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

That makes a ton of sense; thanks. I had worked out the lateral pressure, but did not know that was the primary hold force.

But if that is the case, why would the tangs get 'almost sheared off' with poor fitting parts? Just a bad diameter match between lock ID and valve OD? And how far off is a 'bad fit'? That seems to be RR's main issue at this point: not being able to find an exact diameter match between locks and stem.

It seems to me that if they are not perfectly matched, then it would be better to have the lock's ID a bit smaller than the stem's OD, rather than vice versa. In that way, the retainer would press in on the locks and you would end up with 2 points of pressure from each lock against the stem.