<VV> 3/4" OD Spark plug thread insert

[NicolCS@aol.com]

Here's one for the brain-trust <ggg>.  Does anyone know where I could find a 
large spark-plug thread insert?  I have heli-coil and KD inserts, but they are 
much smaller OD than what I need.  I have a really nice 140 head that had a 
3/4" OD (very fine thread) insert in one of the plug holes.  The insert is gone 
and I'd like to find a replacement.  Anyone familiar with (or have) one of 
these puppies?
thanks in advance
Craig (plugless) Nicol