If there is no one that knows the size of the o-ring, be patient for a week. I will pull mine out and measure. I am going to decommission this car and drive my convertible Dec-Feb so in about a week and i'll have time to do it.

 

 

here comes the exact O-ring size data. i had to buy a new coolant temperature sensor the other day, and it arrived just now. the first thing is measuring it. the outer diameter is 14mm, and the thickness is 3mm, so the central circle is 8mm.
the new sensor:





now the old one(i borrow it from another thread):

 

to get long-lasting effect, i think , maybe 14mm(whole diameter),3.5mm(thickness) also work, and better.

 

Thanks. I bet its the same exact o-ring I had on my instake camshaft position sensor. It came with an extra o-ring so hopefully it is. Looks the same and BMW always uses exact sizes in other parts.

 

why my pictures missing?it's good at first.

 

Just in case anyone needs to replace this o-ring it is #31 and a suitable replacement can be found at Home Depot or ACE Hardware.

 

That might be where I'm losing coolant, getting a low coolant warning every month, 07 530ix

 

Temp sensor gasket is ONE OF the probable source of the leak. Take a flash light and check around all coolant hoses (especially the return hose on the top where it connects to radiator), and the coolant tank. Look for coolant residue around the connectors.
Radiator itself can leak as well. Only way to be sure about the source of the leak is to actually look at the above mentioned parts.

 

I'm hoping its not leaking coolant into the oil through the head gaskets. Other leaks much easier to fix. Actually, I guess the oil sensor might be good at detecting coolant contamination.

Edit update: I just checked with a flashlight, yeah a lot of rubber hoses/connectors, no seepage/leaking seen. ...Just an annoying coolant level warning every month or two. I'll keep adding a 50/50% mixture and grumble.