You knowingly put plugs with different threads into a soft aluminum head? I would worry more about the now damaged threads in the head. If it is running fine and you are blessed that one or more does not blow out, I would never, ever, ever remove them again. June

The critical thing is the thread per inch or mm per thread count - that has to be the same for the plugs and the head. If you can, you can put the threads into each other and see if they mesh - if they do, you are ok. Of course, if you have cross threaded the wrong pitch plug in, don't remove it unt...