Question : Piston cracked... how bad?

[Duck_Hunter_117]

Alright so I am tearing down a 3.4L I pulled from a 1995 Firebird w/ 144k on it. Everything looked great when I tore it apart however once I got the pistons clean I noticed a small crack on 4 of the 6 pistons. All are in the same location on the thrust side of the piston. I have attached a pic of the worst one. Is this a known crack location on these engines? Has anyone ever failed an engine due to a cracked piston in this location?

[SappySE107]

I have never seen anything like that. To have that on 4 of 6 pistons is interesting, as it shows a stress fracture for more than 1 cylinder, in the same location. The pistons must be to a pretty consistent specification to reproduce the same area of fatigue under what I would have to assume are equal cylinder properties.

I would not use those pistons again, and I would check the crank as far as collateral damage from detonation, which is what I am going to guess caused the pistons to react like that.

[Duck_Hunter_117]

If the root cause was detonation do you think I would have seen something on the top of the piston? When I searched for cracks it seemed like the top ring area was the weak point of this piston. Of course there is a ton of information to digest here and I just started last night so I'm sure I'm missing something.

[Superdave]

It's hard telling what caused that.. Maybe the cyls are out of round or piston to bore clearance was too large and you had some nasty piston slap.


They'll make good paperweights.

[dskopek]

I have seen pistons break off the bottom of the skirt before but that was on high compression motor with a lot of cyl clearance (.005") Never on a production motor.

[ForcedFirebird]

This is the second case I have seen on an iron head 3.4 with multiple pistons cracked.

[Duck_Hunter_117]

I spoke with a piston expert at a major automotive supplier to get their take on these cracks and came away with a lot of knowledge. The area that these cracks formed are a known area of high stress concentration for this piston design for two reasons. The first is that it is a strutted design, the strut is the steel piece running along the inside. The second design "issue" is the thin skirt that is needed to cut through for the oil groove drain. According to the expert I spoke with each of these are known to create stress concentrations at the open end of the piston at the skirt to panel transition. Mystery solved and new pistons on order.

[TGP37]

Maybe the engine sucked in some water at some point and almost hydrolocked.

[ForcedFirebird]

Maybe the engine sucked in some water at some point and almost hydrolocked.

In the other case I saw, it was a GM 3.4 crate replacement for s10's (in which it was installed), drag raced occasionally, but just driven. 5 of 6 pistons looked just like the above photos.

I have broken down several 3.4's and haven't experienced this issue to date.