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Piston Damage

10K views 64 replies 15 participants last post by  BuzzLOL 
#1 ·
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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#33 ·
UPDATE: Removed the damaged pistons today. Going to see about purchasing another set. I am trying to identify what pistons these are, so I'll know what pistons I must buy. I know the piston has to match the rod length, which I don't know that either. What is a good way of determining the rod length? A ruler?? LOL. The rods don't look damaged at all so I shouldn't have to replace those.

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#42 ·
. Expensive CP/Carillo forged pistons... over $100 each...

http://www.summitracing.com/parts/cnn-s1303/overview

. Carillo rods and forged crank, also?
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You need to put your glasses on Buzz, or get a bigger monitor LOL

Those are certainly NOT CP forged pistons,...... it is obvious by looking that they are just cheap cast pistons, could be plain old cast $10 each cheapy's , or could be a claimer hypereutectic....but they are NOT forged, there are casting lines everywhere on them, casting numbers, cast waffled surface in the side reliefs, cast mark in the center on the dome, etc.

Looks like they are probably a 5.7" rod piston, but measure center to center on the rod bores to be sure, choices would be 5.565", 5.70", 5.850, and 6.00"...roughly an 1/8" difference in each length so a good eye can do it with a machinist steel ruler or even a clearly marked plastic ruler....even easier with a pair of calipers.

With a 64cc head you will definitely need a dished piston for pump gas in a 383.

At the very least you will want to have the crank crack checked(magnafluxed). I wouldn't expect it to be hurt, but it has to be checked to be safe, as a crack here will be invisible to the naked eye but catastrophic when it causes failure.

The rods with the broken pistons should also be checked for straightness, to make sure one didn't get slightly bent when it was mashing up those big chunks of piston.

If you are going to try to go over 450HP when you rebuild this, a forged piston is good insurance.
 
#36 ·
The rods didn't have any markings on them that I can see. I don't know about the crank either. But $100 each definitely isn't going to happen. I'm replacing the full set with flat top or dished pistons. Hyper pistons probably since I've read that forged pistons aren't necessary for NA engines under 500hp. I've never dealt with pistons before so this is all new to me.

Machine Metal
 
#38 ·
One of the previous replies said that they looked to be 7cc dome pistons so that's what I've gone with. I've never dealt with pistons before. I plan to run 64cc aluminum heads and want 9.5 or 10.0 compression at a MAXIMUM. So dome pistons are off the list.
 
#40 ·
Looks like a 5.7 rod from here.
Calipers are good enough. Your choices are 5.56", 5.7" or 6". Measure from the parting line on the cap to the center of the pin. I'm guessing 5.7 here cause the 5.56 doesn't get into the oil ring and the 5.7 will just touch it and the 6 will be half way into the expander of the oil ring requiring a support rail.
 
#41 ·
You need to take the mess to a machine shop to have it check - rods, block and crank. They'll measure your rods, check your bores, measure your crank and check to see if it needs a line bore for free - possibly, but cheap at least. Personally, I would not put that back together without getting the rods and crank checked at minimum.

Also, if you are running 64cc heads and pop up pistons, then you compression will be higher than 9.5:1 if the quench is acceptable - you need to rethink your piston purchase. What are you doing with the cam?
 
#44 ·
if you're serious about salvaging this mess i'd take it straight to a competent shop and have it all looked at. the amount it costs to check and fix the rods you'd be ahead to buy a new set.

the piston damage looks like a butted top compression ring from improper end gap during assembly. perhaps those pistons are Hypereutectic, if so the top compression ring would require a larger end gap than the 2nd compression ring. for instance, when using a KB hyper piston you can download a master sheet that provides a simple equation that will calculate necessary ring end gap. some of the determining factors would be compression ratio and intended usage. the top of a hyper piston apparently takes on more heat and sort of reflects it back into the combustion chamber, the effect of this is a greater loss in ring gap than that experienced by the 2nd comp. ring. (or something to that effect). the ring gap factor should be rechecked for every piston in this engine as i would suspect them all to be wrong.

you can run a carburetor on the LS engine. there's an intake/ignition box kit that will control the spark and provide you with a carb intake, they're a package deal.

like these:
LS Carb Intake Kit Edelbrock RPM Intake MSD 6012 Ignition Quickfuel 750 Carb | eBay

Holley 300-132K, Holley LS Single Plane Intake Manifolds Carb & EFI | Holley

just be sure you have the correct reluctor on the engine. up till 2005 they're 24 tooth, 2006 and later they're 58 i believe, it'd be easy to figure that out.

i dont know what you've got in this thing already but it could turn out to be a small money pit.
 
#45 ·
Ben S.....the KB Signature Series Hypereutectic pistons have that special larger top ring gap requirement because KB moved the ring pack higher on the piston, making the top ring closer to the top of the piston, (where heat is higher causing the ring to expand more from heat)......it is not because of the hyper material.

Most other company's hypers, like Speed-Pro/Sealed Power, Summit brand, Federal Mogul, Mahle, UEM/KB "claimer", etc have the rings in a more or less "standard" location and have no special ring gap needs, other than allowances for nitrous or forced induction or circle track/endurance race use getting a larger gap.

I still believe that the ring end gap "was" probably perfectly fine on this engine....had it never been run on pump gas, with the too high compression for pump gas causing detonation, which turns into runaway heat in the cylinder/combustion chamber.....and that is what caused the ring gap to close up and break the piston, as those pistons are just an ordinary cast or hyper, they are not KB Signature's with the high top ring placement.

I've seen it many times before....the result of uninformed home "engine builder" parts combination, with no real math or volume calculations done for the compression ratio, just "estimates", or "so-and-so said" or outright guessing on the compression ratio.

That's my educated reasoning on what the OP is looking at on his engine stand. ;)
 
#47 ·
UPDATE!!!!!!!! Okay, got of work today with enough time to make a mad dash for the auto performance shop. I took some of my "mess" to be analyzed for my rebuild basis. The pistons are dome 12cc and are standard bore. The rods are 5.7 standard rods. The crank is from a SBC 400. So I guess I have a 377??? THAT WHORE!!!!! lol smh well, I don't know much about 377's but I WANT my 383 so I guess I will get a .030 over bore. So far, I'm leaning toward hyper dished pistons for my rebuild. At least a 12cc dish and 64cc aluminum heads. AND SO IT BEGINS!!!!!!!!!!!!!!.............SMH
 
#48 ·
Chevy Bore and Stroke Data

Take a look, Your 400 crank is either a 400 or 383. If the bore is 4.0-4.030, then it's a 383, not a 377.

I'm glad to see you have taken it to a machine shop (reputable hopefully). I can't remember or can't find it in this thread, but as several others have mentioned putting money into stock rods and stock heads is good money on top of bad if this is going to be even a mild performance build; also it's not cheaper. If it's a daily driver sub 4500-5000 rpm's then use the stock parts.

My two cents and best of luck to you - Jim
 
#49 ·
. It's the '383' design plan... but at standard bore, it's 377"... not much difference... the usual .030" overbore is just to clean up a used bore block and produce something like a brand new engine...


. The crank and rods are fine for the low compression street re-build planned... the standard cast pistons were the scariest part... go for some KB hypereutectic pistons...


. Did you get a measurement on cam lift? ... Or brand name/grind number?
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#61 ·
Yeah, that's what I wanted to try. Seems like a lot cheaper route. Using the bone plate as guidance, wouldn't have been TOO hard. Plus I work with a machining shop so getting this done wouldn't have been that difficult of a task. I may try it if I run across another cheap block.
 
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