Jim is very correct on this subject. Clean threads are most
in preventing the problem. If you reuse the old plug then it is
to clean the threads on a wire wheel. I usually do this. If I
have one available I use a rag and fingernail to clean them.
usually works. If there is gunk that won't come out a knife
will take care of the problem. If you don't have a thread chaser
can make one from a old plug by filling a couple of notches on
first few threads and running this into the plug hole. By
plug threads clean, most of the time you will not have to chase
threads in the head.