Rebuilt motor w/ broke pistons UPDATE: 11/13/13 1.5yrs later upgrade time!

[dabomb6608]

Hey guys, I figured I would post this here to get some responses. The motor is in an 86 Trans Am but I like the atmosphere here alot more then the forum I go to for it. Its a SBC 400. Just rebuilt it a couple weeks ago, 30 over pistons/.2 on the rods and mains. Rebuilt with new pistons/bearings/rings. Had the block checked/magnafluxed/honed along with the crank and rods checked. The heads werent checked and the cam was reused.(Yes I kept the lifters in order)

Well I put the motor together and now at about 200miles it has some pretty bad blowby(popped the oil dipstick up and spit oil). It also smokes on accel and alittle at cruise. I checked compression and 4cylinders had low compression at around 100psi. The rest were in the 170area. The bad cylinders were #1,3,5, and 8 if I remember correctly. After doing that check I put some oil down #1 and did another check. It went up 125psi.

At this point I have the heads off. The cylinders dont appear to have any scarring. They also look to have some cross hatching left. Now the one thing I noticed, is that the cylinders looked to have some leaking in between the cylinders. The head gaskets didnt look to be damaged any. Just alittle dark in between the cylinders.

So what does this group have to say? Im leaning most towards rings but I dont want to have to pull it...again. With the compression going up with oil, that is what has me looking at the rings. Especially with the oil burning, even though part of that could be oil buring off the headers from the oil coming out the oil dipstick tube. I will be pulling the motor tomorrow most likely so I can inspect the rings.

 

[imiceman44]

A couple of things I can think of, and I am no chevy engine expert but in general engines.
How did you break the engine in, if you didn't break it in properly, the rings wouldn't seat properly and did you check the gap, is it within tolerance, and as for the head gasket, it could have used retorquing after initial start.
When you say the engine was honed, was it 30 over to begin with, and was it within tolerance?
How long did you run the engine for break in?

Hey guys, I figured I would post this here to get some responses. The motor is in an 86 Trans Am but I like the atmosphere here alot more then the forum I go to for it. Its a SBC 400. Just rebuilt it a couple weeks ago, 30 over pistons/.2 on the rods and mains. Rebuilt with new pistons/bearings/rings. Had the block checked/magnafluxed/honed along with the crank and rods checked. The heads werent checked and the cam was reused.(Yes I kept the lifters in order)

Well I put the motor together and now at about 200miles it has some pretty bad blowby(popped the oil dipstick up and spit oil). It also smokes on accel and alittle at cruise. I checked compression and 4cylinders had low compression at around 100psi. The rest were in the 170area. The bad cylinders were #1,3,5, and 8 if I remember correctly. After doing that check I put some oil down #1 and did another check. It went up 125psi.

At this point I have the heads off. The cylinders dont appear to have any scarring. They also look to have some cross hatching left. Now the one thing I noticed, is that the cylinders looked to have some leaking in between the cylinders. The head gaskets didnt look to be damaged any. Just alittle dark in between the cylinders.

So what does this group have to say? Im leaning most towards rings but I dont want to have to pull it...again. With the compression going up with oil, that is what has me looking at the rings. Especially with the oil burning, even though part of that could be oil buring off the headers from the oil coming out the oil dipstick tube. I will be pulling the motor tomorrow most likely so I can inspect the rings.

 

[dabomb6608]

A couple of things I can think of, and I am no chevy engine expert but in general engines.
How did you break the engine in, if you didn't break it in properly, the rings wouldn't seat properly and did you check the gap, is it within tolerance, and as for the head gasket, it could have used retorquing after initial start.
When you say the engine was honed, was it 30 over to begin with, and was it within tolerance?
How long did you run the engine for break in?

The engine was 30 over to start out. They checked the cylinders and told me it was ok to just hone it. They did all of the machine work. The gap was within spec per manual. I let the engine run at idle-2000ish for about 15mins. Then drove it without revving over 3000. At this point im thinking I shouldve been harder on it and done some low speed/high gear pulls and maybe some engine breaking since its a manual.

