It is true that heat can cause some changes in the material properties of a piston, but whether these changes result in permanent growth or not depends on several factors such as the material composition, manufacturing process, and operating conditions.

During the break-in process, the goal is to allow the various components of the engine to gradually adapt to each other, which can involve some wearing and smoothing of the surfaces in contact. The heat cycle method involves gradually increasing the engine's operating temperature over several cycles to promote this process.

However, it's important to note that the heat cycle method is just one of many break-in methods and may not be suitable for all types of engines or materials. The best break-in method depends on the specific engine and manufacturer recommendations. It's always important to follow the manufacturer's guidelines for proper break-in procedures to ensure the engine's longevity and performance.