The pipe threads are tapered, as is the tap. You can chase the threads with a tap, which increases the size of the hole, but at some point the plug will go too far into the case to seal correctly. All you can do is chase it a bit to clean it up and retry the plug. I have seen inserts at some of the bolt supply stores, but they were not cheap.
There is always the option of tapping to the next larger thread size. Most Triumph plugs I've run accross are 3/8" pipe, so the next size up would be 1/2" pipe.