Both SU and Zenith-Stromberg carburetors are measured across the opening on the manifold flange of the body.

Zenith-Stromberg units are identified by the opening’s inch equivalent so that a 175 CD carburetor model has a 1 ¾” opening. An SU carburetor is identified by a number that is equivalent to the number of 1/8” of an inch beyond 1 inch so that an HS4 has a 1+(4x1/8“)= 1+4/8” or 1½” opening.