piston damage (new problem)
 

While doing valve job on my 51 8ba I discovered this damage to #6 piston. It was not there 10 months ago when I had the head off. Never heard anything and the compression was good when checked before I took head off. Looks like a small piece broke off piston top edge and bounced around before being ejected. See attached picture. Not looking forward to dropping pan and removing piston to examin. Any ideas?

Re: piston damage
 

Broken top ring

Re: piston damage
 

Plus top ring groove will be totally eaten out by the pieces of broken ring. (One of which finally ate its way to the top of the piston.)

Re: piston damage
 

like the others, my guess is a broken top ring, I wouldn't run the engine any more till its fixed

Re: piston damage
 

A ring? Shouldn't be.We shall soon see. Lets see the head.

R

Re: piston damage
 

What does the wall look like?? That does not look exactly like the broken ring problems I have seen over the years. I wonder if something came from a valve or port or through a valve and port? I guess anything is possible so it is good that you have the engine apart and can get it reviewed.

Re: piston damage
 

Sure looks like the the mark on top of the piston was made by whatever was (forced ?) out of the hole on the opposite side. It's a mystery.

Re: piston damage
 

picture of cylinder head--cylinder wall looks clean with a couple of very slight vertical scratches on the bottom side where it corresponds to the piston damage. this does not show up in a picture.

Re: piston damage
 

Cylinder head looks as one would expect it to look considering what happened, but no matter how one tries to get around it, the engine should be dismantled to properly check and correct the problem.
If left as is, there is the possibility you could loose the whole engine.

Re: piston damage
 

looks like damage done by the ground electrode that has broken off a spark plug. Marlon

Re: piston damage
 

This is a classic long term broken ring result. When the piston is pulled the ring groove will be well hammered as previously stated , by other people.

Re: piston damage
 

Had that happen to me back in 1964, when I drained the oil I found broken rings in the pan.

Re: piston damage
 

Thanks for all the advice, I will drop pan and remove piston next week and report back to you with the results and pictures. In reinstalling the valves I'm having a problem pulling the intake valve guides down to install the horseshoe clip. The rubber seal around the guide is the problem, even with the proper pry bar they won't budge, any tricks I could try. Maybe i'll start a new thread.

Re: piston damage
 

I don't think you will find a broken ring that caused the damage.the more it is looked at it looks like foreign object damage (fod)

R

Re: piston damage
 

You can try a little lube on them, or just use rtv instead of the seals.

Re: piston damage (new pictures)
 

I like the idea of the RTV is that an acceptable substitute for the rubber seal.

Re: piston damage
 

RTV will work just fine. I think if you had a broken ring, the bore would be pretty well scratched up

Re: piston damage
 

I tend to agree. #6 is right under the carb, any chance a washer or screw went missing?

Re: piston damage
 

Only on the intakes, I think the exhaust would get too hot.
I just pulled apart a 350 I did ten years ago, had rtv on the intake manifold ends instead of gaskets. Was still holding just fine with no leaks.

Re: piston damage
 

Recently had a top ring break. Heading for a car show when all of the sudden I got a rattle sound from the engine. Shut it down, had AAA carry the truck home. Same damage as you showed on top of #5 piston. Pulled the piston and found the broken ring, Just went ahead and replaced all the pistons while I had it opened up.
Good news, no cylinder damage. Sad news is that when I had my machinist do some work for me 5500 miles ago he said that the piston were "about" ready to need replacing. Well no $ put that on hold...Guess what now I have new pistons, rings, gaskets and a "woulda, coulda, shoulda" moment to add to the list.
Those horseshoe clips are a "learning experience". get the claw on your bar to catch the lower portion of the valve guide. Push up on the bar just enough to expose the clip groove and get the clip started in the groove enough to get the valve guide to remain visible. Then gently tap the tab on the clip with a small hammer. It should slip into place and seat in the recess. Because my hands are size 90 and the space size 10 I use a long pair of needle nose pliers to initially maneuver the clip into the slot. You'll get it.

Re: piston damage (new pictures)
 

Removed piston today and this is what I found. see attached pictures.

Re: piston damage (new pictures)
 

Ouch! I guess the guys were right. What does the cylinders wall look like?

