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Old 03-22-2015, 03:34 PM
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Piston Damage

Hello all..

I was tearing down the 383 I recently bought to see what pistons I am working with. I removed the iron heads to find they are dome pistons with no stamping or makings on them. With further investigation, I noticed the #3 piston looked damaged. I inspected the cylinder wall, and the head chamber/valves but no damage at all to any. There was no debris in the cylinder at all.

The champion spark plugs looked good..too good actually. The plug pictured is the #1 plug and was the only one that had at least some carbon on it. The other 7 plugs were fairly clean. From what I know, that could mean it was running very lean, correct?

Could it have hit a valve or melted? Or been damaged during assembly? Detonation?

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Old 03-22-2015, 05:15 PM
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Detonation would be my best guess. That dome looks to be maybe 7cc's and when combined with a 64cc chamber in a 383, would make about 13.0:1. With 76cc chambers, about 11.0:1. All it would take to destroy the whole mess would be pump gas.
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Old 03-22-2015, 06:27 PM
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Yeah, I figured it was something along those lines. So what happened to that portion of piston? Think it melted or blew out the exhaust?

I plan to put either dished or valve relief flat tops. Planning to purchase 64cc aluminum heads so I want a fair amount of compression but am not trying to go outside of pump gas. Is roughly 10.0 compression with aluminum heads okay for mid grade pump gas?
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Old 03-22-2015, 07:46 PM
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That's classic detonation damage, for sure. The piece broken out likely was broken into smaller pieces and went out the exhaust. Like Inspector said, all it would take is run it hard once on pump gas with that dome in a 383, even with 76cc heads.

Champions are the worst plug to use in a Chevy, use AC's, Autolite's, or NGK's
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Old 03-22-2015, 08:10 PM
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Quote:
Originally Posted by Jahblah90 View Post
Hello all..

I was tearing down the 383 I recently bought to see what pistons I am working with. I removed the iron heads to find they are dome pistons with no stamping or makings on them. With further investigation, I noticed the #3 piston looked damaged. I inspected the cylinder wall, and the head chamber/valves but no damage at all to any.
Maybe it's something with the picture, but the bore looks wiped out/scratched deeply. Looks like it's ready for a larger piston. Can you catch your fingernail on those vertical marks in the picture?
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Old 03-22-2015, 11:10 PM
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Quote:
Originally Posted by Jahblah90 View Post
Is roughly 10.0 compression with aluminum heads okay for mid grade pump gas?
Yeah, but you need to engineer the squish at between 0.035" and 0.045" to help prevent detonation. Aluminum heads will want a thicker composition gasket, like a Fel-Pro 1003 that compresses to 0.041, so you may have to cut the decks to get to zero to make a 0.041" squish. Shim gaskets will not work well on aluminum heads due to the moving around of the head because of the differential in heating and cooling between the aluminum head and the cast iron block. They'll cause fretting of the aluminum. Measure your stack of parts and cut the decks to the stack height. For instance, 1.875" crank radius + 5.7" rod length + 1.425" piston compression height equals 9.000".
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Old 03-23-2015, 12:06 AM
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. Am I looking at that picture wrong? Looks like a piece broken out of the piston and a piece broken out of the cylinder wall next to it...
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Old 03-23-2015, 08:44 AM
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Quote:
Originally Posted by Jahblah90 View Post
Hello all..

I was tearing down the 383 I recently bought to see what pistons I am working with. I removed the iron heads to find they are dome pistons with no stamping or makings on them. With further investigation, I noticed the #3 piston looked damaged. I inspected the cylinder wall, and the head chamber/valves but no damage at all to any. There was no debris in the cylinder at all.

The champion spark plugs looked good..too good actually. The plug pictured is the #1 plug and was the only one that had at least some carbon on it. The other 7 plugs were fairly clean. From what I know, that could mean it was running very lean, correct?

Could it have hit a valve or melted? Or been damaged during assembly? Detonation?

Attachment 211658

Attachment 211666

Attachment 211674
Either detonation/preignition of the ring gap is too tight.

Bogie
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Old 03-23-2015, 12:25 PM
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Looks similar to this one, on my 383.



When I got it apart the piston looked like this.



The top ring gap was too tight and it butted, forcing that piece of the piston up and out. Luckily it didn't kill the head or valves. Most of it stayed in the bottom of the cylinder and a quick cleanup of the valve/seat fixed it up. I'd have your cylinder head checked.
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Old 03-23-2015, 06:24 PM
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Quote:
Originally Posted by Jahblah90 View Post
So what happened to that portion of piston? Think it melted or blew out the exhaust?
. Likely the seller removed the pieces, put the head back on, and sold the whole problem to you...
.
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Old 03-24-2015, 04:00 PM
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Went out and checked the bore for scratches. Smooth as glass. I got as close as possible to take another picture of the piston and it looks just like Rick90lx piston does. You can see the 2nd layer of piston between the break.

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So maybe the top ring gap was too tight like you guys mentioned.
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Old 03-24-2015, 06:48 PM
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Yeah, the piece of broken ring is missing too, you sort of got hosed on that deal if he didn't tell you it was broken. He obviously removed the pieces before he sold it, or you have some bent valves. In my case, I was at @ 10#'s of boost when mine let go.
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Old 03-24-2015, 07:15 PM
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Are those hypers or forged?
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Old 03-24-2015, 10:31 PM
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I don't know how to tell the type of piston. All I know is the pistons are dome. And the guy told me it needed piston rings but didn't mention that the piston was jacked up. It was going to be disassembled anyway to figure out the specs for final compression. I have never wanted dome pistons to begin with so swapping pistons was not out of question. Just kind of blows if the guy KNEW and didn't say. Glad the wall isn't destroyed.
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Old 03-25-2015, 12:16 AM
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Quote:
Originally Posted by Jahblah90 View Post
the guy told me it needed piston rings

. Yeah, sellers sometimes give a clue to a concealed problem... I bought a used boat once, and guy said it's always good idea to carry a spare battery, his hint the charging system didn't work...


. Looks like the tight ring was polishing the bore in that spot before it locked up completely and broke out of the piston...

. Once piston removed, prolly some markings on the side of it... also need to know rod length used... check the gap on top ring of a good piston in top of bore...
.

Last edited by BuzzLOL; 03-25-2015 at 12:22 AM.
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