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Old 04-19-2007, 10:52 AM
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Piston Skirt Damage: Consequences?

Hey guys, I'm new here, but I am a long time member of AutomotiveForums.com whick seems oddly similar to this site. Anyway, I was reading a post about piston skirt damage, but since it was from 2004 I figured I'd start a new one with my question.

A buddy of mine's got a Chevy 305, rebuilt for light performance, but he didn't like how agressive the cam was, so while in the process of changing the cam he dropped the pan just for sh*ts. Come to find out the piston in cylinder 5 has a broken skirt on the outer side. We don't know how long the engine's been running with the piston like this, since the rebuild it's got about 3000k on it. He's thinking about just cleaning up the break with a file to eliminate a weak spot What's the worst case and best case scenario of him simply leaving it be and running the engine as it is? Thanks in advance!

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Old 04-19-2007, 12:47 PM
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If it is missing a piece than the balance is off. If it is cracked the heat may cause the skirt to expand into the cylinder Wall causing noise and damage. Replace the piston, if you know what size bore it is you can buy just one piston.
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Old 04-19-2007, 01:02 PM
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That piston will continue to break up till the wrist pin goes thru the cylinder wall and ruins the whole motor. Fixing it now will be a lot cheaper than when it breaks.
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Old 04-19-2007, 02:45 PM
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My mistake, it's a 350... 1969 four bolt specifically and unfortunately we don't know if it's the stock bore, or if it's bored over. The pistons are domed as well. I know that there is different cc's as to how much a piston can be domed. There are some numbers stamped on the bottom of the pistons, but aside form that how can I identify the size of the piston as in bore and height of the dome?
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Old 04-19-2007, 03:41 PM
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Take the piston out and just take it to a machine shop they can identify it for you. They have books with casting # in them that will tell what it is.
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Old 04-19-2007, 06:08 PM
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Doc here,

If you continue to run it, the Piston Can/will Expand and contract from hammering ( normal firing) the crack Can/will probably expand and the wrist pin might break or sieze..If not there, the rings (if the crack runs like a glass crack..) Can/will expand and crack, the Cylinder walls will score , possibly past run out..

If the wrist pin seizes, the rod can come loose, or break, or bend, and lock up a running engine, as well as punch a hole through the piston, as well as the cylinder wall..

The "Off Balance" side of the piston will "Dig" into the Cylinder wall and possibly damage it past repair,

The Rod Bearing and rotating mass bearings will wear from an off balance situation..

OR Nothing at all will happen..Are you good at drawing a straight flush on a 4 card Draw? I'm not...

Look at the bottom of the piston, where the wrist pin goes through the "deck" on one side or the other it will have a stamp, like 020 or 040..ect..that will tell you if an oversize piston was installed..Or you could Mic it..

My advice, since you don't know how long it ran that way, and Since I can't look at it in person, is, Don't reuse ANYTHING from that cylinder..new rod, wrist pin, new piston, new bearings , New rings..Cut the ring groove, Inspect the cylinder, and do a good cross hatch honing before putting it back together.

My nickels worth..

Doc
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Old 04-19-2007, 09:25 PM
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Wow guys, thanks for the advice! I pulled the head today, the cylinder wall still looks immaculate so it couldn't have been running long that way. My guess is that's the whole reason the guy sold the engine. Tomorrow after work we'll work on getting the piston out and identifying it. I'll keep posted as to how it goes.

P.S. We were told by the seller that the engine only had roughly 3k on it since the rebuild. Its cleanliness seems to agree with that statement, but I did notice an ever so slight ridge on all the cylinder walls just detectably by fingernail... Is that normal for that low of mileage? As far as I know it was honed properly due to the fact that you can still see the honed surface inside the crank case where the rings don't touch the cylinder wall.
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Old 04-19-2007, 10:09 PM
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Quote:
Originally Posted by Petea216

P.S. We were told by the seller that the engine only had roughly 3k on it since the rebuild. Its cleanliness seems to agree with that statement, but I did notice an ever so slight ridge on all the cylinder walls just detectably by fingernail... Is that normal for that low of mileage? As far as I know it was honed properly due to the fact that you can still see the honed surface inside the crank case where the rings don't touch the cylinder wall.
Doc here,

A slight ring ridge is Normal after break in..Shows it broke in properly..

You need to put a good cross~Hatch Hone on the new piston cylinder, before installing..Or the rings won't break in, and you may be plagued by excessive blow~by in the future..

Try to do this AS Squeaky Clean as possible .. you Don't want the hone stone dust contaminating the rest of the engine!! Make a depth gauge on the hone just above the bottom of the cylinder, cover the bottom with a plastic type baggie and duct tape it to the bottom of the cylinder hole to catch all the stone dust..build a "Catch Dam" around the top with rags..BE sure you don't nick either while honing, and use cutting oil..When done, Wipe (wash) the cylinder with acrylic paint thinner until Squeaky clean, then remove the baggie and top catch dam, wipe those areas down also..then Install the new piston, rings, rod, in the cylinder..

Run it for about an hour after your done then change the oil and filter to prevent contamination.

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Old 04-20-2007, 04:18 AM
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I have to disagree on the ridge. Most rings today are such that they break in very soon after startup and should not wear the cylinder walls. This means that the bores should be honed to a very fine finish and should not wear much at all when the engine is rebuilt.

I would be it is an old ring groove from wear before the rebuild and they just honed the cylinders when they rebuilt it.

Chris
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Old 04-20-2007, 07:57 AM
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Well, either way I don't see that small of a ridge to be of concern. When running the engine for an hour or so, can some of that hour be at idle? For the first 20+ minutes then engine will have to be ran at 2000 - 2400rpm to break in the flat tappet cam properly.
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Old 04-20-2007, 11:15 AM
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Pretty sure the cylinder wall finish is dependent upon the type of ring used...........................
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Old 04-20-2007, 12:31 PM
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The 305 out of my 86 Chev truck had a skirt broken off when I pulled it apart for an overhaul. I was the first one into that motor which meant the piston could have been like that for 130K miles. Cylinder wall and piston were in relatively good shape.

Cam
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Old 04-20-2007, 12:40 PM
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After you hone it, clean it clean it and clean it some more, the hone dust will settle into the bore, and if it`s left it`ll wear the rings out in very short order. Clean the cylinder then do a check, use a paper towel with some WD40, if you see gray on the paper towel it`s not clean.
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Old 04-20-2007, 12:41 PM
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That ridge could be from before the rebuild. Has this engine been bored or is it still standard bore? If it is std then my guess would be that all the ridge was not removed on the rebuild.

Run a ball hone in there and put in the new piston and rings and be on your way.
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Old 04-20-2007, 04:48 PM
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Doc here,

One other thing..I forgot..

BEFORE you start honing, Seal (wrap) the rod journal up with plastic and tape..THEN rotate it AWAY from the cylinder base..

That way, If you sneeze while honing , you can't nick the journal..(best laid plans of mice and men.. ) And you'll keep the Crud~Bunnies off the journal.

Doc
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