I'm working to remove the fill valve from my (quite old) Kohler toilet. Drained the tank, unhooked the water supply, now trying to remove the locknut. Trouble is (and forgive me, I'm not sure if this is common) the locknut is plastic and two pieces that seem to lock together. So when I attempted to unscrew it, the lower half of the locknut came off (as it kind of unhinged from the top part), but the upper half of the plastic locknut which is up against the porcelain just won't budge. What's the trick to getting that last piece off? I've tried using a wrench but I'm afraid of damaging the plastic or the porcelain. Thanks!
If the plastic part you are removing is headed to the scrap/trash/recycle it might be ripe for "crush it with locking pliers" as a brute force removal technique that should still be low-stress for the porcelain.
At some intermediate level, grab it with locking pliers to unscrew since it does not have much for a wrench to grab might apply, but it might just break anyway.
If you are trying to preserve it, but remove it, good luck.
That piece is essentially a slotted socket wrench. It's probably intended to break away under severe tightening force rather than crack the porcelain or damage the valve parts.
Put the piece back on and use a hex wrench on the assembly. You'll have to hold it there while you turn it, just as you would the hex wrench.