Most engines that have a ringless high silicon aluminum piston, running in a plated cylinder sleeve, have a conically tapered bore.

This would mean the bore is tighter at the top of the sleeve than at the bottom.

The purpose of this smaller internal diameter upper cylinder, is to cater for the differential heating (and expension) of the cylinder, when the engine attains its normal operating temperatures.

The upper part of the sleeve, near the head and the combustion chamber, is much hotter than the lower part of the sleeve. Therefore it expands more and when at operating temperature, the part of the sleeve, in which the piston reciprocates is nearly cylindrical.

The piston in these engines is made of a low expansion, high silicon content, aluminium alloy (hypereutectic?), so it expands no more than the brass (or aluminium in AAC), base sleeve material, when heated.

All these engine's pistons have an interference fit at the top of the bore, so the engine "squeaks" or even seizes when turned over, especially when new.

Because of this fit, these engines need special break-in procedures. David Gierke, Clarence Lee (of RCM) and Dub Jett (http://www.jettengineering.com, read about break-in) wrote about these principles and they work.

If such an engine is broken-in sloberingly rich and four cycling, like you would normally do with with a ringed engine, it could be run-out, or broken, in stead of being run-in.

If the sleeve is allowed to remain tapered, the interference fit at top stroke would get the piston and the plating scuffed, especially in the inferior ABN engines. Lower cost is the only reason some engine manufacturers prefer nickel plating to hard chrome... The plating can easily be damaged in these engines.

Also, in every non-firing revolution the piston is pushed, into the seize at the top of the bore and then pulled back down, out of the seize, by the conrod. These repeated compression-tension cycles can ultimately cause the conrod to fail, due to metal fatigue (this was common in some older K&B ABC QM engines).

This type of engine must be broken-in in a somewhat rich two-stroke mode, beyond the point it starts to two-cycle, but not too lean.
It must be run with the throttle wide open and shouldn't be idled during the break-in.

It must not be allowed to run for any length of time in four-stroke mode, during the break-in. After the break-in is completed, there is no problem with four-cycling because the interference is minimized and is no longer an issue, even when the engine is run lukewarm.

Use a prop with the same pitch, but with one inch less diameter, than the prop you would normally use. This, to lessen load on the engine without compromising cooling.

In addition, to heat treat the piston (for size stability), the first 20 runs, or so, must be short; starting from 10 seconds and gradually getting longer, to about 1 minute.
The engine must be allowed to cool down completely between these runs.
Harry Higley of MAN (and Harry Higley model products) developed this method years ago and it works.

After that, allow the engine to run for 10-15 minutes, still in a somewhat rich two-stroke mode.

After that the engine should be leaned gradually for short periods and then richened back (still two-cycle) to cool.
After about 10 minute doing that, try to lean the engine carefully to maximum RPM. If it can hold this RPM without sagging, for 60 seconds straight, you have completed the break-in.
If it sags richen it immediately, to prevent damage, repeat the first part of this paragraph for 10 more minutes and then try again.

I have had ample experience with breaking-in ABC engines made by Rossi and MVVS, as well as ABN engines made by OS and Webra.

The only non-success was with an OS40FP, before I adopted this break-in technique. It peeled the nickel off about 25% of the bore.
It still ran for years afterwards, though, until it broke in a bad crash.