I bought a Fluidmaster bottom entry float valve to replace an old ball
valve in a toilet. While I was trying to tighten it onto the pipe
connection I didn't notice a fibre washer had deteriorated and was
bulging on one side, and as I tightened the connection it pushed the
valve thread out of alignment and caused me to slightly strip the
plastic thread. I managed to correctly rescrew everything after
replacing the washer, but now I can't get the connection tight enought
to stop it leaking. I've tried PTFE tape and Bosswhite, but it seems
to need so much force to tighten the nut to stop leaks that the other
end of the valve connection keeps unscrewing (where it enters the
cistern). I can't find anyway to get both ends of the valve to screw
in without leaking (to the cistern and to the water feed). I've
probably ruined the valve thread and should buy a new one, but is
there any way to fix it?

Thanks,

Ken