OK, on pre 96 engines, the exhaust valve and intake valve stems were 6mm, from 96 on, they all got 7mm stem sodium filled exhaust valves. You can change the guides out. I recommend it highly, getting 6mm exhaust valves seems to be neigh impossible these days. I've not seen any wear on the chilled cast iron guides. They are hard as hell. Usually just the seats need dressing.