I've been working on a stationary pump engine that has been giving us problems for 5 years. It is a 1978 Cummins NT855 F1  for fire pumps. The engines it has ~ 2K hours on it, most of that time is from exercising 30 min per week pumping at the given rate of the system.  
The problem we are having is that the # 5 piston is over heating  with in the cylinder causing scuffing on the cylinder wall and or the crown is being melted to the point of seizing the engine. The cooling system seems to be drawing off the heat of the engine in general, the temp. gage (tested) is running in the range of 180 degrees This engine has been rebuilt now for the 4th time. New cylinder kit, (piston, rings, liner) new main bearings and the rod bearings through out.
Each time we get about 200 hrs on it before failure. The engine has been rebuilt by Cummins trained personnel from 2 different companies, and no one can give us a definitive  reason for the cause of the failures.

Can any one try to shed some light on what the causes might be?

Does the engine run rough?  First I'd suspect the #5 injector, but not over fueling, rather not atomizing the fuel and spraying raw fuel onto the cylinder wall washing down the lubrication, unless of course it was the following:

I DID have a 1978 Cummins 855 once upon a time in an automotive application and I DID have some pistons that looked exactly like that but it was because the driver had monkeyed around with the injector pump and over-fueled it so much that it literally melted itself.  Only a couple of the pistons looked like that.  But you should have heard the stories up and down the road about how well it pulled (it was distinctively marked so it was easy to determine that it was my truck being bragged on).  Has anyone monkeyed around with the pump on this one and "turned it up?"

Is this a turbo'd engine?  And if so, is it a 300HP version or a 350HP?  Someone may have tried to make a 300HP run like a 350 by turning up the pump.  The timing and advance is different between the two and you can't get from one to the other by just 'turning up' the pump.

The photo is a typical thermal failure. The piston temp has increased causing expansion to the point where it no longer fits the bore, resulting in lubrication failure resulting in more heat generation rsulting in . . . . etc.
The root cause is often overfuelling or late timing but can be lack of lubrication or lack of cooling (oil squirter blockage) etc.

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