No, the reamer does not fit the threads. It is simply a reamer that is shaped to fit into the hole in which the glow plug fits. It is smaller than threads just as the body of the glow plug is smaller than the threads.

I have never bought one before. The few times that I've ever needed one, I was able to get the crud out with an ice cream stick. It is much more time consuming but it worked for me.

That said, the times I've done it, I was dealing with goo rather than crust. If you have crust, then the only way to safely remove it is probably with the reamer.

It won't hurt anything, but if you don't have so much goo or crud that it is pressuring against the glow plug, it is not necessary. Also you will probably need to remove the injector tubes to get access to use it.

Good luck,