Question Re: Chipped Pistons

I have a question about chipped pistons - these are NOT from my engine. The question is, what are some possible scenarios that could cause these two pistons to be cracked as they are? These are cylinders #3 and #5 (front bank). We don't have the heads to look at, but the cylinder walls are not scored, still in good shape.

Various angles of the same two pistons. Click on images to magnify:











Thanks for any help!

I remember reading some threads about this back in the day over at regalgs.org. I can't exactly remember what the reasons were, but it always seemed that these two pistons were the first to chip on these 3800 engines and in those exact spots. I kind of remember that it may have something to do with these cylinders have a tendency to run a little leaner than the rest. I wish I could remember the reasons better and it is too bad those threads are probably long gone over at regalgs.org.

I would guess it has something to do with the restrictive exhaust manifolds.I know the rear manifold gets way hot! I bet there are hot spot issues in the heads.It's a good idea to enlarge the water jackets in the heads.If you put the gasket in place and mark where it sits in relation to the head you will see allmost a good 1/2 inch at places that can be ground away. And port the exhaust manifolds.If youre careful and after you get the engine back,you can enlarge the water ports with a Dremel and either stone burrs or sand cones.Just stop up the ports so as not to get junk in the heads.The heads will run cooler and the manifold will flow better.

I've seen exactly that on many 3800s where people dropped pulley sizes carelessly.

That is detonation my friends, pure and simple. Cracks the ring lands off.

Thanks everyone for your opinions. Much appreciated.

98riv,

I remember the same about two cylinders being prone to detonate. But I remember it being #3 and #4 (inner two pistons), because they tend to get hotter than the rest.

robotennis,

These damaged cylinders are both on the front bank. The rears looked fine.

I guessed detonation as well. However, I was wondering if anyone would suspect a couple of failed rod bearings, causing the cylinders to make contact with the head?

AA wrote:

Thanks everyone for your opinions. Much appreciated.


I guessed detonation as well. However, I was wondering if anyone would suspect a couple of failed rod bearings, causing the cylinders to make contact with the head?

Aaron,
Detonation will indeed beat the hell out of the rod bearings, but that would not cause contact with the head. Production motors are never 0 decked as a rebuilt engine might be. Most production engines have the pistons .050 or so in the hole at TDC. Piston to valve contact could also do that, but I would also expect to see damage to the valves in that engine. Usually, that results from using a long duration cam without piston valve notches, and without checking piston to valve clearance. The piston usually gets closest on the exhaust stroke where the piston is in effect, chasing the exhaust valve up the bore.

Interesting. I talked to the engine builder working on my engine, who finished it yesterday. His diagnosis for my engine (not the above): rod bearing failure on cyl #1 causing the piston to contact the head, chipping the piston much like pictured above. There was actually a piece of piston embedded in the underside of the head. The #1 connecting rod was also damaged. No other problems at all with the engine.

So, knowing this happened at between 5500-5800 RPM under approx 12-14 PSI, there was some spark knock, but not very much, and not sustained. KR was reading 0-3 deg during WOT most of the time. There were a couple of scary spikes of 8+ deg, but they were very brief, and possibly caused by the 3rd gear shift (I noticed the spikes occurred just after the shift).

Assuming there was enough detonation in cyl #1 sufficient to cause rod bearing failure AND piston chipping in the same moment, is it possible the chipped piece could embed into the head at high RPM, without the piston itself making contact?

Or does it seem more likely the rod bearing failed first, from whatever cause, which then led to the piston making contact, resulting in the chip? I know this may not seem mechanically possible, but this is the suggested theory from the engine builder.

AA wrote:

Interesting. I talked to the engine builder working on my engine, who finished it yesterday. His diagnosis for my engine (not the above): rod bearing failure on cyl #1 causing the piston to contact the head, chipping the piston much like pictured above. There was actually a piece of piston embedded in the underside of the head. The #1 connecting rod was also damaged. No other problems at all with the engine.

So, knowing this happened at between 5500-5800 RPM under approx 12-14 PSI, there was some spark knock, but not very much, and not sustained. KR was reading 0-3 deg during WOT most of the time. There were a couple of scary spikes of 8+ deg, but they were very brief, and possibly caused by the 3rd gear shift (I noticed the spikes occurred just after the shift).

Assuming there was enough detonation in cyl #1 sufficient to cause rod bearing failure AND piston chipping in the same moment, is it possible the chipped piece could embed into the head at high RPM, without the piston itself making contact?

Or does it seem more likely the rod bearing failed first, from whatever cause, which then led to the piston making contact, resulting in the chip? I know this may not seem mechanically possible, but this is the suggested theory from the engine builder.

Normally when rod bearings fail, they spin, and make noise. If the failure is severe enough, the rod could break at the big end, and that causes a lot more damage to the crank and block. For the piston to hit the head, the stroke would have to increase by .040-.050. That's a lot, but it is possible. How was the crank? I suppose anything is possible, but I bet detonation and temporary oil starvation caused the damage, with that chunk of piston flying around, it could have embedded itself.

When the damage occurred, I felt loss of power, heard rod knock, then pitted, shut down about 30 secs later. It did not sound like anything was clanking around in the cylinder, just the knocking, which an automotive engineer confirmed (from listening) was from the bottom end. I did not start the engine after that.

