From the Parts Counter — spreadbore

The Short Answer Spreadbore and Squarebore are terms used to describe the 2 major designs of 4-barrel carburetors.  In a spreadbore design, the primary bores are smaller than the secondary bores.  In a squarebore design, both the primary and secondary bores are the same size.   Fig 1: Squarebore vs spreadbore The Longer Answer The bores (also known as barrels) are parallel chambers going from the top of the carburetor body to the bottom (base) of the carburetor body.  They are the chambers where fuel and air are mixed together to create the combustion charge that makes the engine run.  ...