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?

 

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.

 

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 ?

 

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.

 

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 :thumbup:

 

well the clearance thing might be but the pistons were stock hypers

 

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

 

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?

 

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.

 

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.

 

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

 

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.

 

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

 

Parts

Get a magnifying glass and look at the ends of the broken rings and see if they were butting...

Can you post a picture of a broken piston...

Do you have the tools to check the bore size and the piston diameter...

I personally do not think that is the problem... I have run cast pistons at .008 clearance.... They hammered like heck at cold start but did not break the skirts...

 

I think this is a bunch of baloney

hOW CAN THERE BE SO MUCH DAMAGE!?
Let's see some pictures! imp:

 

first i do have the tools to check bore size but haven't had an opportunity
second, scrot, this is no baloney, why would i get on a site and ask a question like this and not have a legit problem? lastly i do not have any pictures yet b/c i haven't had time to do that either. as soon as i can i will let you guys see all the damage. but no joking aside, all 8 pistons are broken (skirts on one side and rings on the other.

 

HE SAID in #1 and # 14 that ALL the pistons were broken on the CAM SIDE....

If all the pistons are broken on the cam side then 4 pistons are broken on the thrust side ...... and 4 are broken on the NON thrust side..... hhhmmmmmmmm?????

and the rings broken on the other side......


SCRATCHING HEAD......

 

Sounds like the block was maybe clearanced for forged pistons.

.

Clearance is built into the pistons, not the cylinder walls.

A 4.030 hole is the same regardless of piston type. The forged pistons are made smaller for more clearance.

 

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

Geez. You need to buy a better brand of pistons..... I have NEVER EVER seen this problem except on cheap-junk-rebuilder-pistons.

Just my 2 cents.

 

let me clerify alittle more, #1,3,5,7 pistons are broken, skirts on the side of the piston facing the cam (the inside of the motor) and the rings are broken on the opposite side of the same pistions. #2,4,6,8 pistons are broken on the side facing the cam (the inside of the motor) and the rings are broken on the opposite side of the same pistons, making all 8 pistons broken exactly the same way. what about detonation beating them all apart, i didn't hear any pinging but that doesn't mean it couldn't be does it? o yea, these were NOT junk pistons they are stock 94 350 hypers from the factory

 

I don't have an opinion but I do have a question. Did you have the block bored out??? If not, could it be that the cylinders are tapered and when they reach the bottom end they rock enough to cause this problem?

 

let me clerify alittle more, #1,3,5,7 pistons are broken, skirts on the side of the piston facing the cam (the inside of the motor) and the rings are broken on the opposite side of the same pistions. #2,4,6,8 pistons are broken on the side facing the cam (the inside of the motor) and the rings are broken on the opposite side of the same pistons, making all 8 pistons broken exactly the same way. what about detonation beating them all apart, i didn't hear any pinging but that doesn't mean it couldn't be does it? o yea, these were NOT junk pistons they are stock 94 350 hypers from the factory

Like I said in # 19..... that is really wierd.

By what you describe (and what I said in # 19), all the pistons are NOT broken in the same place since the thrust side is towards the cam on the left bank and away from the cam on the right bank.

That is, the thrust is towards the right side of the engine (opposite crankshaft rotation). Soooooo, the right bank pistons (even numbers) are broken on the NON thrust side.

If the pistons have offset pins, this is backwards. If the pistons have centered pins, it is still wierd.

This has to be clearance related.

I would like to hear a feesible hypothesis.

 

You don't suppose the crank counter weight's broke them off do ya?

 

That's what I was thinking. Would help explain the position of the breakage.

 

You don't suppose the crank counter weight's broke them off do ya?

He says the engine has 15,000 miles on it and racing passes too (post # 5).... If it were a crank strike, that would have happened the first rotation.

Regarding Jim's post # 11:
Most pistons and rods fragment at the end of the exhaust stroke/beginning of the intake stroke. That is the point that there is NO LOAD pushing on the piston/rod and the piston hits the top of its travel like a ball on the end of a string -with a substantial jerk.

But that still will not explain the odd breakage pattern.