An overbore and underbore are terms used to describe two different engine modification techniques that involve altering the internal diameter of the engine cylinders in a motorcycle:
Overbore: Overboring refers to increasing the internal diameter of the engine cylinders. It involves machining or replacing the cylinder block with larger diameter cylinders. By increasing the cylinder diameter, the displacement of the engine is increased, allowing for more air and fuel mixture to be compressed and combusted. This modification can result in increased power and torque output from the engine. Overboring is typically done to increase the engine's overall displacement and is often performed as part of an engine rebuild or performance upgrade.
Underbore: Underboring, on the other hand, involves reducing the internal diameter of the engine cylinders. This is typically done by machining the cylinder walls or using smaller diameter cylinders. The purpose of underboring is often to restore the engine's cylinders to their original specifications by removing minimal material. It can also be done to correct cylinder damage or wear, allowing for a fresh surface for piston rings to seal properly. Underboring is generally not performed as a performance modification since it reduces the engine's displacement and can potentially decrease power output.
Both overboring and underboring are precision machining processes that require expertise and careful measurements to ensure proper fitment and performance. These modifications are typically carried out by experienced engine builders or machine shops. It's important to note that modifying the cylinder size of a motorcycle engine can have implications for other components, such as pistons, piston rings, and gaskets, which may need to be replaced or modified accordingly. Consulting with a knowledgeable professional is advisable before undertaking any overbore or underbore modifications to ensure compatibility and desired outcomes for your specific motorcycle engine.