273, or 318?

Rat Bastid said
Thats because they are NOT LS valves or magnum valves.

They are VALVES. They have no idea what size they are or what they fit.

3/8 stem valves are stupid. Even 11/32 stems are quickly becoming obsolete.

Of course, in the upside down, *** backwards dumbed down world we live in, we will get stuck with metric stems. All because part of the world needs math dumbed down for them.

A 5/16 stem should be the standard in ANY performance build.

Its not 1962 any more. That’s why the Hemi got 5/16 valves.

I refer to the valves as LS or Magnum as the OEM source. I am using the Melling 5.3 LS valves in my Ford 289.
As to stem diameter, the difference between 5/16" and 8mm is only about 0.002". A 5/16" ID guide can be installed and the option to run 5/16" or 8mm stems is open. All that needs to be done is hone the guides to proper clearance.
I think a number of 4 valve engines are running 5mm valve stems.