Mahle piston wrist pin bore weird?

[powermaxx]

Is it just me or has anyone else seen this on thier Mahle pistons? The wrist pin bore has 2 distinct areas where the pin cannot touch. It is not "Egged Out" as the 2 smaller "Mouse Ears" (you'll see what I mean in the pics) have machining (broached) marks that go along the length of the bore (90 degrees opposite of pin rotation). The surface(s) that actually touch the pin have radial marks that match the pin rotation. The other pistons from this set, 2 have perfect bores (no divots or mouse ears) and the 3rd has a a simular pattern of "Mouse Ears" but are much smaller.

Now it seems to me that the wrist pin bore should be a full circle with no interuptions for even loading. Since the "Ears" start at the "3 and 9 o'Clock" positions of the bore and the cracks start/end there it is making me suspicious. Since I'm a Mechanical CAD designer I have modeling these in our FEA system and we'll see what happens when under load (should be interesting).

[Ben]

I wonder if that crack could occur if the piston was in the early stages of seizing, and was cracked on the intake stroke. At that point, the rod (and therefore) pin are pulling the piston down. Looks like the pin was trying to pull the skirt off. The top ring looks a bit rough as well. Also, the location of the cracks - both above the centor of pin bore, like you said - cracked along the reliefs (where the bore is out-of-round) - but I think this supports my thought. You know what I mean? Just thinking out loud..

[glhsken]

From the looks of it, I'd say you are on to something Ben. That is definately odd. I've seen a ton of detonated pistons. NONE EVER affected the pin bore.

Defective pistons??

[powermaxx]

I won't be able to run an FEA until late today or tommorrow (I love these CAD tools!). So far most folks who've seen these have said it's weird, looks like a "Tension" break vs "Compression" (especially since the top ring is undamaged). The cracks are on both sides of the piston, pretty symetrical but this is the worst side The inside of the crown is almost cracked all the way around, this was close to seperating and totaling the engine.

[turbovanman]

Ha, that was my other suggestion in your other thread, :big grin: , that the piston pin could have seized!! Looking at your pics, it looks like the piston pin is offcentre and that would cause the pin to bind and crack your piston. What do the others look like?