Parco is a leading manufacturer of O-rings. An O-ring is a solid-rubber seal shaped like a doughnut or torus. When compressed between mating surfaces, an O-ring blocks the passage of liquids or gases. Parco O-rings are available in a wide range of standard and non-standard sizes suitable for nearly all sealing applications.

O-ring sizes are described by inside diameter (ID) and cross section (W). There are 369 standard sizes of O-rings with inside diameters ranging from 0.019 to 25.940 inches and cross sections ranging from 0.040 to 0.275 inch. Standard sizes are available in 5 popular cross-sectional diameters (0.070, 0.103, 0.139, 0.210 and 0.275 inch). Each O-ring size is identified by a "dash number." Each Parco "dash number" is the same as the Aerospace Standard (AS) 568A designations used throughout the O-ring industry.