Threaded inserts on front of Ford heads

[Humbucker]

Anyone know what size these are? I have one that must have backed out
and fell along the road somewhere a couple days ago. I thought I may
have a header leak until I felt the exaust coming out of the hole.

If I'm not mistaken, there are two sizes on the front (and rear as
well) of the heads. This is the larger size. Looks to be about 3/8ths
to 1/2 inch. I've been told by many it's a very non-standard size, so
a Lowe's or Home Depot remedy is out. I'm not using the opening to
mount anything, so a bolt would work if the thread size is right.

Any ideas? Thanks!

Jake - #1408