6.2l broken piston!

[lmocibob]

about a month ago i posted a question on engine knock coming from valve cover area.
i dropped the oil pan & found 5 pieces of aluminum (broken piston) it came from no.# 6 cylinder the whole skirt was missing.
can someone tell me why these pistons fail?

also can someone tell me why main bearing webs crack?

[john8662]

I don't know why some pistons crack, maybe excessive piston to cylinder bore clearance, piston slap = busted skirt. I really don't know, I've seen cylinders that had bad rod bearings below do this too. Could that cylinder have swallowed someting, like a glow plug tip or become hydrolocked from a leaky gasket or injesting water?

So is your block cracked?

There is NO straight answer on why the blocks crack. We all have our "theories" why they crack, some of the theories have more weight than others. One that's pretty good at explaining sudden death crank failure is a bad harmonic balancer, and I'd assume that the cracking main web's could have been related or contributed.

Mainly most of the 6.2 blocks that crack are due to poor casting, which includes the majority of the blocks in service. Some of the early (most 82 red) blocks had a better metalurgy with a higher nickel content that seems to be a bit stronger.