O-ring backup rings: when and how to size them

What causes O-ring extrusion

Extrusion happens when the O-ring is forced into the diametrical clearance gap between the piston and bore, or between the rod and gland, under system pressure. Higher pressure, larger clearance gaps, and softer or heat softened compounds all increase the risk. Once the O-ring nibbles into the gap, the extruded edge tears on each cycle and the seal eventually leaks. A backup ring closes the gap so the O-ring has nothing to extrude into.

One-way versus two-way pressure: single and double backup rings

The number of backup rings a gland needs depends on which direction pressure acts on the seal. A single ring installed on the low-pressure side of the O-ring is enough when pressure only pushes one way. Reciprocating hydraulic cylinders and other alternating-pressure applications need two rings, one on each side of the O-ring, because the extrusion gap can open on either face during the cycle. See the backup ring product line for ring cross sections available across standard AS568 sizes.

Two rings should be matched so both fit the same groove width without over-filling it.

AS568 dash numbers and backup ring sizing

Backup rings are sized to match the O-ring they support, so the AS568 dash number the O-ring uses is the starting reference for the backup ring cross section and inside diameter. Adding a backup ring widens the groove: the ring adds its own thickness to the groove width, on the low-pressure side, while the groove depth and the O-ring's intended squeeze stay as designed for the O-ring alone; the groove design guide covers the dimensions. The print is the authority on final dash number, groove dimensions, and backup ring part number for a given assembly.

MS28774 and MS28782 backup ring specifications

MS28774 and MS28782 are two of the military backup ring specifications in Parker's ORD 5700 cross-reference, which also lists AN6244, AN6246, MS27595, MS28783 and MS35803. MS28774 covers the single-turn, scarf-cut PTFE backup ring, while MS28782 covers the double-turn style. Parker's ORD 5700 backup ring cross-reference sets MS28774, MS28782 and MS27595 under SAE AS8791, the PTFE backup ring document, so all three are PTFE; the print names the specification and dash number. Conquest supplies backup rings sourced to the print, including MS28774 and MS28782 dash numbers, alongside the O-ring itself.

What to record on the print

Because backup ring configuration sets the groove width, every requirement belongs on the engineering print rather than left to assumption. The print should call out the O-ring dash number, the backup ring specification and material, the ring count, and which side of the gland each ring occupies. The print governs on all of these details, and no dash number or material should be substituted without a documented change.

Sources

1. DLA ASSIST-QuickSearch MS28774 (ident 41723): MS28774M and Notice 2, Retainer, Packing Backup, Single Turn, Polytetrafluoroethylene, read 2026-09-28
2. SAE Mobilus public standard page: SAE AS8791, Retainer, Packing, Hydraulic and Pneumatic, Tetrafluoroethylene Resin, read 2026-09-24; current revision AS8791D record read 2026-09-27
3. Parker O-Ring Handbook ORD 5700, section 6 Table 6-2 Back-Up Rings Cross Reference (handbook pages 6-8 and 6-9), read 2026-09-25
4. SAE Mobilus current-revision record: SAE AS568F, Aerospace Size Standard for O-Rings, read 2026-09-27