Truck Repair: 855 cummins, head gaskets, perfect head

Question
I have a 855 cummins big cam in a ford 976 tractor, i bought the tractor last fall used it for a half hour in the field and number 1 and 2 pistons started to score along the sides and seized up, i rebuilt it (2 new pistons and sleeves) the other 4 looked perfect i checked all the oil passages, the tractor has 1500 hours on complete rebuilt from engine to tranny everything was rebuilt 1500 hours before i bought it, so i put the two new pistons and sleeves, checked everything out got the injectors done and put it together, it ran perfect for about an hour of idling, driving then last week a went to do some harrowing and number 6 piston seized up. what is going on? its not from working cold i let it warm up really good like hot before i even moved it, the top of the piston is perfect head is perfect but sleeve and piston are scratched and scored terribly please help me out

Answer
From experience, I know that overheating due to malfunction of the cooling system may lead to piston/sleeve scoring. I advice that you confirm that your cooling system components,i.e. water pump; driing belts, thermostat and the coolant itself are ok.
Also I need to confirm that you used new and correct head gaskets when you replaced the two scored pistons.

you may also check your connecting rod small end bushings for excessive play.

kind regards,