O-Ring Groove Dimensions for AS568-218

AS568-218 O-Ring Specifications

• Dash Number: AS568-218 (200 Series)
• Inside Diameter (ID): 1.234″ (31.34mm)
• Cross-Section (CS): 0.139″ (3.53mm) — 1/8″ nominal
• Outside Diameter (OD): 1.512″ (38.4mm)

Groove Dimensions for AS568-218

The groove dimensions below are based on Parker O-Ring Handbook recommendations for standard 1/8″ (3.53mm) cross-section o-rings. These provide the proper squeeze ratio for reliable sealing.

Face Seal (Static Application)

Face seals are the most common type — used when the o-ring sits in a groove on a flat surface with no relative motion between parts.

• Groove Depth: 0.106″ (2.69mm)
• Groove Width: 0.187″ (4.75mm)
• O-Ring Squeeze: 0.02″ to 0.033″ (14.4% to 23.7%)
• Gland Fill: ~75% maximum (allows for thermal expansion)

Piston Seal (Dynamic Application)

Piston seals sit in a groove on a piston that moves inside a bore. The o-ring seals against the bore wall.

• Groove Depth: 0.109″ (2.77mm)
• Groove Width: 0.187″ (4.75mm)
• O-Ring Squeeze: 0.017″ to 0.03″ (12.2% to 21.6%)
• Bore Diameter: O-ring OD (1.512″) + diametral clearance (typically 0.002″-0.005″)

Rod Seal (Dynamic Application)

Rod seals sit in a groove in a bore, sealing against a rod that passes through. The o-ring compresses radially against the rod.

• Groove Depth: 0.109″ (2.77mm)
• Groove Width: 0.187″ (4.75mm)
• O-Ring Squeeze: 0.017″ to 0.03″ (12.2% to 21.6%)
• Rod Diameter: O-ring ID (1.234″) minus diametral clearance

Why These Groove Dimensions?

O-ring groove design is a balance of three factors:

• Squeeze (compression): The groove must compress the o-ring enough to create a seal. For the AS568-218 (1/8″ CS), the target squeeze is 19% of the cross-section. Too little squeeze = leaks. Too much = excessive friction and premature wear.
• Gland fill: The o-ring should fill no more than ~75% of the groove volume. The remaining 25% is free space that allows for thermal expansion of the rubber compound.
• Width clearance: The groove width (0.187″) is wider than the o-ring cross-section (0.139″) to prevent pinching and allow the o-ring to move slightly under pressure.

Material Considerations

The groove dimensions above work for standard o-ring materials (Buna-N/Nitrile, Viton/FKM, EPDM, Silicone). For harder materials (90+ Shore A durometer), you may need to adjust squeeze slightly. Softer materials (below 60 Shore A) may require tighter grooves to prevent extrusion at high pressure.

Surface Finish Requirements

• Static seals: 32 microinch Ra (0.8 μm) maximum on sealing surfaces
• Dynamic seals: 16 microinch Ra (0.4 μm) maximum on sliding surfaces
• Groove bottom: 63 microinch Ra (1.6 μm) maximum
• Break all sharp edges — use 0.005″-0.015″ radius or 15° chamfer at groove corners

Related AS568 Sizes

• AS568-215 Groove Dimensions — ID: 1.046″ (1/8″ CS)
• AS568-216 Groove Dimensions — ID: 1.109″ (1/8″ CS)
• AS568-217 Groove Dimensions — ID: 1.171″ (1/8″ CS)
• AS568-219 Groove Dimensions — ID: 1.296″ (1/8″ CS)
• AS568-220 Groove Dimensions — ID: 1.359″ (1/8″ CS)
• AS568-221 Groove Dimensions — ID: 1.421″ (1/8″ CS)