Re: piston damage (new pictures)
 

I'd be worried about where the rest of the ring went. There's a lot missing it looks like. Did it all go out the exhaust?

Re: piston damage (new pictures)
 

Cylinder walls look good except for a few miner scratches. I guess the missing piece of ring went out the exhaust. It's not in the pan where else could it be?

Re: piston damage (new pictures)
 

No surprise here! Used to see a lot of identical damage back in the day.

Re: piston damage (new pictures)
 

That looks pretty conclusive. Some of the marks on the piston appear to be longer than the opening would allow to pass. Obviously not. Thanks for the pictures.

Re: piston damage (new pictures)
 

Wonder if the ring may have broken into several small segments and then spit them out of the opening one by one.

Re: piston damage (new pictures)
 

seeing how many pits in the top of piston, I agree the ring broke into a lot of small pieces and went out thru the exhaust valve, check valve to make sure its not bent or the valve seat is not damaged

Re: piston damage (new pictures)
 

Sure looks familiar...refer post #20
I was also amazed, and felt very blessed not to find any collateral damage to the cylinder or valve.

Re: piston damage (new pictures)
 

Would something like that occur during installation?
Or, as a result of something else?

Re: piston damage (new pictures)
 

I imagine many reasons. Just a few;
Ring clearance wrong (end, side)
Not removing a ridge properly (sounds like not this engine)
careless insertion of piston and breaking ring on assembly.(to me, most likely)
Detonation.
Overheating (ring partly welding to bore causing it to break) may have done thousands of miles with no obvious problem since.

We can guess endlesly, but at the end of the day, something was out of spec.

Re: piston damage (new pictures)
 

A common cause of broken top rings is from having the ring hit the ridge at the top of the cylinder. If an engine is babied for many miles and never taken to high revs, the ridge will be in a certain place. If that engine is then run at much higher revs, the higher stresses will allow the piston to go much higher in the bore. The top ring then hits the ridge and breaks ring and/or piston.

Re: piston damage (new pictures)
 

I've had this car for 2 and 1/2 years. That head was off 10 months ago for inspection and the piston was fine, no damage visible. The former owner tells me the heads were not removed during the 20 years he had the car. I've put about 4,000 miles on the car since I bought it. It always ran fine but the compression was slowly dropping. Some exhaust valves were leaking and in the process of doing a valve job I discovered this problem. The pistons look original and the block has never been re-bored. I am assuming this is normal wear and tear.

Re: piston damage (new pictures)
 

It seemed strange to me but there is very little ridge in the cylinders and this cylinder had almost no ridge and the piston came out easily without using a ridge remover.

Re: piston damage (new pictures)
 

10 months ago that broken ring could have been within .010" of breaking through and you would never see it!

Re: piston damage (new pictures)
 

If the ring was broken in that cylinder shortly after assembly (or even before or during assembly) in that cylinder, that would explain why there was less ridge in that cylinder where it was expected to be. (less or no spring pressure)

Re: piston damage (new problem)
 

51 MERC
Your explanation sounds like a real possibility. I found an original piston and will hone the cylinder, fit it up with new rings and new rod bearings and give it a go. I know I should do a complete rebuild but at 73 I'm pushing my luck going this far.
Bill

Re: piston damage (new pictures)
 

I've got 10 years on Ya' and I remember back in the day that's the way it would have been done. Can remember standing in a grease pit and watching one or two (as required) crankshaft rod journals being ground while still in the car.

Re: piston damage (new pictures)
 

Billfish, Try to get the weights the same. same model piston and wrist pin?
Also What sort of hone will you be using? If there is any sort of ridge left in that bore, have the top ring groove moved down by machining the top groove for a spacer. The groove is widened on both faces (top and bottom) then a steel spacer goes in above the ring. This lowers the top ring down, slightly off the ridge.

Re: piston damage (new problem)
 

After initial startup with smooth quite operation the cylinder with replaced piston has developed a knock. The knock only occurs after engine sits overnight and stops after a few minutes of warm-up. Knock also stops if that cylinder's plug wire is pulled off. Piston was replaced with new rings and a new rod bearing of standard size. Not sure if it is piston slap, rod bearing or something else. Like I said - engine runs and sounds great after about 3 to 4 minutes of warm-up.