be this drastic?

In your experience, what could be the most common bad compression cause? Is it the valves or piston rings? If I were to open the engine could it be solved by a head reparation and piston ring exchange, or is it necessary to change the bores?

I would appreciate any insight by someone who has had experience with this engine. I think that I could do a head reparation and piston ring exchange, with all the seals for like 150eur. This is an amount I am willing to invest, but 900eur for a complete rebuild is twice the cars worth, so not a good idea.

Any help very much appreciated!

You can put oil in the cylinder before the compression test: if the compression is good with oil that mean the rings are bad, if the compression is still bad it's valves or head gasket.

Maybe you can just change the rings but you have to check the shape of the cylinders and with 600k km you probably better seach another cheap engine.
Loosing compression in only 20k km seems strange.

I am honestly not familiar with this method of putting oil in the bores. Can you explain to me how much oil should I put in? Is that through the spark plug hole? And how will the engine start after that, is the starting than difficult?

How much are these engines in France? I have a friend there who might be able to bring me one.

I tried the method explained by Baptiste and it works. I did have some doubts about a XU 1.6 engine because it used to burn a lot of oil.
I measured the values first, engine middle hot and I got 8 bars for each cylinder. Then, I put some oil into the gaz chamber (just put some into the oil drum cork and put it into through the spark plug hole).

Wait a short time (20 s), try again the measures and then follow Baptist's recomendations. In my case, I got 12 bars... engine out of order!

In France you can find some, the best idea is to buty the whole car. It is not so expensive if the car is non working. ( no TÜV for example)

Thanks very much!

You're welcome!