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Old 03-01-2004, 07:01 AM
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??? for engine builders

I have one that kinda got me lost. The motor is a 355 sbc. It has kb hypereutectic pistons in it. I did this motor for the customer like 2 years ago. Saturday i was working on another problem and wile it was at my shop i put a set of plugs in it. Well one of the plugs had the ground strap bent straight out. So i leaked it and got 50 percent. I also did a compression check and it was down 30 lbs in that cylinder. So i yanked the motor and pulled the heads. I found 4 pistons that were broken at the top ring land........ I know what your thinking the rings butted....... I am pretty sure that they did not,but i did not pull the pistons yet. I had the top rings at .033" I have built many motors with these pistons and never had this happen. There are no signs at all of detonation.....

The car is a 4 speed and i have herd from other people that the owner likes to run it up on the chip. I was thinking if he did this repeatedly that it might cause this to happen???? Any body ever have a similar problem????

Thanks Keith

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Old 03-01-2004, 07:15 AM
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It sure sounds like the top rings butted with predictable results. Keith Black's hypereutectic pistons have the top ring groove high on the piston and it requires a greater ring gap for that reason. The .033 you used should have been enough though.

Keep us posted on what else you find.

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Old 03-01-2004, 07:41 AM
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rings

Yea that is exactly my thoughts... That the rings butted. I have built like10 motors with these pistons and 2 of them are being sprayed with nitrous and i have had zero problem up-till now.I am very careful with my top ring end gap to make sure this does not happen.I am going to tear the rest of the bottom end apart tomorrow night and i'll get back with what i found. If the rings did butt then i need to look for the reason why because with .033 gap i can't hardly believe that it happened...

Keith
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Old 03-03-2004, 06:26 AM
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I yanked the slugs out last night and the more i look at them the more i am lost. For those of you that have worked with the kb pistons........ The od of the piston between the top ring and the top of the piston has grooves around it(they look like a fine pitch thread) Those grooves on every piston have melted aluminum in them that reminds you of what detonation would look like on the top of a piston. The tops of the pistons look fine. They are marked up from the pieces of shrapnel running through them but no signs of detonation on the tops????? The thing that is crazy is some pistons(4) look like they just came out of the box on the tops and the other 4 are all busted up. One piston even had the top ring totally gone(must have blown out the headers) The plugs were in bad shape dirty and fuel fowled so i can't really read them for signs of detonation.It don't look to me like the rings butted but i have never really looked at a set that i knew for sure were butting...

Any ideas??????? Any thing else for me to look at???

I am positive the pistons were not at fault but i would like to know what happened.

Keith
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Old 03-03-2004, 06:53 AM
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I'm stumped. Did you check the cylinder wall taper? Not that it makes a lot of sense but it's just a thought.

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Old 03-03-2004, 07:03 AM
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Have a close magnified look at the ends of the broken ring...if they butted they should have a wear mark on the ends. It may be a very small mark so use a magnifying glass.
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Old 03-03-2004, 07:06 AM
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I don't think the guy that you built it for is being totally honest with you (imagine that). Was there any transfer of piston material to the cylinder walls (streaking)?
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Old 03-03-2004, 08:06 AM
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If he had to file the rings to get them at the .033" would they still show the wear mark on the ends from butting? I don't know but they might not be perfectly clean to start with and it could be hard to tell if they are showing markings.
I would have to agree with BstMech, I don't think the owner is being very honest. With one ring completely missing and they rest totally screwed, it sounds like it was severely abused. I wouldn't have a clue as to why the spiral marking are on the top of the pistons. Hope someone else has seen this before and can help. Rick
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Old 03-03-2004, 08:09 AM
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What kind of compression ratio are you running? The melt down around the tops above the rings suggests high speed detonation. Poor fuel distribution, vac leak, leaned out nitrous system. Also check that no part of the head gasket was protruding into the chamber.
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Old 03-03-2004, 08:30 AM
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pistons

Centerline; I finished the bores on this block so i know there is no taper in them.. What are your thoughts on the taper if there was some?????

