[rzrektd]

What can cause a broken piston skirt? I also found two other pieces of steel in the oil pan, but I can find where it came from. Pistons are aluminum and everything else is accounted for including piston skirt that broke off. The steel pieces are square/rectangular in shap 1/8" thick and the rectangular one looks to be folded over. I am thinking maybe someone swapped the oilpan from a different motor that was blown.

I am more concerned with the broken piston skirt and what can cause it. I am thinking maybe the steel piece hit the piston and caused the piston damage. Has anyone ever ran a motor with a broken piston skirt?

[formulabird428]

Someone correct me if I'm wrong, but I've always heard that if the bore size is a little too big, the piston can wobble and bust the skirt. I could be wrong, but I believe someone told me that some time ago.

[PONTIAC DUDE]

Factory parts?
Recent build?
Cast or Forged pistons?
Pin tight on the piston?
Used or new pistons?


Hard to do with a Forged piston, but quite a norm for cast and what I call

Star Trek'in the engine.................. taking it (rpms) where no man has gone before.

Have also seen it when the rods are installed wrong and the pin end of the rod pressed sideways against the piston and causing excessive load on the piston. Had a customer way back in the day build his stroker 383 SBC and had two on the same journal done this way. Had a knock, the shirt broke in a wedge and stayed on the piston till it was pull out of the cylinder.
The cylinder would have to be excessively oversize to cause that with a Forged unit.

[rzrektd]

Originally Posted by PONTIAC DUDE

Factory parts?
Recent build?
Cast or Forged pistons?
Pin tight on the piston?
Used or new pistons?


Hard to do with a Forged piston, but quite a norm for cast and what I call

Star Trek'in the engine.................. taking it (rpms) where no man has gone before.

Have also seen it when the rods are installed wrong and the pin end of the rod pressed sideways against the piston and causing excessive load on the piston. Had a customer way back in the day build his stroker 383 SBC and had two on the same journal done this way. Had a knock, the shirt broke in a wedge and stayed on the piston till it was pull out of the cylinder.
The cylinder would have to be excessively oversize to cause that with a Forged unit.

From what I can tell they are factory cast units. 69 428 XF code engine.

The other interesting thing I found was two steel chunks in the oil pan of this motor. One about 1" X 1" and the other 1" by 3" and folded in half with a 16N stamped on it. I cannot for the life of me find a place they came from in this motor. I can only speculate that this oild pan was swapped out at the junk yard some time before the engine was pulled and piston skirt failure was a coincidence. I did fint the piston skirt in the oil pan, so that can really rule out the pan swap unless skirt was in windage tray until I rolled motor over on engine stand.

Interesting stuff.

BTW, I like the star trek comment!

[Geoff]

Could the steel pieces be from the windage tray? They have a reputation for cracking. If pieces broke off, could have jammed/broken the piston skirt.

[PONTIAC DUDE]

Cast pistons and run hard. Not an abby normal thing to see.

Seen it a bunch of times. Cast pistons are that, Cast.
BTW: So are hypertectics, just a different process. And have a limit as to rpm, compression, etc.

Throw em away or ash trays, start over and build you a killer 428.

[Mr. P-Body]

When we took the pan off our '69 428 to reseal, we found as broken skirt. Detonation was the culprit. 10.25:1 and 93 octane simply don't "mix". No one heard the first ping. All the pistons showed pitting on the head, and the skirts were collapsing. If this engine is entirely original, lower the compression when you put it back together.

Jim

[67ConvFB]

When I was younger, I had a 389 that would spark knock sometimes. When it started smoking out the exhaust, I pulled it apart, and found broken ring lands and a broken piston skirt. They were cast pistons.

[Mr. P-Body]

67,

Classic symptoms of detonation. Forged pistons would have done the same thing.

Jim

[PONTIAC DUDE]

Originally Posted by Mr. P-Body

67,

Classic symptoms of detonation. Forged pistons would have done the same thing.

Jim

Ditto. See it all the time with people trying to 'push the edge' thinking they need max compression on 93 octane to make power on the street and pump gas.

The difference in 1/2 a compression point can be more then made up in cam or head flow. 90% can't feel the 1/2 to 1 point anyway.