"Anyone of you heard something about such series of O-235 ?"
For what ir is worth, yes I have. Came across this a number of years ago on a 235 in a Traumahawk. Aviator1512 has given you the cause far more clearly and lucidly than I could have.
If the aircraft was your own, I would support Aviator1512's statement in its entirety.
However, as you have 'borrowed' it, I would recommend in the strongest terms possible you do as Aviator 1512 says and DO NOT FLY IT. Do your very best to ensure the owner(s) actually understands this point and why. Make your point in writing to ensure your back is covered if the engine should fail when someone else is flying it (a sad reflection of our present-day litigious society).
As to the overhaul shop, could you please PM me who it is.
camlobe