Spend the time and money to get the correctly sized coolant level sensor
O-ring from the dealer.  A couple of years ago I bought one along with a defective
level sensor from a List parts merchant.  He sent one which was oversized; it
seemed to be the English measurement system size which was closest to the
correct metric size, but still it was too big (thick).  The merchant replaced it
with another one of exactly the same wrong size.  Even after giving his crack
(on crack?) staff the correct metric dimensions, I kept receiving the same
lame part, three times in all.  I finally jumped out of that circle jerk and went
to the dealer who looked up the part number and simply handed me the correct
item.

If you look at the construction of the coolant level sensor, you will see
that it is intended to seal on the ID of the sensor well, not the top edge.  So,
if your coolant sensor O-ring is visibly squished between the mouth of the
sensor well and the hex of the sensor, it is the wrong one.

George