O ring for quick connect on expasion tank
 

Does anyone know what size the o-rings are for all the connectors for the radiator and expansion tank? There are no part numbers for the o-ring in the connector. Thnks

I'm not positive, but I don't think those are replaceable from BMW. You can go to lowes and such to find an o-ring but you are better off replacing the entire hose. If the O-ring is leaking, then the hoses are probably shot.

This is for a 2001 x5 4.4L with M62. Just swapped out expansion tanks, old one had a pin hole leak. Tired of seeing coolant crusted on the sides of the expansion tank. Connectors worked for old tank, now leaks with new tank, hoses are in good condition. O-rings are cheaper to replace than whole assembly.

O-rings are cheaper yes but, they aren't available as a service part from BMW or any aftermarket suppliers.
You might buy a kit of metric o-rings and find the right size.