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Old 04-09-2006, 06:51 PM
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all my piston skirts are broken

hi, i just pulled my piston out of my 350 (94 chevy truck) and all the pistons had skirts on one side broken off (the side facing the cam) and all the rings were broken on the opposite. i put the pistons in with the notch facing forward and had the right rings and bearings (std size). what would make this happen? any help would be great.

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Old 04-09-2006, 08:14 PM
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what sort of noise did the engine make before you dismantled it?

Was it knocking?

Wrist pins could be a likely culprit...but it would be odd for all the postons on one side to have the same damage.

Got any pics of the damaged pistons?
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Old 04-09-2006, 09:09 PM
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the engine had a bad knock to it when it started up but after about 10 sec or so it would go away, and after it was warm and i shut it off and started it back up (like if i went into store) it didn't knock. and by the way ALL of my pistons are broken the same way. i could understand one but ALL EIGHT! and sorry no pics, i just pulled them today and was so baffled i didn't think about it.

Last edited by deltadog; 04-09-2006 at 09:18 PM.
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Old 04-09-2006, 09:23 PM
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is it a motor that you built yourself ? if not, then maybe they were broken a long time ago ?

not to steal your thread, but i have a question regarding broken piston skirts: can a skirt be reattached to a piston(welded/brazed) without affecting performance or rigidity/integrity ?
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Old 04-09-2006, 09:40 PM
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no i built it myself and this happened after about 15,000 miles and some quarter passes (street not strip) the reason i tore the motor down was because i was leaking oil like a sieve. i was putting about a quart and a half everyday for about 2 and a half weeks.
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Old 04-09-2006, 09:41 PM
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all my piston skirts are broken

my guess it would be from too loose of piston to cylinder wall clearence or used a really cheap japaniese piston with to much clearence. i think chevy was running hyperneutenics pistons at that time so replace them with a set of speed-pro's hyperneutenics and machine the block to the right clearence, and you'll have 300,000 mile motor
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Old 04-09-2006, 09:49 PM
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well the clearance thing might be but the pistons were stock hypers
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Old 04-10-2006, 06:19 AM
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Were there any marks on the tops of the pistons like they were hitting the head?

Also sounds like too much clearance. Hypers only need like .0015
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Old 04-10-2006, 06:54 AM
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there were no marks on top or bottom. if i find it is too much clearance what then? bore it or just a good honing?
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Old 04-10-2006, 07:19 AM
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The problem is likely to much clearance. You'll have to measure the bores then see if you can get pistons to fit. If not you'll have to bore the block to the next size piston. Sounds like the block was maybe clearanced for forged pistons. Forged require more clearance than hyper, which require the same clearance as plain cast AFAIK. You could likely go to forged pistons without boring.
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Old 04-10-2006, 07:35 AM
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It id definitely a piston/cylinder clearance issue.

The pistons are either too small for the bore,or the cylinders have become egg shaped at the bottom from the side ways motion of the crank,this same thing is very common on old 300 sixes with a long stroke.

The skirts break off on the side that slaps against the wall on the upstroke during compression when there is no resistance on the piston,that's why only one side breaks.

The cylinders are probably larger at the bottom than the top.

I would not rebuild it again until it is bored out and honed properly.
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Old 04-10-2006, 11:17 AM
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Just a thought here. Could the crank have been improperly ground in the area of the rear thrust bearing? If so, the crank could "walk" forward and backward, putting a front to back load on the rods/pistons causing them to over stress and crack off the skirts?
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Old 04-10-2006, 01:02 PM
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Pistons

Lets get some more detail,,,

You say all the pistons from one side are broken... Do you mean like 1,3,5,7 or do you mean one side of the skirts on all 8 pistons???

What rings are broken?? All 8 top rings?? All 8 second rings??? $ top on one side??

How many miles since you assembled the engine??? 5 or 50,000

Did you change anything else in the engine?? Like connecting rods,crank pistons????


Keith
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Old 04-10-2006, 08:16 PM
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all 8 on the side of the pistons facing the cam and mostly the second ring on each (some had top and second) only about 15-20,000 on the motor. the only adjustment made was installing roller rockers. all the rods and pistons were stock assembled from the factory all i did when rebuilding was ghange the main and rod bearings and put new rings on.
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Old 04-11-2006, 07:40 AM
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I alway have the pistons in hand when i take the block to the machinist.... I have him measure the piston and then bore the cylinders base on the actually piston diameter + clearance. It eliminates errors. I always file fit the rings too because my engines are not always bored exactly .030, 040, etc. due to this. But when your done, you need not worry about piston rock.

Just my method, and $0.02
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