"Richard J Kinch" wrote in message
. ..
There's an odd thread, called the Leica thread, that is used on
photographic enlarger lenses. It is specified as 39mm in nominal diameter
but 26 tpi in pitch.

I'm wondering what other monstrous metric-English combinations are out
there?

Shaft nuts---the thin round nuts with notches in the rim used to retain
bearings
on shafts. Metric diameter, English pitch.