I'll ask the engine rebuilder more details when I pick up the engine tomorrow. He will show me the damaged parts, too. He said my crankshaft was not reused. A reconditioned core was installed.

.040" isn't too far a distance. I suppose if the spun bearing were loose enough, it could have made contact. Or, if the chipped piece became wedged between the piston and head, maybe that could explain what happened. I guess what I really want to know is which happened first, and could/did it result in the other event?

Detonation


/thread

And some people wonder why we obsess about KR.

Remember that in the situation you were in, multiple repeated WOT runs without any cooldown time, you made the engine internals as hot as they've ever been. Combine that with 200K miles and maybe a touch of KR and this is the result. I've seen it happen on more than one, and without warning.

Maybe your fuel pump wasn't keeping up, maybe the FF wasn't flowing fully, maybe the injectors were maxed out. A lot of people prefer to make their WOT a little rich for some extra cylinder cooling, this is a situation where that may have been a necessity. Coolant doesn't do a lot of cooling, oil temps are very important too - the oil helps cool the bottom end and rotating assembly...

There are LOTS of maybes that can factor in to this situation. Which one was it? No way to tell if you weren't actively scanning. Plus, at WOT only a wideband O2 sensor would be telling us an accurate A/F ratio. Plus these cars don't have an oil temp sensor - you would have to add one to your external oil cooler line, when you get that installed

Good points. I was actively scanning most of the day, but not at the exact time of the failure. KR seemed to be in check, but as you say, cylinder temps may have been getting hot even though coolant temp never got much over 200ºF. In addition to oil temp, I should probably be monitoring exhaust gas temp.

Based on discussion with the engine builder, and various opinions from members here, BillBoost, Ed Morad, and a few others, here's my theory on what happened:

Detonation in cylinder #1 caused a small chip (about the area of a dime) to the edge of the piston. This chunk of piston came free and wedged between the piston crown and the head during the following upstroke(s), pancaking into a round slug about .030-040" thick, embedding it to the top of the the head. The increased stress to the connecting rod from this event caused the #1 rod bearing to fail.

Sound reasonable?

I'm still contemplating why the detonation occurred in #1 and not in the other cylinders (all other pistons and rod bearings were in good shape). One idea is a lean condition developed as a result of a clogged #1 injector. I'm sending out my injectors soon to have them flow tested.

Injector flow test results are in. As suspected, cylinder #1 (my engine) was receiving the least amount of fuel compared to the others. This likely caused detonation only in cylinder #1, chipping the piston. See the injector cleaning thread for more details:

https://rivperformance.editboard.com/t1731p30-fuel-injector-cleaning#148307

I think oil starvation also played a part here, We only have 4.5 quarts of oil in our engines. The oil pump is very good. I think the pump may have emptied the pan and the engine experienced momentary oil starvation. I think the oil got pretty hot too. If I was you, I'd find a way to run more oil. A bigger pan with baffles would do it. If you intend to take the car racing like that, you need this.

Oil analysis is out. We'll see if Blackstone notes any characteristics that point to the oil getting hot or breaking down it's composition. The engine builder mentioned all the other bearings were in very good shape; the engine showed no other signs of damage other than the piston chip, which we now think led directly to the #1 bearing failure.

Are you still going to do something like the Accusump anyways for extra insurance? Or perhaps an oil cooler too for cooling and extra volume?

I've used the PF-52 filter for a long time, so that gives me 5 quarts total oil volume. Also, the oil-restricted push rods I use are supposed to keep oil down near the crank at high RPM. Not much, but a start.

Whether or not I do the Accusump or oil cooler will depend a lot on the Blackstone report. If the oil got too hot on track, its composition should show signs of breaking down, and the lab tests will reveal this. If oil starvation caused the bearing to fail, there should be some evidence of metallic particles to confirm it. However, if the oil still had its lubrication properties, and there are no signs that oil caused the bearing to fail, I can conclude the failure of piston #1 was knock induced, as evidenced from the injector flow analysis.

I want to try and avoid making assumptions that can't be verified by testing. If it turns out I have reason to believe the oil played a significant role in the failure, I will counter with the needed mods. If the Blackstone guys don't find anything substantial, I will ask them a few more questions to confirm things. Right now, they haven't been made aware of the piston or bearing failures. I want to see if they can tell anything from reviewing the oil sample.

Aaron,
Were you able to see the bad rod bearing? If it went from lack of oil, it will be black (burned). I don't believe a PF 52 gives you another 1/2 quart capacity, especially the new ones. They are shorter than the old stock PF-52's. If it was my car, I'd look for a bigger oil pan. I think we have plenty of clearance for it, and you could keep the oil away from the crank. If you keep the stock oil pan, I think it's a good idea to run 5 qts when you race.

I can fit 5 qt in, but you're right it's a little above full. I still find the longer PF 52 filters around.

I didn't see the bearing, but I did ask the builder about it. He said the bearing just failed (from force and old age), and the crank was scored enough that it needed welded and reground. I don't know if that's very common or not. He didn't say it was burned, or that lack of oiling was the cause. He seemed like he was not actually 100% sure of the exact cause. I'd like to think if he saw anything unusual, he would have pointed it out.