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

This is for single groove valves......
The thing most people get wrong is assuming the “tang” on the locks are taking all the load from the spring.
They’re not.
The squeezing of the locks against the valve is what does most of the work.

The retainer, locks, and valve should all “lock” together under tension.

It’s not that uncommon for me to see locks with the tangs almost sheared off because of a poor “interfit” with the retainers/locks/valves.
And many times when that’s been going on, the upper edge of the groove in the valve gets rounded over...... so then you also need new valves.

If you take the stock magnum valve, retainers and locks, assemble them together with a little upward pressure...... they’re “locked”.
You can bang the valve down on the bench firmly and they won’t come undone.

That’s how they should be.

Sometimes with aftermarket parts it takes a bit more force to get the parts locked together.
If you can get the pieces to lock using a height mic on the heads when you’re setting up your springs, even if it takes a little tap with a brass hammer........ that’s okay.
But when you do it that way, and you back the height mic off....... it should take a little effort to unlock the pieces.
If you undo the height mic and the pieces fall apart....... and you put it together like that........ don’t be surprised if it fails, or if the parts are all toast when you pull the heads apart next time.

Even with springs that are close to 400lbs closed, being used with .900 lift cams and over 1000lbs open load...... if everything has the proper interfit, there will be essentially zero wear marks on the tangs in the locks when the heads are taken apart for freshening.

I’ve also seen poor fitting parts run with springs that are only 100/280 that were on the verge of shearing off what little of the tang was left.

When the factory decides they want the valve to float in the locks(instead of acting like a collet), and are going to rely on the tangs to bear the load........ they use 3 or 4 tangs(like the stock 4 groove ex valves) ..... and then keep in mind they’re also being used with really mild cam profiles and modest spring loads.