Overbore (rebore)

Overboring means machining the cylinder to a larger diameter. It is done for two reasons: to repair a worn or scored cylinder by fitting an oversize piston, or to increase displacement as a tuning measure, often with a big-bore kit.

Many manufacturers and piston makers offer oversize pistons in steps, for example +0.25 mm and +0.50 mm. Cylinders with a hard coating such as Nikasil cannot simply be bored; they are usually replated or replaced, or fitted with a liner.

A larger bore raises compression and may need fuelling changes, and too thin cylinder walls can lead to overheating or cracking. Always have the bore measured by an engine shop and follow its piston-to-wall clearance and running-in advice.