My house has a drain pipe for the kitchen sink, and the pipe gets
plugged sometimes. Then you have to remove the plug or bung from the
cleaning hole and run a snake down the pipe.

Last time I had to unplug the drain, I wanted to replace the (60mm
plastic) plug. The drain hole is in a fitting that looks like cast
iron, and the house is about forty years old.

I had a lot of trouble getting the new plug to engage the threads of the
cleaning hole because the threads were rusty. I took a stainless-steel
wire brush (and I think some Liquid Wrench) to the threads, but that
didn't seem to do much.

So I just put the old plug back in---that wasn't too hard to do. But I
am afraid of cross-threading the plug and ruining it sometime.

Next time I have the plumber over to do some work I thought I might ask
him to do something about those threads.

Would he have some kind of tool to clean out the threads without much
risk of messing things up?