Heat cycling for stress relief
Stress relief is accomplished by repeatedly heating and cooling the engine-short engine runs followed by complete cooling repeated many times. ABC engine break-in involves such heat cycles, betit is not the same proceedure as is used for non ABC engines.

ABC break-in the inside story
Abc engines require a shorter break-in (heat cycling) than engines of the past- less than 45 min. for most. To attain and maintain maximum power levels, however, requires adherence to an entirely new set of break-in rules.
The primary objective of ABC break-in is to maintain the delicate top of the piston pinch zone while allowing the internal engine components to heat cycle with some minor smoothing and allignment (crankpin, wristpin, rod journals, etc.).

Here is how this is achieved:
FUEL- Use the same fuel for break-in as you will for normal operation. If you break-in your engine with low nitromethane fuel, e.g., 5% and then jump to 20% for actual running, the piston / cylinder fit will be too loose. Five % nitro doesn't generate as high a combustion temp., so the cylinder won't expand as much, and the pinch zone will wear more than if the cylinder had pulled away farther, as it would with the hotter burning, higher nitro fuel.

2- vs. 4-cycling operation
If you run a 2-stroke engine then you have heard of 4-cycling. When a two stroke engine is operated very fuel-rich or at somewhere below 1/2 throttle, it begins to fire on alternate crankshaft revolution. As the mixture is richened at wide open throttle, the ehaust sounds like this as it breaks from 2-cycling to 4-cycling: " RRREEEAAAHHHHHHH."
As the throttle is reduced to about 1/2, the exaust sounds like this, as it breaks from 2-cycling to 4-cycling: "RRRRREEEEETAT-TAT-TAT-TAT."
With both examples comes a simotanios loss of rpm with the change in exuast sound. This is 4-cycling. Learn to avoidit. Because the ABC-type engines were originally designed as WOT racing engines, their pistons and cylinders operated happily. Today, ABC engines are also expected to idle and throttle reliably. Unfortunatly this allows them to cool excessively, especially below 1/2 throttle, where poor cylinder scavenging (clearing) causes them to 4-cycle and wear away the critical pinch zone as the piston sleeve cools and tightens around the piston
Throttling is a necessary phase of running nitro engines in cars, so you can't simply eliminate it from your routine, but do not allow the engine to 4-cycle during break-in. Afterward, when engine components have been stess-relieved and bedded in through heat-cycling, the wearing effect of 4-cycling will be minimized.

Tight ABC piston/cylinder assemblies
Some new Abc-type engines are so tight at TDC that for the first few times the engine is cranked over and run, the possibility exist of damaging the connecting rod, crankpin, and piston. To avoid this, preheat the cylinder and the head with a heat gun or a hair dryer. This preheating expands the piston sleeve, and that minimizes the potential for stress on all the engine components during initial startup.