 

[imiceman44]

Well I don't know about these days but when I built engines, I fired them up, high idle for 10 minutes, dumped the oil, then ran the engine at about 2k rpm for 30 minutes, dumped the oil, then drove it for 1000 miles at around 2500 rpm, at that point the rings are seated and you can rev it higher.
I hear now the tolerance are getting better and no need for that much care for break in, but then you get things like what you got.

The engine was 30 over to start out. They checked the cylinders and told me it was ok to just hone it. They did all of the machine work. The gap was within spec per manual. I let the engine run at idle-2000ish for about 15mins. Then drove it without revving over 3000. At this point im thinking I shouldve been harder on it and done some low speed/high gear pulls and maybe some engine breaking since its a manual.

 

[80' 427]

To have it have that much blowby I wonder if you don't have a broken ring.

 

[imiceman44]

To have it have that much blowby I wonder if you don't have a broken ring.

I don't know but 100PSI is a lot of pressure for a broken ring, it seem smore like not seated well.

 

[76zimmer]

What type of pistons, and what type of rings, and what finish stone did they hone with. Did they check out of round AND taper to each cylinder? What gap did you set the top and 2nd rings at? Should be .017-.020 for a 4.125 cyl. bore. Moly rings should seat quickly, cast iron, maybe not for 1000mi on a used cylinder in good shape.
You could put the heads back on, do a cylinder leak down test to determine exactly where the compression is going.

 

[dabomb6608]

Pistons are speed pro hyperuetectic, the rings were speed pro and I believe cast. It was a complete rebuild kit that included the rings and pistons from summit. I'm not sure what finish hone they used. And I assume they checked for out of round and taper. Because they mic'd the cylinders as well as magna flux. They are a pretty popular shop that's been in business since the 70's so that's were I'm making my assumption. We checked the gap as per my manual for my blazer since they have info for a 400ci and they were in spec. I did hook my compressor up to #1 before pulling the heads and could tell it was leaking and it felt like the air was coming from the oil holes in the oil gallery. So that means its going in the crank case. I'm pretty sure that was all your questions.
The thing is, is that it started out running fine with no problem burning oil or blowing the oil dipstick up. Just all of a sudden one day it started that, and then progressively got worse. It was even getting hard to start, I would have to keep at wot until it started. And it's fuel injected and would start super easy before with no throttle input.

What type of pistons, and what type of rings, and what finish stone did they hone with. Did they check out of round AND taper to each cylinder? What gap did you set the top and 2nd rings at? Should be .017-.020 for a 4.125 cyl. bore. Moly rings should seat quickly, cast iron, maybe not for 1000mi on a used cylinder in good shape.
You could put the heads back on, do a cylinder leak down test to determine exactly where the compression is going.

 

[4X4HIGH]

Sounds to me like your issue is in the heads. The fact that you have smoke on acceleration is the key point. Also why would you or anyone else for that matter ever do work to the bottom end and not do anything to the top end?

 

[dabomb6608]

Sounds to me like your issue is in the heads. The fact that you have smoke on acceleration is the key point. Also why would you or anyone else for that matter ever do work to the bottom end and not do anything to the top end?

I know what you are saying in terms of why do the bottom end but not the top. My reasoning behind this was because I was planning on upgrading later down the road to a Holley Stealth Ram intake or something along those lines and was going to get better heads then. So it didnt make any since to me to have work done on these crap 70s emission heads.

On a side note, I have found my problem....EVERY single piston looks like this....


The pistons cylinders that had higher compression werent as bad as this one pictured. That is #1 cylinder. But they all had cracked in that ring area. So they were on their way out just like that one and they all wouldve eventually had around the 100psi compression or lower.

 

[4X4HIGH]

This is caused by detonation.

 

[dabomb6608]

The datalogs never showed any excessive amount of knock. Only time it knocked was on startup or shutdown if i recall, and that was minimal.

 

[4X4HIGH]

The datalogs never showed any excessive amount of knock. Only time it knocked was on startup or shutdown if i recall, and that was minimal.

I don't care what any data logging tells you, that was caused by detonation. Remember that I was an automotive machinist for 22 years and have built thousands of engines over those years.

