Bent Conrod

So today was spent renewing the valve guides.......not a problem.....and then trying to remove number 4 piston and conrod.
The rod was bent to such an extent that it would not pass up the bore. The width of the bigend is only 1mm smaller than the diameter of the bore. Any way, I managed to weaken one 'ear' of the bigend by drilling a series of holes until I could break it off.
Below is a picture of the offending rod next to a straight one...and the piston as it came out.

The 'eagle eyed' will immediately notice that the top piston ring is absent!!

So this is how I reckon the rod got bent.....

The engine had a tendency to runaway in clouds of smoke.
Number 4 cylinder was soaked with oil on removing the head.

Clearly it was getting engine oil up passed the piston, not helped by missing a ring and the oil control ring was excessively worn. I think then, that when the conditions were right it would diesel the engine oil but where the diesel injectors are timed to allow combustion at or just past TDC, the engine oil combustion would happen earlier trying to drive the engine backwards......this being resisted by the inertia of the flywheel and the other 3 cylinders. So not being able to win against this it bent the rod.
My theory anyway. It was good though to find something definitely wrong. How though, anyone could omit a ring I dont know.
Onward, ever onward.

Your theory sounds feasible, perhaps just a build up of oil above the piston would be enough to bend the rod just like injesting water?
Where the ring went is a mystery. With my 2.25 the remains of the ring were embedded in the top of the piston. Any head damage ?

I too would suspect oil above the piston to be the cause.