I am assuming you are replacing valves,seats and checking all the normal stuff. If so who is supplying the parts? Is Caterpillar doing so? If they are you should contact them. They are going to tell you to check your intake air for proper filtration and want you to check normal stuff like gov. settings, fuel air ratio control etc, tell them you prefer they do those tests and provide results in writing. If the problem persists tell them you are wondering about the quality of the metal in the seats and valves. Tell them you are considering sending samples to an independent metallurgical testing facility for tests. <br>Engine manufacturers subcontract a lot of their fabrication of parts and sometimes the quality control at the subcontractors is sub par but most engine manufacturers appreciate their attention being called to a recurring problem. Give Cat headquarters in Illinois a call if the problem continues.<br>Good luck.