O-rings
Backup rings: MS28774, MS28782 and MS27595 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, and an MS28774 number carries the AS568 dash of the O-ring it backs. Commercial PTFE and hard-rubber rings cover the same job on industrial glands.
- Material
- PTFEalso hard rubber; leather on old military parts
- Mil-spec families
- 3MS28774, MS28782, MS27595
- Document
- SAE-AS8791rev D (2020), reaffirmed 2025
- Position
- Low-pressure sideone per pressure direction
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 needs the tabulated clearance cut by 50 percent (the Parker® handbook, section 4.2). 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' published guidelines disagree on the exact threshold, and the disagreement is worth knowing. For a static seal, Trelleborg sets the line by ring size, 725 psi above a 50 mm inside diameter and 1,450 psi below it; for a reciprocating seal it sets 725 psi whatever the diameter. Parker ORD 5700 section 4.2 says to look at backup rings above 1,500 psi for every seal type. SAE AS4716, the aerospace gland design standard for AS568 sizes, draws the same line in its scope: its glands take one or two anti-extrusion rings above 1,500 psi and none below. Precision Associates' worked example puts two rings, one on each side, in a gland over 1,500 psi. The lowest published threshold is Trelleborg's 725 psi, for any reciprocating seal and for static rings over 50 mm inside diameter.
| Source | Service | No backup ring up to, psi | With backup ring(s), psi | Condition |
|---|---|---|---|---|
| Trelleborg | static | 725 | above 725 | ring inside diameter over 50 mm (1.969 in) |
| Trelleborg | static | 1450 | above 1450 | ring inside diameter under 50 mm (1.969 in) |
| Trelleborg | dynamic, reciprocating | 725 | above 725 | any ring inside diameter |
| Precision Associates, Inc. (PAI) | any | 1500 | above 1500 | two rings, one on each side, above the line |
| SAE AS4716 (gland design standard, scope) | static and dynamic, AS568 sizes | 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 ring maker's sheet sets the exact width, and the groove design guide covers the rest of the gland.
Materials and styles
The backup rings in Parker's cross reference come in three materials: PTFE (the AS8791 families), hard rubber (Parker compound N1444-90) and, on older military parts, leather (MIL-W-5521). The same table lists three cuts: continuous, single-turn scarf cut and double turn. Parker's PTFE seal guide names PEEK among the other engineering polymers used for backup rings.
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; the current revision is AS8791D, revised January 2020 and reaffirmed 2025-07-16, read from the SAE Mobilus record on 2026-09-27. Parker ORD 5700 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 same table sets all three under its AS8791 PTFE columns, 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.
| Designator | Document | Material | Status | Size suffix and pairing |
|---|---|---|---|---|
| MS28774-xxx | SAE-AS8791 | PTFE | AS8791D current (revised 2020, reaffirmed 2025-07-16) | three-digit suffix; quote with the O-ring dash it backs |
| MS28782-xx | SAE-AS8791 | PTFE | AS8791D current (revised 2020, reaffirmed 2025-07-16) | two-digit suffix; quote with the O-ring dash it backs |
| MS27595-xxx | SAE-AS8791 | PTFE | AS8791D current (revised 2020, reaffirmed 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 (revised 2020, reaffirmed 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.
MS28774 and MS28782: what each one is and how the dash reads
MS28774 is the single-turn, scarf-cut PTFE packing retainer under SAE-AS8791. Its dash number is the AS568 dash of the O-ring it sits beside, so MS28774-214 backs an AS568-214 O-ring, and with it any mil-spec ring in that size such as MS28775-214 or M83248/1-214, in the same gland. That pairing runs across the span of Parker ORD 5700 Table 6-2, from -006 to -425, and MS27595 uses the same dash numbers for the continuous, uncut ring.
MS28782 is the double-turn PTFE ring under the same document. Its two-digit suffix is a sequence of its own and does not repeat the O-ring dash, so this site never derives an MS28782 number from an O-ring size: send the number as printed, or the O-ring dash, and the retainer is matched from the document. All three families are PTFE (the AS8791 title names tetrafluoroethylene resin), and the retainer section dimensions sit inside SAE-AS8791.
