O-rings

Backup rings (anti-extrusion rings)

A backup ring is a hard ring, usually PTFE, that sits beside an O-ring on the low-pressure side of the groove and closes the clearance gap so the O-ring cannot extrude into it under pressure. The military part families MS28774, MS28782 and MS27595 are PTFE backup rings made to SAE-AS8791, the successor to MIL-R-8791; commercial spiral, split and solid PTFE rings cover the same job on industrial glands.

Material: PTFEalso PTFE blends, hard elastomer, PEEK
Mil-spec families: 3MS28774, MS28782, MS27595
Document: SAE-AS8791rev D, 2025-07-16; was MIL-R-8791
Position: Low-pressure sideone per pressure direction

Line drawing of the section, not to scale.

Why an O-ring extrudes

System pressure pushes the O-ring toward the low-pressure side of its groove, and if the clearance gap there is wide enough the rubber flows into it. The result is a chewed or nibbled edge on the low-pressure side, and eventually a leak. Three things drive it: the pressure, the size of the gap, and the hardness of the compound.

Hardness on its own is not a fix. The published guidance is plain that a harder ring resists extrusion better but cannot compensate for an excessive gap, and that silicone and fluorosilicone need the tabulated clearance cut by 50 to 60 percent (ERIKS 50 percent, Parker® 60 percent, per the extrusion modifiers on file). A backup ring closes the gap instead, which is why it is the answer once pressure climbs past what the gland can hold.

When you need one: published thresholds

The manufacturers' guidelines on file disagree on the exact threshold, and the disagreement is worth knowing. ERIKS starts backup rings at 750 psi in dynamic service and 1,000 psi in static service; Trelleborg sets the line by ring size, 725 psi above a 50 mm inside diameter and 1,450 psi below it; the AS568 industrial convention that AS4716 follows adds one or two anti-extrusion rings above 1,500 psi. The lowest published threshold on file is Trelleborg's 725 psi for rings over 50 mm inside diameter; for a moving seal ERIKS starts at 750 psi.

Published manufacturer guidelines (ERIKS, Trelleborg, Precision Associates, Inc.), read 2026-09-06; the print or the standard governs
Source	Service	No backup ring up to, psi	With backup ring(s), psi	Condition
ERIKS	static	1000	6000; special construction to 30000
ERIKS	dynamic, reciprocating	750	above 750
Trelleborg	any	725	above 725	ring inside diameter over 50 mm (1.969 in)
Trelleborg	any	1450	above 1450	ring inside diameter under 50 mm (1.969 in)
Precision Associates, Inc. (PAI), AS568 industrial convention	any	1500	above 1500	one or two rings above the line

Which side, how many, and what it does to the groove

A backup ring goes downstream of the pressure, on the low-pressure side of the O-ring. One ring is the mainstream convention when pressure comes from one direction; two rings, one on each side, when the pressure alternates. One published source prefers two rings by default even for one-way pressure and accepts a single ring only where the groove has no room for two.

The groove is widened by the thickness of each backup ring, and the O-ring size does not change. The widths on file for one and two rings on each AS568 cross-section are tabulated here; the groove design guide covers the rest of the gland.

Groove width with backup rings, inches, by AS568 cross-section: the range across the published guidance of ERIKS and Precision Associates, Inc., read 2026-09-06; the ring maker's sheet sets the exact width
Cross-section in	Groove width, 1 ring	Groove width, 2 rings
0.070	0.138 to 0.152	0.204 to 0.210
0.103	0.171 to 0.197	0.238 to 0.256
0.139	0.208 to 0.244	0.275 to 0.307
0.210	0.311 to 0.350	0.410 to 0.429
0.275	0.408 to 0.476	0.538 to 0.584

Materials and styles

Backup rings are made of PTFE and PTFE blends, of 90 to 95 durometer elastomers, or of polyamide or PEEK where the temperature calls for it. Spiral and split (scarf-cut) PTFE rings install over a piston or into a bore without a two-piece gland; solid PTFE rings take higher pressure but only in two-piece assemblies; contoured elastomer rings suit glands where the backup must stretch on installation.

Conquest Seal supplies PTFE backup rings in the mil-spec families and in commercial styles, and quotes PTFE and PEEK rings machined to your print by the shops we source from. The machined plastic parts page covers those.

