O-Ring Extrusion Gap Chart

Maximum diametral clearance between mating parts before the O-ring extrudes into the gap — by pressure, hardness, and backup ring configuration.

Quick answer

Extrusion gap is the diametral clearance between piston and bore or rod and gland. At ≤7 MPa (70 bar), 70 Shore A NBR tolerates about 0.25 mm gap without a backup ring; at 14–21 MPa, limit gap to 0.10–0.15 mm or add a PTFE backup ring. Above 35 MPa dynamic, use dual backup rings and precision-fit gaps ≤0.15 mm. If gap ÷ cross section exceeds 0.15, add a backup ring regardless of nominal pressure.

High-pressure O-ring failure is often extrusion, not chemical attack. System pressure pushes the elastomer into the clearance gap between metal surfaces. Each pressure cycle nibbles material from the low-pressure side until the cross section collapses and the seal leaks. Controlling extrusion requires three variables: system pressure, O-ring hardness, and the diametral extrusion gap. Backup rings block the gap entirely when groove geometry or machining tolerance cannot hold clearance tight enough. Use this chart as a design starting point. Verify your actual gap with calipers or drawing tolerances, then cross-check squeeze and fill rate with the Compression Calculator.

How to Measure Extrusion Gap

Extrusion gap (also called diametral clearance or E-gap) is the total diametral space between the two metal surfaces that bound the O-ring groove on the low-pressure side. On a piston seal, it is bore ID minus piston OD. On a rod seal, it is gland bore ID minus rod OD.

Measure at the location of maximum clearance — worn rods, oval bores, and misaligned glands often exceed nominal drawing values. For used equipment, measure after 5,000–10,000 hours if the application is high-cycle.

Compare your measured gap to the tables below. If the gap exceeds the limit for your pressure and hardness, either tighten machining tolerances, increase hardness to 90 Shore A, or add PTFE backup ring(s).

Backup Rings →High-Pressure Design Blog →

Dynamic Service — Reciprocating Hydraulics

Rod and piston seals in cylinders and actuators. Values are maximum diametral extrusion gap in millimeters.

Dynamic service is less tolerant of clearance because motion and side load increase gap on one side of the gland. Add a backup ring when dynamic pressure exceeds 10 MPa (100 bar) even if gap is within the no-backup column.

Bidirectional pressure (double-acting cylinders) requires backup rings on both sides of the O-ring.

Pressure	70 Shore A	90 Shore A	With Single Backup Ring	With Dual Backup Rings
≤ 7 MPa (70 bar)	0.25 mm	0.30 mm	0.50 mm	0.60 mm
7–14 MPa	0.15 mm	0.20 mm	0.30 mm	0.40 mm
14–21 MPa	0.10 mm	0.15 mm	0.25 mm	0.35 mm
21–35 MPa	Not recommended	0.10 mm	0.20 mm	0.30 mm
> 35 MPa	Not recommended	Not recommended	0.15 mm (precision fit)	0.20 mm

Hydraulic System Selection →Hydraulic Cylinder Application →

Static Service — Flanges, Ports and Manifolds

Face seals and static glands with no relative motion.

Static glands tolerate slightly larger gaps because there is no friction heating or side load. Still add a backup ring when pressure exceeds 150 bar or when flange separation under bolt load increases effective clearance.

For static service above 200 bar, prefer 90 Shore A compound with a PTFE backup ring on the low-pressure side.

Pressure	70 Shore A	90 Shore A	With Backup Ring
≤ 10 MPa (100 bar)	0.30 mm	0.35 mm	0.55 mm
10–15 MPa (150 bar)	0.20 mm	0.25 mm	0.40 mm
15–20 MPa (200 bar)	0.15 mm	0.20 mm	0.35 mm
20–35 MPa	Backup ring required	0.15 mm	0.30 mm
> 35 MPa	Dual backup rings recommended	Dual backup rings	0.20 mm max gap

Static & Flange Seals →

Quick Decision Rules

Rule 1: If extrusion gap ÷ O-ring cross section > 0.15, add a backup ring regardless of pressure.

Rule 2: Dynamic pressure above 10 MPa (100 bar) — specify at least one PTFE backup ring.

Rule 3: Dynamic pressure above 35 MPa — dual backup rings and 90 Shore A minimum.

Rule 4: Elevated temperature softens elastomers — reduce allowable gap by one column or add a backup ring when continuous temperature exceeds 100°C.

Rule 5: Worn equipment — remeasure gap periodically; chrome wear on rods increases clearance over time.

Condition	Action
Gap unknown on legacy equipment	Measure ID/OD; assume worst case until verified
Pressure spikes > 1.5× continuous rating	Design for peak pressure, not average
Soft elastomer (60–70 Shore A) in high pressure	Increase hardness or add backup ring
Large CS (5.33–6.99 mm) in high pressure	Backup ring sized to match CS; scarf-cut PTFE typical

O-Ring Selection Checklist →Compression Calculator →

Frequently Asked Questions

What is the difference between extrusion gap and groove depth?

Groove depth is a designed dimension that controls squeeze. Extrusion gap is the unintended clearance between mating metal parts on the low-pressure side of the seal. Both affect sealing, but extrusion gap determines whether the O-ring is forced into the clearance under pressure.

Do backup rings eliminate the need to control extrusion gap?

No. Backup rings occupy the gap but require enough groove width to fit O-ring plus backup ring(s). Extremely large gaps can still extrude the backup ring material itself, especially under shock pressure. Target precision fits even with backup rings installed.

How does temperature affect extrusion resistance?

Elastomers soften at high temperature, which lowers effective hardness and increases extrusion tendency. At continuous temperatures above 100°C, use the next lower gap column or add a backup ring. FFKM and FKM high-temp grades maintain hardness better than NBR but still benefit from tighter gaps.

Can I use this chart for metric and imperial sizes?

Yes. The gap limits are in millimeters and apply to any standard cross section — AS568, ISO 3601, or custom. Convert inch clearances to mm (1 mm ≈ 0.039 in) before comparing to the table.

When should I switch from O-rings to a different seal type?

Consider U-cups, lip seals, or spring-energized PTFE when pressure exceeds 400 bar dynamic, speed exceeds 0.5 m/s with elastomer O-rings, or chemical compatibility excludes all elastomers. O-rings with backup rings remain the default for most hydraulic static and moderate dynamic service.

Not Sure If Your Gap Is Safe?

Send gland dimensions, pressure, and fluid. We recommend hardness, backup ring style, and cross section.

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