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.

 

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?

 

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".

 

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

 

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?

 

. 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...

 

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?

View attachment 211658

View attachment 211666

View attachment 211674

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

Bogie

 

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.

 

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.

So maybe the top ring gap was too tight like you guys mentioned.

 

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.

 

Are those hypers or forged?

 

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.

 

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...
.

 

The sellers name is't "Brian" is it. There's a thread on another board posted by a guy who got burned by "Brian" in N.C. Pistons are probably some cast "claimer" style pistons, and that one could't take the detonation. I am Butch/junior stocker.

 

No, the guy I bought the engine froms name is Jerry. But I'm in Jacksonville if that narrows down the "brian" list in NC lol But he DID say the truck was his son's originally sooooo, Maybe. I bought the engine from the dad...Mid-late aged man, maybe in his 60's. He seemed to be a really cool dude so I figured I could trust him.

Another re-ring horror story lol I'm passing on any more "engine needs rings" sellers. Or i'm going to start pulling heads before cash is even touched!

 

. Yeah, if any hint at all that something is wrong with an engine, just start offering 'core' price for it...

. Don't think that's detonation damage... in the wrong spot... butted ring ends makes the most sense... I love high compression engines... would just put a new domed piston in and keep going... check gap in other top rings... inspect for shiny patches like that in other cylinders indicating rings were butting... check for any other pistons with broken ring lands... check for rod bearing damage/bent rod from locked piston... run some kerosene or other octane booster in Premium gas...

. 64cc, 70cc, 76cc heads?

http://www.summitracing.com/popup/calcsandtools/compression-calculator
.

 

UPDATE: I decided to go pull the other head and rotate the engine over to get better looks at the pistons.

Here is a closer view at piston #2

It had a few tiny dints in it, i'm assuming from valve damage.

 

Pulled the left head off and #3 piston has the same type damage with dints in the head. Took a picture of the #3 chamber and the valves look exhausted.

Cylinder wall isn't damaged and no other pistons or chambers are damaged. Still spins over very freely. I really do believe in respecting my elders but I honestly feel like cursing the old dude out. He could have been honest with me. I would have been a bit more prepared for the repairs instead of buying other crap. I was a few bucks short when I purchased this and gave him my word that I would bring him the rest later and I did. I was a complete stranger and could have just not took him the rest of the cash but I have integrity. Guess he doesn't.

 

. Not valve damage... from the pieces of rings and pistons that were removed...

. Valves don't look damaged... there's just been some moisture/water in there that caused a little surface rust...

. What's the heads casting numbers(s)...?
.

 

They are the hecho Mexican 993 heads. Port matched and such. But that makes me feel a little better knowing the valves weren't hitting the pistons. I had not plan to reuse the heads anyway.

 

Another attemp to make a sows ear (993 heads) into s silk purse. Send 'em to a recycler and start over.

Bogie

 

Yeah, the dents on the pistons are caused by the pieces of the broken rings and piston ring lands being repeatedly mashed at TDC until they were broken into small enough pieces to go out the exhaust.

If I were to bet, I'd say massive detonation and the resulting runaway combustion heat caused the rings to run much hotter, causing them to expand more and closing the gap, so that the ring ends butted end wedged in the bore tearing one end of the ring off and the top ring land/piston top with it. In one of your pic's you can even still see one intact ring end right at the break point on the piston.

A 383 with that much dome and even a large 76cc head like the 993's is still over 11:1 compression, pump gas would kill it in short order.

 

crank damage ?

I have seen more damage on the bottom end when pistons are damaged, time to see if the crank is bent and conditions of the rods.
Time for a complete tear down and carefull inspection of everything.

 

UUUUGGGGHHHHHH!!!!!!!!!! If the crank is bent, someone OWES me something! If nothing but an explanation. I borrowed money to buy this POS!!! I will get into the bottom end next Saturday, if not today. I'm going to give the guy a call today. Need to ask him a couple of questions.

 

This is beyond frustrating. I have been burned many times, so I feel your pain.

 

Why did you borrow money to buy something and not verify it's condition before purchasing? Sometimes people lie, sometimes they remember differently, sometimes they repeat what they were told, sometimes they make up stuff. There are no guarantees with the human factor involved.

 

. I get about 10.7 - 10.9 compression ratio depending on how far the pistons are below the deck... maybe 350 - 375 HP depending on cam... max power limited by head flow... definitely needed to be using octane booster in Premium gas or racing fuel... did you see what ring gap they were using?

 

Every time I've done that it was from too much boost and not enough ring gap. The dents are from the pieces of piston and ring getting squished.

Kevin

 

Again, when those pieces made it out of the chamber, they were probably smashed here and there with the valves. Not to add fuel to the fire but your more than likely going to need a valve job. If the heads are nothing to write home about it, you may be better off finding another set to put the money in.

I don't know if you mentioned what you were putting it in but a junkyard LS motor may be a much better option. I'm a Ford guy but those motors work really well.

 

I swear to you' an LS swap doesn't look too bad right now. It's just that darn wiring that gets me! I tried to do an LT swap on a 91 Camaro I had once. The wiring gave me such a headache that I sold it all. Price isn't the problem. If I just had someone to do the wiring, I would definitely drop an LM7, LS1, or LQ9 into my Formula.