I just replaced the radiator in my M50 engine, however I am not sure where to locate the o-ring for the coolant sensor. I removed the old one and didn't see an existing one. I inserted the o-ring over the sensor inside the plastic nut, however I can't seem to thread it on to the radiator after doing so. Am I doing something wrong? Is there supposed to be a slot for this o-ring inside the nut or do I really have to force it on to get the thread started? For those of u that have done this, your input is greatly appreciated.

TIA,
Brooch

1991 525iA
M50