2000 with 210,000. Bought with broken #5 piston, appears to have had a bad injector that bled into cylinder and cracked the piston. Cylinder continued to fire as evidenced by the hole in piston and underside of the piston both exhibiting some aluminum melting. Bored .020, new Ford Master Rebuild kit. Noticed on 3,5,2 and 6 (middle 4 cylinders) that the exhaust valves indicate some mushrooming on the tops of the valve stems. Several mechanics looked at it and didn't seem too concerned. Engine machine shop clean up the mushrooming and did a valve job and didn't seem too concerned. Mushrooming was not super extreme but would not allow the valves to be removed without first cleaning or they would have damaged valve guides. Lifters appear good, did not disassemble. Rollers are excellent and no wear patterns appear on the camshaft lobes. Does anyone have any insight or experience with this situation. The lifters are hydraulic so where is the 'slap' coming from? Engine knock, fuel delivery, timing? I am replacing all 8 injectors during the rebuild. Thanks for any feedback.