8mm vs 5/16" valve stem

It’s pretty basic really......

A lot of 5/16 valves have a stem diameter of .3110.
Which would provide .0015 clearance if used in a .3125 guide.

The typical 8mm valve stem diameter is .3130, which won’t even fit in 5/16” guide that’s sized for .0015 clearance with a .3110 valve.

How is that “interchangeable”?

It’s really more of a case of “some modification required”.