 

[dabomb6608]

Im not trying to say your completely wrong here Scott. I just am going off of the fact that before I rebuilt it, it was showing a LOT of knocking. Which resulted in,

now that for sure is detonation. It was also showing up on the datalogs and would max out the knock counts. That was what caused this rebuild. Then 200 miles later, and barely any knocks counted, this happens.
My thoughts were that it mightve been that the rings were too tight causing the gap to get closed up when they got warm.
Does that seem like a plausible source for this as well or are you saying this was only detonation?

 

[76zimmer]

I know what you are saying in terms of why do the bottom end but not the top. My reasoning behind this was because I was planning on upgrading later down the road to a Holley Stealth Ram intake or something along those lines and was going to get better heads then. So it didnt make any since to me to have work done on these crap 70s emission heads.

On a side note, I have found my problem....EVERY single piston looks like this....
View attachment 129914
View attachment 129915
The pistons cylinders that had higher compression werent as bad as this one pictured. That is #1 cylinder. But they all had cracked in that ring area. So they were on their way out just like that one and they all wouldve eventually had around the 100psi compression or lower.

Wow, that could do it....I really like to run Forged just for reasons like this.

 

[1-ton]

If you assembaled this engine yourself...did you make sure you lined up the ring gaps properly? The top and bottom ring gap is supposed to be opposing at aproxamitly 4-O, clock and 6-O, clock respectivly.

 

[Fordum]

Man, I really hate to disagree with Scott. He has more experience than I will ever have.
And, I agree that looks like detonation.

But, in 200 miles? And you could drive without having to turn the radio up all the way?
And all the pistons?

Forget the det counter. The sensor might be defective. What did the engine sound like?
You should have heard that thing coming down the road for miles. Should have sounded like someone hitting the block with a hammer.

Get somebody to mic that worst cylinder, and the piston. And look for a scar on the cylinder wall.
I saw one like that many years ago when a guy rebuilt an engine and used standard pistons and rings in a bored block.
The rings slid out too far and turned sideways when the power stroke hit. There were scars in the cylinder where they dug in.

A couple of pieces went between the piston and the wall.

Of course, if you were hearing lots of knocking but were going by the det counter and not thinking it was detonation, then Scott is right.
Its just massive detonation.
I just find it hard to believe that you would drive it that far with that much noise going on........

 

[BoondocK5]

I have not built as many engines as Scott, but, I've been doing the engine thing for a long while.
That very well could be detonation.
I have a question, where did you get those pistons? Name brand?
Possibly defective?
If I was you, I would break out your micrometer, measure piston diameter, cylinder diameter, re check your ring gap, the whole deal.

I've had a few engines develop some bad detonation. Non forged pistons I've dealt with have always melted holes in the upper surface where the "flame propagation" happens.
The aluminum just gets super heated there, gets soft and the bang from the detonation, along with being extremely hot, also knocks a hole in the now softened aluminum.
Its a vicious cycle, the high cylinder temps increase the likelyhood of lean condition which causes higher temps, which causes more of a lean condition, which causes detonation. It only takes a very short amount of time before you've got damaged pistons.
Only time I ever saw them break like that was improper installation/ defective pistons rings

When I hone a cylinder, I use a hone first, get the desired diameter minus about .003 Then I run a glaze breaker. It looks like a miniature chimney sweep. A couple of passes to develop a non-directional finish. Then clean with rubbing alkyhol, then oil it with marvel miracle oil, then drop in piston/ring/rod assemblies.

AND just because I'm curious, were you running that ditch water and kerosene that passes for gasoline in your neck of the woods?

 

[Fordum]

The aluminum just gets super heated there, gets soft and the bang from the detonation, along with being extremely hot, also knocks a hole in the now softened aluminum.

That was the other thing that made me wonder about detonation, but I forgot to mention.
From the side, the piston top and edge looked pristine. Of course it was only one picture of one piston, but I would have expected to see damage in that area.

I certainly do not say it was not det., but given the low miles, and mild running, I lean more toward a mechanical/assembly problem.

 

[4X4HIGH]

This is CLEARLY detonation. Yes if the top ring gap is too small the ring gap can close up and it will break the head of the piston above the top ring but in this case you also have the land above the second ring broke as well. Also keep in mind that a stock ring gap is "usually" on the loose side and unless those are KB pistons (doesn't appear to be by looking at the design) the gaps would need to be slightly larger than normal but you wouldn't butt the rings gaps unless there was severe heat in the cylinders.