The mil-spec families under SAE-AS8791

MS28774, MS28782 and MS27595 are the three part families for PTFE packing retainers, the military name for backup rings. All three are made to SAE-AS8791, which superseded MIL-R-8791; the current revision is AS8791D, dated 2025-07-16, read from the SAE Mobilus page on 2026-09-06. Parker ORD 5700 (2018) Table 6-2 footnotes the three cuts: MS27595 is the continuous ring, MS28774 the single-turn scarf-cut ring and MS28782 the double-turn ring, and the MS titles agree. The legacy specification chart on file pairs all three with MIL-R-8791 / SAE-AS8791, which is where the part-family-to-document pairing comes from.

The MS28774 and MS27595 numbers take a three-digit size suffix and MS28782 takes a two-digit one. Send the number as printed together with the dash number of the O-ring it sits beside and the retainer is matched from the document, never from a guess. Conquest Seal is AS9100 certified.

Status read from SAE Mobilus on 2026-09-06; part-family pairing from the Conquest legacy specification chart
Designator	Document	Material	Status	Size suffix and pairing
MS28774-xxx	SAE-AS8791 (was MIL-R-8791)	PTFE	AS8791D current, 2025-07-16	three-digit suffix; quote with the O-ring dash it backs
MS28782-xx	SAE-AS8791 (was MIL-R-8791)	PTFE	AS8791D current, 2025-07-16	two-digit suffix; quote with the O-ring dash it backs
MS27595-xxx	SAE-AS8791 (was MIL-R-8791)	PTFE	AS8791D current, 2025-07-16	three-digit suffix; quote with the O-ring dash it backs
SAE-AS8791	Retainer, Packing, Hydraulic and Pneumatic, Tetrafluoroethylene Resin	PTFE	AS8791D current, 2025-07-16; original 1997-10-01	the governing document for all three families

The section dimensions are inside SAE-AS8791 and are not reproduced here; the cut styles above are as Parker's cross-reference and the MS titles print them. Conquest Seal is a distributor: the parts come from manufacturers qualified to the document, and the certificate of conformance names the manufacturer and the revision.

Frequently asked

What does a backup ring do?

It closes the clearance gap on the low-pressure side of an O-ring groove so the O-ring cannot extrude into the gap under pressure. The O-ring still does the sealing; the backup ring is a hard, non-sealing ring that keeps the rubber where it belongs.

When do I need a backup ring?

The published guidelines on file put the threshold between 725 and 1,500 psi depending on the source, the service and the ring diameter: ERIKS at 750 psi for a moving seal and 1,000 psi static, Trelleborg at 725 psi above a 50 mm inside diameter and 1,450 psi below it, and the AS568 industrial convention at 1,500 psi. Compound hardness moves the line as well: a harder ring resists extrusion better, though it cannot make up for an excessive gap.

What is the difference between MS28774 and MS28782?

Both are PTFE packing retainer families made to SAE-AS8791, the successor to MIL-R-8791, and both are quoted against the current revision AS8791D. MS28774 takes a three-digit size suffix and MS28782 a two-digit one; MS28774 is the single-turn scarf-cut ring and MS28782 the double-turn ring (Parker ORD 5700 Table 6-2); the section dimensions are inside the document, so send the callout as printed and the retainer is matched from it.

Which side of the O-ring does the backup ring go?

On the low-pressure side, downstream of the pressure. Use one ring when the pressure always comes from the same direction and one on each side when it alternates; one published source prefers two rings whenever the groove has room.

What are backup rings made of?

PTFE and PTFE blends most often, in spiral, split (scarf-cut) and solid styles; 90 to 95 durometer elastomers where a contoured ring that stretches on installation is wanted; polyamide or PEEK for high temperature. The mil-spec families are PTFE.

Does the groove change when I add a backup ring?

The groove gets wider by the thickness of each backup ring and the O-ring size stays the same. The one-ring and two-ring widths on file for each AS568 cross-section are listed on this page; the groove design guide covers depth, squeeze and fill.

Related guides

Sources

Backup ring thresholds, side convention, materials, styles and groove widths (ERIKS, Trelleborg, Precision Associates, Inc.; read 2026-09-06); SAE-AS8791 title, revision and supersession of MIL-R-8791; Part-family pairing MS28774 / MS28782 / MS27595. Dimensional facts are stated in our own words; no standard's text or table is reproduced.

Parker is a trademark of Parker Hannifin Corporation. No affiliation or endorsement is implied.

Trelleborg is a trademark of its owner; no affiliation or endorsement is implied.

ERIKS is a trademark of its owner; no affiliation or endorsement is implied.

Send the retainer number and the O-ring dash it backs

The MS or commercial part number as printed, the dash number of the O-ring beside it, the quantity and any certificate the order needs. A Conquest representative replies with the quote.

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Last updated September 25, 2026