M54 upper coolant hose O-Rings size?

[coolpapabel]

Hello guys can anyone tell me what size O-ring is required for the upper coolant hose on M54B30? My hoses are in excellent condition but I have a slight drip from the deformed O-rings around the tip of the expansion tank. I'm sure they are o-rings because I was changing the expansion tank soon and I found them to be deformed but still connected them but started to swim... Please if you can give me the size in millimeters because i am from Europe I apply a picture of who I am talking about. OEM No. 11537500733
In red circle:

[coolpapabel]

Come on guys I'm sure some of you know and can give me an answer.