Cracked Pistons, but why?

-
straightlinespeed

straightlinespeed

Sometimes I pretend to be normal
Joined
Oct 21, 2012
Messages
10,630
Reaction score
4,399
Location
Uvita, Costa Rica
So these are the pistons that came out of my 360. They are stock dish pistons, around 2500 miles on them. 4 of the 8 have cracks in them, which go all the way thru to the inside, all in the same location. It was hard to capture the cracks with my phone's camera. The pic with the yellow circles is by far the worst. The 4th piston just wouldnt show right.

Im curious, what would cause more than one piston to crack in the same location? Sorry I cant tell you the side or cylinder they came out of. The builder actually found the cracks.

View attachment IMG_20160128_173707123.jpg

View attachment IMG_20160128_173733874.jpg

View attachment IMG_20160128_173834838.jpg
 
With respects,
The 'tooling' marks indicate a cast piston??
 
oh snap - that is crazy stuff.. and with only 2500 miles! Just tossing a thought out - if they were all on the same side maybe they somehow contacted the head.. did your builder say anything about cylinder scoring?
 
Wild *** guess, bores in the motor beyond tolerance, stress cracks to to the pin.
Maybe too much endplay in the crank..
What do the skirts on the pistons look like.
 
contact with the head is my first thoughts as well
 
stock 360 pistons are not hitting the heads. extra crank endplay will be absorbed no problem by the wristpins. those are cast pistons cracking at a weak area-the big oil ring groove slot. probably too much piston to wall clearance-measure it and get back to us
 
ScampMike said:
oh snap - that is crazy stuff.. and with only 2500 miles! Just tossing a thought out - if they were all on the same side maybe they somehow contacted the head.. did your builder say anything about cylinder scoring?
Click to expand...

They sat .107 in the hole. I was getting new pistons anyways, and he noticed that after he cleaned them and was getting ready to remove them from the rods. Now the cylinders is another story which I will continue in the next quote.

sireland67 said:
Wild *** guess, bores in the motor beyond tolerance, stress cracks to to the pin.
Maybe too much endplay in the crank..
What do the skirts on the pistons look like.
Click to expand...

Yes, this was a issue when they started measuring the bore. The bore ranged anywhere from .030 to almost .040 and egg shaped. Skirts have some scuffing (not sure how to describe it) on them, but didnt look terrible. I was thinking it was caused because the piston rocked in the bore, but not sure.

dano said:
What brand?

What do the rod bearings look like?
Click to expand...

Silv-o-lite 1279

Rod bearings looked good, as did the crank bearings.
 
so it's a result of the pistons essentially "slopping" around in the cylinder? sort of banging into the walls if you will?
 
They have all cracked at the end of the oil return slot. The worst one almost looks like the area of the skirt between the cracks has shifted down a bit from the rest of the piston skirt, making me think there perhaps was a high spike of pressure down on the rings. Possibly detonation. Were there any broken top or 2nd rings?

Or... was the ring ridge removed at the prior rebuild? (It would have been if it was bored.)
 
Wild guess..........stress fractures from mounting the pistons on the rods, pistons not well supported??
 
ScampMike said:
so it's a result of the pistons essentially "slopping" around in the cylinder? sort of banging into the walls if you will?
Click to expand...

Certainly could be with the condition the cylinders where in.

nm9stheham said:
They have all cracked at the end of the oil return slot. The worst one almost looks like the area of the skirt between the cracks has shifted down a bit from the rest of the piston skirt, making me think there perhaps was a high spike of pressure down on the rings. Possibly detonation. Were there any broken top or 2nd rings?

Or... was the ring ridge removed at the prior rebuild? (It would have been if it was bored.)
Click to expand...

I noticed that one area shifted down as well. When I first had the engine built, I was breaking it in. I heard a snap.. shut it down.. found nothing wrong and started it again, ran fine. I finished the break in and drove it home. About 500 miles later something was wrong, I did a leak down test to find the #6 cylinder with like 80% leak down. Brought the engine back to the shop and they found broken pistons rings. They said they fixed it but I wonder if they just threw a new set of rings in and called it good. It most certainly could be the same piston that broke the rings and honestly wouldnt surprise me.

They did remove the ridge on the original build, but if they didnt bore the cylinders good, I suppose that large difference could had caused this.

JBurch said:
Wild guess..........stress fractures from mounting the pistons on the rods, pistons not well supported??
Click to expand...

Certainly could be.

Coronet 500 said:
If they are pressed pin and assembled in a press without proper heat could cause this.
Click to expand...

This would also make sense.
 
I do have to say.. I have the name of a engine shop NOT TO USE if you are in Minnesota and thinking of having something built. Each time I talk to the new engine builder he tells me more things of what he has found wrong. Now I have some interesting garage art.
 
S.W.A.G ,
Scientific Wild *** Guess---
Bad batch of silv-O-lite Chicom pistons.
 
"Pressed pins" are interference fit with the rods - not the pistons. That would be from the bores being too loose. Cast pistons need tight wall clearances and my money's on them slamming around. Why someone would put new pistons in a worn, out of round bore is beyond me.
 
mech1nxh said:
S.W.A.G ,
Scientific Wild *** Guess---
Bad batch of silv-O-lite Chicom pistons.
Click to expand...
Me too. To me, in proper install procedure, and, or, Def pistons. I think if slug was slopping around the bore, the skirt would take the most beating.
 
Hydrolock?
Do the cracks run to the pins?
Are the cracks on the inboard or outboard sides of the pistons?
 
When you press a pin into a rod, you put a huge stress on the piston itself because you are using it as a 6-20 ton stop for the rod..unless you use a rod brace which no one has ever seen before (I cant even find a pic of one, looks like a little anvil) in these days of rod heaters. Now we just heat the rods a dull red and slide the pin in, bing-bang-done. Of course you have to press them out now....
 
straightlinespeed said:
I noticed that one area shifted down as well. When I first had the engine built, I was breaking it in. I heard a snap.. shut it down.. found nothing wrong and started it again, ran fine. I finished the break in and drove it home. About 500 miles later something was wrong, I did a leak down test to find the #6 cylinder with like 80% leak down. Brought the engine back to the shop and they found broken pistons rings. They said they fixed it but I wonder if they just threw a new set of rings in and called it good. It most certainly could be the same piston that broke the rings and honestly wouldnt surprise me.

They did remove the ridge on the original build, but if they didnt bore the cylinders good, I suppose that large difference could had caused this.
Click to expand...
Well if they found broken piston rings at 500 miles, that is a pretty good sign that either the ridge was not properly removed or the was some bad detonation going on for some reason. But I would expect marks on the piston tops with detonation. Almost sounds like they just honed the cylinders. That all fits with that one broken area being shifted down. I can't quite imagine broken rings from the bore being .010" loose or ovalized....but stranger things have happened!

I like the term 'garage art' LOL
 
And I suppose you are going to let the first shop keep the money you paid them for building that motor?

Some of you guys need to start putting your foot down with these shops that charge but can't do the job.
Getting paid for a job they can't do correctly isn't good enough and I'm seeing WAY too much of it.
Not just in the auto industry either.
 
I have to ask, what prompted a rebuild at 2500 miles?
 
You must log in or register to reply here.
-

Back
Top