Standard MGB engine has a bore of 80.26mm and a stroke of 88.9mm. Say an overbore is plus 40 thousandsth of an inch, or 0.04 inch which converts to 1.016mm, so the bore of the engine is now 80.26 + 1.016 = 81.276 mm
The swept volume of a single cylinder is the circular area of the cylinder multiplied by the stroke of the piston. Circular area is PI x radius squared.
Radius = 81.276/ 2 = 40.638 Squared = 1651.447
Multiplied by PI (3.14159) = 5188.169 sq mm
Multiplied by the stroke (88.9) = 461228.27 sq mm or 461.2 sq cm (cc)
Multiplied by 4 (four cylinders) = 1845 cc
Probably represents an additional 2 or 3 bhp