Bore - Thermodynamics:

The inner diameter of engine cylinder is called as bore. It can be measured by a vernier calliper or bore gauge. As engine cylinder wears out with the passage of time, so the bore diameter changes to larger value, therefore the piston becomes lose in the cylinder, and power loss occurs. To over come this problem reboring to therefore next standard size is done and new piston is placed. Bore can be denoted by the letter 'D'. It is measured in mm (S.I. units) or inches (metric units). It is used to calculate the engine capacity (or cylinder volume).