This ambulance has a 6.0 liter diesel came in with a dead miss on# 6,relative comp. Was down on# 6,powerbalance showed a dead miss on# 6,fuel press good inj.buzz test passed.decided to do a compression test all were good accept #6was very low,removed head to find bent valve and piston had made contact with with valve,there was no signs of debris in cyl.or exh.any body have any idea what could havecaused this kind of damage,or anything else I should be looking for.I think this is looking like retail if I can't find cause of failure.any. help muchly appreciated.