This International Standard specifies the dimensions of the seal cavity for step-balanced mechanical seals, mechanical seals suitable for straight shafts, and for soft packing, to be used with horizontal end-suction centrifugal pumps, including those conforming to ISO 2858. Although applicable to general-purpose and heavy-duty process applications, this International Standard is not intended for use in processes with slurries or with high levels of solids. This International Standard offers two options: a) a range of stuffing boxes suitable for soft packing or mechanical seals for general-purpose duties up to a seal chamber limit of a gauge pressure of 16 bar; b) a range of larger cavities suitable for mechanical seals, including cartridge mechanical seals, for general and heavy-duty process applications, up to a seal chamber limit of a gauge pressure of 40 bar.