Broken Piston Rings - School Me Please

Long story short...

I tore apart an AMC 360 that I got with a vehicle, no knowledge of its history.

When I started to dismantle it I noticed that it was very clean inside But then I took the heads off I noticed that the cylinder walls were well worn but there was no ring ridge at all. I thought that seemed a little strange, but OK. When I took the stock, standard bore, pistons out I found that every top ring was broken, every one . The second rings were fine. The main/rod bearings had a date of '99 and I believe this engine is a '74 so someone has been in there before.

So, school me:

I don't see marks were someone used a ridge reamer so, why is there no ridge?

And, more importantly, what the heck broke every top piston ring if there is no sign of a ring ridge?


Marks on the cylinder walls are from me using a snap gauge.

Top ring gap to tight or maybe a detonation problem?

Are the faces of the top ring pieces worn as if they had been run, or are they "virgin"? If they show no face wear maybe they were broken when installed.

The 318 that was in my 87 Diplomat had a similar issue. Minimal bore wear, no ridge and broken top rings.

In this case the exhaust flapper was stuck shut forcing the exhaust from that side to use the crossover in the intake as it's path out. Engine ran hot, but not boiling over hot. I assume, I know, that the added heat caused the rings to expand enough to break. New set of moly rings and fixing the flap was the solution.

The engine has been ran (all rings show wear).

I hadn't thought of detonation, I didn't know that could effect the rings like that.

Over heating, maybe?? Just seems weird that all the second rings were fine but all the top rings were broken.

I was thinking of re-ringing it and putting in new bearings but I'm unsure if the same thing would happen again??

First thing - look at the ring ends and see if they are shiny. If so they butted, either should have been file fitted or it was overheated.

I'll bet they butted together. Someone didn't pay attention to end gap when they were installed. Top ring is closest to combustion temps. Lucky it didn't take the top corner off the piston....I've seen that before.

Some ring compressors don't keep the top ring well compressed as it enters the bore...chance to break the ring.

A few unanswered questions (but maybe I missed them?). Are there any signs that this engine was rebuilt? Original bore and pistons? If it was bored then maybe that's why no cylinder ridge? If it was rebuilt then it's as said incorrect ring end gap or rings broken during install? What's the end gap look like or measure on non-broken rings? Is it just one cylinder (I guess I need to re-read that) with A broken ring, but the title of this thread reads rings How about defective or poor quality piston rings ?

The engine wasn't rebuild but someone put new bearings, and I assume, new rings in it (standard bore, original pistons, standard bearings). All eight top rings were broken when I disassembled the engine, none of the second rings were broken.

Post a few pictures of the rings on their ends and the other ends where they are broken so we can help you figure out exactly what happened to those rings

First picture is of the factory ends of one of the top rings (No dot/bevel on the top).
Second picture is of the same ring (broken ends)
Third picture is of the bottom ring (dot on top with bevel on the top). I did find one of the second rings in multiple pieces as well.

Maybe the engine seized from sitting and the rings broke when the engine was freed-up.

Cylinders looks nice and clean. No rust, etc.

The fact that all eight top rings broke IMO that would seem like an install or ring quality issue?

The shiny ends of the top ring tell me they were either ground on for getting the end clearances proper or they butted and that led to much tension leading to that one breaking on the other side of the gap or something I haven't seen like that yet
It doesn't look like the two broken half's touch each other while it ran also
That small black dot on the top edge of broken ring may have caused it to fail while being installed or shortly there after
Did you check all the rings for proper back clearances in the piston ring grooves by pushing them in far enough to clear the pistons sides? If they are too thick and won't slide back into the grooves all the way that can cause them to break also

Ring installation error? Cracked them on installing on the piston and then they broke when running?

I'd get the block measured at the top of the cylinders to see Itf they are tight at the top. That's hard on rings.