Block drain bolt

[David Tulchinsky <tulch@ccsalpha3.nrl.navy.mil>]

HI,

Trying to drain the coolant out of my 77B 18V  engine.

I think I found the main block coolant  drain near the distributor
and the rear oil cooler/filter lines.  Its is the only bolt back there.

Here is the question.  What size is it??? I tried a 3/4" socket.  Too big.
I tried a 11/16" socket. Too small.   I tried 18mm socket.  Just right.
I thought this was a non metric car.  Or did the PO just bugger this one up.
What size should it be??  Most of the other bolts don't seem to be one of
the other Brittish standards (i.e. Whitworth, etc.)

for ref:
11/16"    0.6875"
18mm    0.709"
3/4"         0.750"

Thanks for any help.

-Dave