Looking for a plug wrench that will fit the OS 40. None of mine will fit into the cylinder head (too large)
Any suggestions.

I picked up a cheap one at the LHS and just used a bench grinder to reduce the outside diameter not pretty but it works martin

I bought a 5/16" nut driver at the hardware store. Didn't have to modify anything. I have probably a half dozen different 5/16" ones. They are all different brands.

G'day Mate,
A standard 8mm socket, is the right size for glowplugs.
I use one on a screwdriver type socket handle, won't strip threads, not enough leverage.

An 8mm socket and or nut driver is .0025" larger than a 5/16" but will work fine with no more torque than you are using if that's what you have.

G'day Mate,
A glowplug, is 8mm AF, or 5/16AF, that's Across Flats, measure a plug & see for yourself.

You are right, but that's really splitting hairs

Well, it started out that I thought there was a much bigger error. My math was wrong.

I went back and corrected the actual difference (Which there is little difference)

I was just trying to smooth out my mistake.

The 8mm should work just as well as a 5/16"