The table groups the MS28774 and MS27595 numbers by AS568 cross-section series; the O-ring W column is the O-ring's cross-section, not the retainer's. Every O-ring size page from -006 to -425 prints its own MS28774 and MS27595 number beside the mil-spec callouts for that dash. The groove is the O-ring's AS568 gland widened for the retainer, as described above.
| AS568 series | O-ring dash range | MS28774 (single-turn, scarf-cut) | MS27595 (continuous) | O-ring W in | O-ring W mm | Sizes |
|---|---|---|---|---|---|---|
| 0xx series | -006 to -050 | MS28774-006 to MS28774-050 | MS27595-006 to MS27595-050 | 0.070 | 1.78 | 45 |
| 1xx series | -102 to -178 | MS28774-102 to MS28774-178 | MS27595-102 to MS27595-178 | 0.103 | 2.62 | 77 |
| 2xx series | -201 to -284 | MS28774-201 to MS28774-284 | MS27595-201 to MS27595-284 | 0.139 | 3.53 | 84 |
| 3xx series | -309 to -395 | MS28774-309 to MS28774-395 | MS27595-309 to MS27595-395 | 0.210 | 5.33 | 87 |
| 4xx series | -425 | MS28774-425 | MS27595-425 | 0.275 | 6.99 | 1 |
Related pages
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 put the threshold between 725 and 1,500 psi depending on the source, the service and the ring diameter: Trelleborg at 725 psi for any reciprocating seal, and for a static seal at 725 psi above a 50 mm inside diameter and 1,450 psi below it; Parker ORD 5700, Precision Associates and SAE AS4716 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, and both are quoted against the current revision AS8791D. MS28774 is the single-turn scarf-cut ring and carries the three-digit AS568 dash of the O-ring it backs; MS28782 is the double-turn ring and takes a two-digit suffix of its own (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 O-ring does an MS28774 backup ring fit?
The one with the same dash number. MS28774-214 backs an AS568-214 O-ring, and the same holds for MS28775, M83248/1 and the other mil-spec rings in that size, across the -006 to -425 span of Parker ORD 5700 Table 6-2. MS27595 uses the same numbers for the continuous ring; MS28782 does not, because it runs its own two-digit sequence.
What material is an MS28774 backup ring?
PTFE. MS28774, MS28782 and MS27595 are all tetrafluoroethylene resin retainers made to SAE-AS8791. Commercial backup rings also come in hard rubber and, on older military parts, leather, but the mil-spec families are PTFE.
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, hard rubber or, on older military parts, leather, in continuous, single-turn scarf-cut or double-turn form (Parker ORD 5700 Table 6-2). The mil-spec families MS28774, MS28782 and MS27595 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 ring maker's sheet sets the exact width, and the groove design guide covers depth, squeeze and fill.
Related guides
Sources
Backup ring thresholds, side convention, materials and styles (Trelleborg, Precision Associates, Inc.; read 2026-09-24); Parker ORD 5700 section 4.2 (1,500 psi, silicone 50 percent), read 2026-09-24; materials and cuts: Parker ORD 5700 Table 6-2 (read 2026-09-25) and Parker PTFE Seals Design Guide PDE 3354 (read 2026-09-25); SAE AS4716 title, revision AS4716D (2021-11-09) and scope: glands for AS568 O-rings with one or two anti-extrusion rings above 1,500 psi and none below (SAE Mobilus, read 2026-09-24); SAE-AS8791 title and revision (SAE Mobilus AS8791D record, read 2026-09-27); Parker ORD 5700 Table 6-2 (read 2026-09-25), for the MS28774 / MS28782 / MS27595 part-family pairing; SAE AS8791 record (SAE Mobilus, read 2026-09-24); Parker ORD 5700 Table 6-2, Back-Up Rings Cross Reference, handbook pages 6-8 and 6-9 (read 2026-09-25), for the cut styles, the MS28774 and MS27595 dash pairing with the AS568 O-ring dash, -006 to -425, and the MS28782 own sequence. Dimensional facts are stated in our own words; no standard's text or table is reproduced.
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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. Quotes answered within one business day. A person answers (714) 632-9830, Pacific business hours.
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