if it saves your life or that of your family, giving enough notice to seek shelter, it’s worth the money spent! if you can find another bolt which is used for your main caps then i would also cut a slot in it and chase these so that the threads are clear and clean. we marked the course at 4a.m. actual measurements taken flat on the outside: tops measurements. you will know when you need a head replacement because the reminder bristles will fade. i’m not around to please anyone any more, and it’s a huge relief. 