cavatation is one thing, when you hit the throttle, and the engine revs up and ski really doesnt accelerate to quickly, also cooling sytem, cause its based on pump output, so the normal high end spec is .040 of an inch, then anything over that it needs a new wear ring, some "grooves" are ok, but not good by any means, around .015 is a nice clearance between wear ring and impeller, take a pic and post it maybe, but a wear ring is not a real tuff job to do, and not real expensive
