RE: Interference Engines
The simple way to see if an engine would have a clearance issue would be to hold a valve open and VERY slowly turn the crank. If you can turn a complete revolution without touching the valve, there is no interference. If there is a problem, it would occur if the cam somehow stopped turnning, slipping a few teeth would probably not be enough for anything to come into contact. The only failure that would cause interfernce would be if the cam somehow stopped turning completely with a valve open. With a direct gear cam drive (no belt or chain) that doesn't seem like the most likely situation.