all the 7/8-18 taps I've been looking at have been in the neighborhood of $80, than I ran across this:

http://buy1.snapon.com/catalog/item....re&dir=catalog

a Tap, Spark Plug Hole, 7/8"-18 NF for $25.

Of course, I don't think I've ever come across a 7/8" spark plug, that's why the question.

It matters cause I am using some 340 barrels for a project.