In the context of motorcycle engines, overbore and underbore refer to modifications made to the cylinder bore diameter, which is the inner diameter of the engine cylinder where the piston moves.
Overbore: Overboring involves increasing the cylinder bore diameter beyond its original specification. This is typically done to increase the engine's displacement, which can result in higher power output and improved performance. By enlarging the cylinder bore, the engine can accommodate larger pistons, allowing for a larger combustion chamber and increased displacement. However, it's important to note that overboring requires precision machining and careful consideration of factors such as piston-to-cylinder clearance and cooling capacity to ensure proper engine function.
Underbore: Underboring, on the other hand, involves reducing the cylinder bore diameter below its original specification. This is less common in motorcycle engines and is usually done for specific purposes, such as reducing engine displacement for racing classes with displacement limits. Underboring reduces the engine's displacement and can have an impact on power output and torque. However, it requires expertise and proper tuning to maintain proper engine performance and reliability.
Both overboring and underboring can affect the engine's performance characteristics, including power delivery, torque, and responsiveness. These modifications should be undertaken by experienced professionals who understand the specific requirements of the engine and have the necessary knowledge of piston-to-cylinder clearances, cooling, and other factors that can influence the engine's operation.