As you know, it is ideal to replace the orings in the hoses that are connected to the expansion tank. If they are the original hoses, they may be flattened or smoothed over from all of the pressure they have had to deal with. I am not sure of the oring sizes and if you can not find them and you get a leak, you will have to replace the hoses that connect to the tank.

Most people are fine not replacing the orings, but there are plenty that get leaks from the connection points and it is usually related to smoothed over orings.

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2004 E60 545i Sport 6MT. Black/Black Build Date 6-2004 102k miles