RE: About O.S #8 plugs!
BW,
An extra plug-washer will generally not do it.
Using two washers is generally not recommended and could be more succeptible to leaking.
It is also. probably too thick, to take up 1/64" of thread (one-half a thread, as Brian wrote) of the long plug.
I agree with Brian on the very high quality of Enya glow-plugs... Too bad they are also mid-length, making them less suitable for larger engines (but OK for smaller ones).