My kingdom for an O-ring!

I have had a hell of a time getting the right temperature sender for our
Syncro from the Bus Depot. First they sent the wrong Sender II for the
Syncro; then they told me I was wrong; then the List pointed me to the
right part #; then the Bus Depot said "oh! that Temp II sender"; they
shipped it without the O-ring; and finally after two emails they still
haven't sent any O-ring.

So tell me "What size is the blasted O-ring" so that I can go to the
local shop and buy me one.

Larry