rebore
The process of modifying the bore of an engine.
rebore: enlarging an engine cylinder to restore or increase displacement
A rebore is the machining operation that enlarges the internal diameter of an engine cylinder. The cylinder bore, already worn or damaged by years of operation, is honed or bored out to the next standard oversize, typically 0.010 inch, 0.020 inch, or 0.030 inch larger in diameter. New pistons and rings matching this oversize are then fitted to restore compression and seal, bringing the engine back to working condition or raising its displacement.
The process requires precise work on a boring machine or hone. The cylinder must be opened up to remove the damaged surface layer, scoring, or wear ridge that prevents proper ring seating. Cast iron blocks can usually tolerate three or four rebores before the cylinder walls become too thin; aluminum blocks with cast iron liners may allow only one or two. Some blocks cannot be rebored at all once they reach maximum oversize limits set by the manufacturer.
Why rebore instead of replace
A complete engine replacement costs far more than the machine work and new piston and ring kit needed for a rebore. For industrial engines, agricultural equipment, and older vehicles, reboring is the standard way to extend service life. The alternative, sleeving a cylinder (installing a steel or iron tube inside the damaged bore), is more expensive and requires different tooling but allows restoration when no oversize limit remains.
Reboring is also used to increase engine displacement deliberately, raising horsepower or torque output. A 0.030 inch rebore on a four-cylinder 2.0 liter engine increases displacement to roughly 2.1 liters. This is common in engine rebuilding for performance applications, though it requires matching pistons, careful balancing, and recalculation of compression ratio.
The term comes from the verb bore, meaning to drill or enlarge a hole; rebore describes boring the hole again, deeper or wider. The work sits between routine maintenance and major engine restoration, and is often paired with regrinding the crankshaft, replacing bearings, and fitting a new or rebuilt cylinder head.