4 jaw; I'll look real close tonight with a glass to see if i can spot something. I did look at one last night under a light and i can still see the marks on one side of the ring from my wheel where i fit the rings and the other side was still stock looking. One thing i noticed is that the rings that were on broken pistons that the gaps were black on both sides.

Best mech; I know this guy pretty well. We are getting to be friends. I think he is being truthful up to the point that if he did something wrong he just didn't realize he did it. He is not upset with me or anything like that. As a matter of fact he told me he wanted something to be wrong so we can do a better motor. There is some aluminum transfer in one of the cylinders. What are your thoughts on this??????

I was thinking about this some today..... How about this. Say the guy ran it on the chip (7000) alot . Every time the msd box drops a cylinder it was still filled with fuel. The next time around it fires that cylinder. If the motor was on the fat side (and it was) and that un-burnt fuel got down around the rings when it fired could it have caused an explosion around the rings??????? Just a thought??? Or maybe enough of fuel was there to super heat the top part of the piston from the ring side????

Keith
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Old 03-03-2004, 09:46 AM
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Re: pistons

Quote:
Originally posted by k-star
Centerline; I finished the bores on this block so i know there is no taper in them.. What are your thoughts on the taper if there was some?????
I was just thinking that if there was a significant difference (now) that it might have contributed to the failure. It was just a shot in the dark. Does the engine show any evidence of being seriously over heated?

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Old 03-03-2004, 11:25 AM
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pistons

Centerline: I see what your saying now. I'll check the bore sizes now and compare them to the recorded sizes i have for that motor. I don't see any outstanding evidence of overheating,all though he did have some problems with it pitching belts right after i put the motor in. But that was 20 months ago. Maybe it did some damage and it just built on it????

Rick; There is no doubt that this guy spanked this motor like a 3 year old in walmart,,but i also have the same set-up in other motors that get the same abuse (some with 150 shot of nos)and i have no issues with them.

Bobcrman; I can get on board with the high speed detonation answer....... Is this what happens to a piston when that occurs????? Because of to low of an octane???This motor was never sprayed,head gaskets are good with no overhang in the bores,no vacuum leaks in the motor, the static comp was 10.32:1 the cylinder psi was 160 The owner told me he ran 100 octane in it most of the time with 92 octane every now and then... Maybe the 92 was more now then then???????? The carb was a 750 edelbrock,timing was 36 total. I am still wondering how detonation could do that to only 4 pistons and how the tops of the pistons look so good????

Keith
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Old 03-03-2004, 08:10 PM
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Yes detonation will do that to the outer edges, sometimes it makes them look like overheated aluminum foil around the sides of the top ring land. Sounds like you may have a mixture distribution problem with the carb/intake or maybe a vac leak on the bottom side of the intake
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Old 03-04-2004, 05:57 AM
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just read back through the posts and I don't see anywhere that you state which 4 pistons were damaged. It might not mean anything but if it was some sort of fuel distribution problem then which four would help or if was just the 4 in the center (4,6,3,5) then it might be a heat issue, or all on one bank then it could be something else.
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Old 03-04-2004, 06:47 AM
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pistons

engineczar; the pistons were random 1,2,6,8. with 8 being the worse.

With everybody's input i am now leaning to detonation as the problem. I looked in the KB catalog last night and there was a short statement in there about how detonation can cause the tops of these pistons to grow enough that the od of the piston can actually touch the bore. After i looked at the pistons again i think this is what happened. This explains why "all" of the pistons have those small pieces of melted aluminum on the top od at those grooves......... I looked real close with a magnifier at the rings and none of them look like they butted. Also this would explain why there is some aluminum transfer onto the cylinder walls...... Does this sound reasonable to everyone???????

Now i need to see if i can find out why it was detonating...... I may never find this one........

Keith
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