The thread diameter/pitch for the O2 sensor is 18 x 1.5 mm.

I went to my flaps and they were useless, although I give them credit for
trying.  I went next door to the Goodyear shop and the front desk put me
on the phone with one of their mechanics.  I asked him if he knew what
will fill the O2 sensor hole. He said, "Do you got a spark plug?"  I said
I did but it was too small.  He said, "Let me look around and see what I
can find."  A few minutes later has came out to me with a new Autolite
size 46 spark plug.  He said he didn't have any used ones, but that I
could have this new one, and that it would fit.  Nice. I brought it home
and it does fit.  A little while searching on the Internet for the exact
thread size of this particular spark plug and I finally got it.  Now I'll
go back to the flaps and buy an 18 x 1.5 mm oil drain plug.  Then, on to
the next thing...

Brian
84 Westy