This document specifies the housing dimensions for class A O-rings for general industrial applications conforming to ISO 3601-1, as well as housing dimensions for class B O-rings used on selected metricdimensioned hardware, e.g. fluid power cylinder bores and piston rods. These O-rings are for use in general hydraulic and pneumatic applications without and with anti-extrusion rings (back-up rings). The dimensions of the O-rings (d1 and d2), size codes (SC) and tolerances conform to ISO 3601-1. This part of ISO 3601 also addresses different design approaches in industry, and this is reflected in the table structures for hardware dimensions. Using the O-ring as a starting point to design the optimal hardware dimensions is reflected in Tables 2, 4, and 7. Using fixed hardware dimensions (i.e. bore / rod) and choosing the most appropriate O-ring to fit is reflected in Tables 3 and 5. Housing dimensions for the O-rings intended for aerospace applications that are specified in ISO 3601-1 are addressed in informative Annex A. NOTE 1 It is expected that O-ring housing dimensions for special applications be agreed upon between the O-ring manufacturer and the user. NOTE 2 The term “housing”, in this part of ISO 3601, is used to describe the groove or cavity, into which the O-ring is fitted, and the mating surface, which between them confine the O-ring.