What would cause a broken piston ring?

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DartVadar

DartVadar

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I am just taking apart the old 318 that was on my dart before I swapped in my 360. The 318 ran fine but burnt some oil. As I started taking it apart I started to see that it wasn't the original engine, it's a 84 with a roller cam. The top of the motor was perfect looking, nice and tight, the cam and lifters were in really good shape too. The heads looks pretty good, but two cylinders looked odd, one on each side.

I started taking the Pistons out, the bearings all looked brand new still, and the cylinder walls didn't have a ridge or anything on them. Upon taking the Pistons out everything looked good, except two of them, pieces of the rings fell on the floor for both of them, explains the burning of oil.

What would cause this to happen? And what type of Pistons did a 84 318 have? This one has flat tops and they don't seem to sit very far in the hole. They also all had a very small notch in them on the side, does this serve a purpose?
 
The notch is to tell you front from rear. Some pistons have the pin offset. It used to be "suggested" to turn the pistons around for better top end. Of course you cannot do that with domed / valve relief pistons.

Top rings broken might be carbon, detonation or rust damage to cylinders I'm not sure frankly what would break oil rings, maybe badly worn bores
 
Probably just lack of oil that caused it. Main bearings/seals leaking ?
 
Maybe it sat for a long time & fired up basically dry. This is why I squirt some oil in the cylinders and rotate with all the plugs out to try and get the rings moving smoothly before lighting off a motor that's been sitting.
 
Pinging or detonation most likely. The 80s Mopars were famous for those two running issues.
 
I'm not sure if it sat and was fired up, it's possible. And it leaked in a few places not really too bad though. But detonation could have been it. I turned it by hand once I got it on the stand, it seemed like it was still pretty smooth even after sitting outside under a tarp for over a year.

The valve on the head were all good but number 1 and 8. They looked really dark and kinda oily, those were where the Pistons rings were broken.

Also I thought the roller cam was introduced in 85 or so, why do I have an 84 with a roller cam? Bearings have "84" writing on them as well.
 
I had a 340 that I did a ring and bearing on when I was much younger. I got stuck out of gas, an nice old lady let me buy the stale stuff from her gas can, but I had to climb some hills to get out of the area and reach a station... It pinged the whole way and cracked three cylinders' worth of top rings. Also broke two ring lands.
 
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