Help Probably Blown Head Gasket?

[ALBERTO]

Hy,
My CLK 430 has 230.000Km never ver heated max 103 C for a while in traffic jam, baut recently i noticed that i have to add soem water...........apparently NO LEAKS.
So i deconnectet the klittle hose that came from the radiator to the expansion tank, and put it in a glass fulled with water, i started the car and waited untill the thermiostat open and with my big surprise with the water i can see may very very little bubbles liek when water begin to boil but the temperature was not 100 C .
I suspect this eare gases on the cooling..........so one of my head gaseket is blown.
really not happy.........

Does anyone have alredy done this jon on a 113 why I am afrain about lose the timing, and I do not know if a can do the job Without demonted the CHAIN....
Help
I really apreciate your helps guys
Many thanks
Alberto