I think the heater itself is 1 3/4 inch (1 and 3/4 inch) socket. The adaptor is another story...its huge.

Dad and I were working on getting the plug out of my engine.
we already had the manifolds off for installing a pyrometer and whatnot.
Holy HELL that was really tight in there...had to break out the oxy-acetelene and all...it was not a happy baby...
The adaptor is steel I believe and the heater is brass..

Atleast the new ones are