AdmiralHam,
If it's ringed, and still running well, it's fine.

ABC stands for Aluminum, Brass, Chrome. It means an Al piston running in a Chromed Brass liner. The liner is cut in a taper, so that the bore is smaller at the top than the bottom of the piston stroke. That's why new ABC engines have that "bump" or "grind" at the top of the stroke. That's a GOOD thing, not a mistake. As the engine heats up, the liner expands faster than the piston, and at full temp, you get the right piston-liner seal and the right compression. You want to avoid running them cold because you don't want to wear down the fit when the engine is cold, as all you will do it loose power when it hits full temperature. That's why you don't run an ABC engine at idle or overly rich for long when new. All you are usually breaking in is the con rod and wrist pin fits, you aren't adjusting the piston-liner fit, you want it tight. ABC engines also handle being run overly lean fairly well, since the overheating condition just causes the liner to over-expand so the engine looses compression and runs poorly. ABC engines produce a bit more power than ringed engines in the smaller engine sizes, but they do wear out a lot faster, a ringed engine will usually last about 3 times as long under the same conditions. As a side note, OS engines aren't actually ABC, they are ABN, they use nickle instead of chrome on the liner, and early versions of the FX line had a problem with the nickle flaking off on overly hot/lean runs, eating the engine. As far as I know, that's been fixed though.

Ringed engines (usually) use an aluminum piston and a liner that is steel. They have an iron or steel ring around the top of the piston. There is a slight gap in teh ring, so when the engine is cold, they appear to have less compression, the total opposite of an ABC engine. As the engine heats up, the ring expands and seals against the liner and closes the gap. When breaking in a ringed engine, you are seating the ring to the piston and liner, and that's best done with lots of cool oil, running slowly at a lower temp, exactly the opposite of ABC. If you overheat a ringed engine bad enough, the ring can "seize", or expand too much so that it develops too much friction with the liner and stop the engine. After this, it's usually time to replace the piston/ring/liner. Sometimes you can break a con rod this way, though it's less common. So you don't want to overheat your ringed engines. Oh, because you have the ring in there, ringed engines also handle eating a bit of dirt better than ABC engines. The ring will tend to help seal up really small gouges in the liner. ABC engines can't do this, so you have to be more careful about dirt.

As an amusing experiment, find two engines, one ABC and one ringed in about the same size. Flip them through compression a few times cold. Then start em up, and run them to get them good and warm. Stop them, and then flip though compression again. You should see that the ringed engine has MORE compression when hot, while the ABC engine has LESS than when it was cold.

These days, I think all engines in the .46 and smaller classes are ABC or ABN. If someone knows of a ringed .40, let me know, please? It used to be that you could get the same engines in ABC or ringed. At the time, the ABC ones costed more and produced a bit more power. The funny thing is that ABC is actually cheaper to make, which is a big reason that the small engines are all ABC now. I have two OS .32F engines from years ago. One is ABC, the other is ringed. I can't tell them apart with out taking off the muffler So, in the end, it really isn't a huge deal either way.

As long as you don't routinely overheat your engine, or do an extremely long and cold breakin, or overly fast non-breakin, you really won't hurt most engines either way. You may not produce absolute max power from it, but generally, most guys don't care if they loose 2k rpm because they broke it in overly fast. (I have 3 Mag .28 ABC engines. Last weekend, one tached at only 16.5k, the other was over 18k. But I fly combat, so I noticed that power drop. Flying sport, most guys wouldn't notice or care, and wouldn't be pushing 18k anyway. But it looks like time for a new piston/liner in one of my engines).

(for all that blather, I don't consider myself an engine expert. I don't rework them (other than parts replacements))