I have taken my mates TT head to get the valve guides replaced. Now the machine shop says they need to grind the valves and seats as the valve angle will not be the same with the new guides. I may be wrong but this doesnt make sense to me. Surely if you remove the existing guide and press the new one in, the valves are going to be in the same position. Are they correct or are they spinning me a story to make an extra buck?