My cooler nozzles have some type of white sealant on the thread. Did a search to see if anyone knew what was best to use and came across an interesting but long link from BMW forum.
E32 Cooling system -- Radiator Replacement
These Beemers have rads with tanks on sides and from that link, it seems the transmission cooler connections are a problem for them. (Keep scanning through the link - parts on transmissions nozzles are in several places.)
Even new Nissens rads failed at top cooler nozzle after very short life. In their case, the plastic tank cracked where the nozzle exits. Behr appears to now have reinforced rads to solve that BMW problem.
Could be that on BMW because of side tanks, the upper nozzle gets more stressed?
I am about to go out and install either the one or both nuts. But I WILL check closely that the plastic tank does not have cracks! And if I ever have to buy a new one, if the new type is re-inforced. From on-line pictures of Nissen and Behr for W123, it doesn't seem rads have been reinforced.
My Behr radiator nozzle exits look a lot like the old BMWs. You can see the old and new types below.
