What is an engine cylinder parent bore?

The engine cylinder parent bore refers to the original, factory-machined diameter of the cylinder bore in an engine block before any modifications or oversizing are performed. It serves as the baseline measurement for the cylinder's dimensions, ensuring proper fitment for pistons and rings. If the engine undergoes wear or damage, the cylinder may be honed or bored to a larger size, but the parent bore remains the starting reference point for these alterations.