8MM retainers/locks on 5/16 valves: OK or not OK?

Very interesting.... I had seen that illustration but had not caught that it showed contact of the bead lock only at the top; thank you. Well, RR seems to be saying that he has a lock bead that is about .020" shorter than the lock groove, based on how the lock can slide up and down the stem a bit. Maybe he will chime in and confirm that, but it sounds like that his vertical clearance between lock bead and groove is OK.

So IMHO, that leads back to what the acceptable tolerance in the 2 diameters: lock ID and stem OD.

And RR refers to the locks not falling off the stem but perhaps that is a misinterpretation of this statement from your linked Yellow Bullet thread attributed to Ferrerea: " With the valve sitting on a table you should be able to put the locks on, pull the retainer up into the locks and let go and the retainer shouldn't be able to fall down". So perhaps RR is not checking them correctly.