overbore
To bore out more than is usual for the design; especially, to bore out to a degree that pushes the limit regarding the wall thickness of the casting.
overbore: boring an engine block past safe limits
Overboring is the practice of enlarging a cylinder bore beyond the factory specification, typically by 0.020 to 0.060 inches (0.5 to 1.5 mm) in passenger engines, or further in competition builds. The cylinder wall becomes thinner as material is removed, and the casting itself sets hard boundaries: go too far and you breach the original wall thickness, creating thin spots prone to cracking under combustion pressure.
The main purpose is to allow a larger piston and ring package, which increases displacement and often improves volumetric efficiency. A 0.030 inch overbore on a 350 cubic inch V8 adds roughly 18 cubic inches. Production runs typically offer one or two stock overbore sizes, with corresponding pistons and rings manufactured as matched sets. A machinist working in a rebuild shop can bore to intermediate sizes, but must verify wall thickness with calipers or ultrasonic measurement before finishing the bore.
The casting constraint
Cast iron blocks vary in wall thickness by design and foundry process. A typical automotive casting leaves 0.200 to 0.260 inches of material between the bore wall and the outside surface or coolant jacket. Racing blocks and aftermarket castings sometimes have thicker walls (0.300+ inches) to permit aggressive overboring. Aluminum blocks require even more caution because thermal cycling induces larger stresses than iron, and aluminum's lower stiffness amplifies deflection under load.
Failure modes include piston slap (excessive clearance), ring flutter, blowby, and worst case, a casting fracture that forces an engine teardown. A crack typically initiates at a thin spot and propagates under the thermal and pressure cycling of normal operation. Some rebuilders take a conservative approach and bore only 0.010 to 0.020 inches oversize to keep wall thickness well within bounds, accepting the displacement penalty.
The term appears in parts catalogs as overbore pistons or overbore kits, and machine shops often advertise their bore capability: "bored to 0.060 overbore" or "ready for +60 pistons." Knowing your casting's original wall thickness and the intended final bore diameter is essential before any cutting begins.