These are the best valve seats I’ve found. They seem to last forever. No pitting or recession. They require a tight interference fit of .007″ in Norton alum heads but thats not a problem when you heat up the head. I’ve used them in street bikes, race bikes and propane V8 engines which eat up regular valve seats. I’ve never had to replace them. These are hard seats and I use Neway cutting tools to face them (machining is slow due to their hardness – so you want seats with the basic…