Greetings. I am a very non-handyman trying to replace the flush mechanism in my toilet. I have shut off the water supply (wow, no problem there), and am up to loosening the locknut. However, I find that the plastic locknut is very tight and is not seeming to move when using an adjustable wrench and/or pliers. Is there a way to loosen the nut so the wrench/pliers will work? Any answers would make my day.

Mark

Try spraying it with WD 40 or PB blaster
I don't know if a rachet and socket would work.

I usually tackle stuff like that with my 16'' Channellocks. You can also try turning the internal parts. Even a partial turn could loosen it enough to be able to turn the outside nut then.

Try cutting it, hammer and chisel, hacksaw, utility knife, Dremel, anything goes.

I find that the plastic locknut is very tight and is not seeming to move when using an adjustable wrench and/or pliers. Is there a way to loosen the nut so the wrench/pliers will work?
I follow Labmans advice and put a set of channel locks/ pump plers on the nut while holding the ballcock from turning. If that fails I take my mini-hacksaw and make a cut across the nut and peel it off. Good luck, Tom

Tom and Labman are correct about the Channellocks. Many home owner do not have a wrench large enough for that nut but we all have Channellocks.