Small Engines (Lawn Mowers, etc.): different cylinder sizes, piston sizes

Question
i have an old briggs and stratton engine (1976) and it does not say the H.P on it so i looked up the serial # and it says its a 5 hp ( serial # starts with 13 , for 13 cubic inches right? ), and i bought a 1993 5 H.P briggs and stratton , and when i took both of them apart and i measured the diameter of the cylinders, the cylinder of the old engine(1976) is 4mm bigger then the newer engines cylinder, why is that ?

Answer
Hi Alain,


Briggs & Stratton engineers figured how to gain some additional power by placing the valves in the cylinder head so they are facing the piston.  OHV [OverHead Valves] make for a more symmetrical combustion chamber and result in more efficient burning of the air-fuel mixture.
\
There are other ways that an engine can be designed so that more is done with less.  Hence the cylinder of the older engine is 4 mm larger
than the newer engine.

Good question.

Howard