Extrusion Gap and Other Groove Details — Designing O-Ring Grooves That Do Not Fail Under Pressure

Up to this point in the series we have been protecting the O-ring against itself — wrong ratio, wrong stretch, wrong fill. Now we look at the failure mode that comes from outside the ring: high pressure pushing the elastomer into the clearance gap between the bore and the piston or rod. This is called extrusion, and it is the typical end-of-life failure for O-rings in hydraulic service above ~750 psi.

This post also rounds out the groove design with the details that the previous chapters quietly assumed: wall angle, edge break, surface finish, groove radii, and installation chamfers.

What Is Extrusion?

Imagine a piston in a bore with the O-ring sitting in a groove on the piston. There is a small radial clearance between the piston OD and the bore ID — the extrusion gap. Under pressure, the O-ring is forced outward toward that gap. If the ring is soft, the pressure is high, or the gap is wide, the elastomer is literally squeezed into the clearance like toothpaste out of a tube.

Once a piece of O-ring protrudes past the gland land, the next pressure cycle shears it off. The seal starts to leak. The debris (called "nibbles") may also score the bore wall.

Extrusion is mainly a concern for radial seals (piston- and rod-type). For face-type seals where the mating faces are line-to-line metal-to-metal contact, the gap that the ring can extrude into simply does not exist.

The Concentricity Trap

Even if your average bore-to-rod clearance looks small, real hardware is not perfectly concentric. Unless something positively aligns the parts, you have to assume the entire available clearance shifts to one side:

Extrusion Gap (piston-type) = Bore – Piston
Extrusion Gap (rod-type)    = Bore – Rod

That is the gap the O-ring must withstand. Half that gap centered around the O-ring would be friendlier to sealing; all of it stacked to one side is the realistic worst case.

Recommended Extrusion Gap — Inches and Millimetres

The following pressure-vs-hardness matrix gives the recommended maximum gap:

Two takeaways from this table:

• Harder O-rings resist extrusion better. A jump from 70 to 90 durometer roughly doubles the allowable gap at any given pressure. Softer compounds (60 durometer) are already unusable above ~1250 psi.
• At the top end, 60 durometer is not enough. Even 90 durometer is starting to get tight at 1500 psi. For pressures above 1500 psi, contact your seal manufacturer for guidance — you will likely need a back-up ring.

In metric units. Same matrix, column by column: .25 / .38 / .51 / .64 mm at 500 psi; .05 / .20 / .30 / .46 mm at 1000 psi; etc.

Other Groove Details That Matter

Even with great compression and gland fill, a poorly machined groove ruins the seal. The remaining design points to get right:

Wall Angle, Edge Break, Surface Finish

The groove walls, edges, and bottom radius must be free of nicks, scratches, and burrs — every one of those is a leak path, and most of them are invisible to the eye.

Practical note. "Static sealing surface finish 32 µin max" sounds abstract, but it corresponds roughly to a honed hydraulic surface. If your part is being milled, you will need a secondary finishing pass.

Groove Corner Radius by AS568 Series

A generous radius is easier to machine on larger cross-section O-rings. Sticking to the range matters: too tight a corner cracks the O-ring at the bottom of the gland; too large a radius leaves the ring unsupported at the wall.

Installation Aids — Don't Let Assembly Kill the Seal

A perfectly designed O-ring seal is worthless if the O-ring is damaged during installation. The single most common cause of new-assembly leaks is a nick from a sharp gland entrance.

15° Installation Chamfer (Piston and Rod Seals)

For both piston- and rod-type seals, a 15° chamfer on the bore or rod entry is recommended. The chamfer has to be long enough that the O-ring only ever sees the chamfered portion of the hardware during assembly — never a square edge.

Tip. A short chamfer is the difference between an O-ring that slides in and an O-ring that gets a half-moon bite out of its ID.

Face-Type Seals Need No Extra Chamfer

For face-type seals, the mating faces are designed line-to-line and the O-ring sits in a flat groove. The standard groove detail recommendations above are sufficient — no additional chamfer is required.

Pre-Production Checklist

Before you cut the first chip, run this list:

• Pressure × durometer extrusion gap inside the matrix in this post.
• Wall angle within 0°–5°.
• Break edges present at every gland corner.
• Sealing surfaces finished to ≤32 µin (0.8 µm).
• Containing surfaces finished to ≤64 µin (1.6 µm).
• Groove radii inside the per-series range.
• Installation chamfer present, length per the table above.
• Machining inspected for nicks, scratches, and burrs.

What's Next

The hardware side of the design is now complete. Part 6 switches to the other side of the seal: the elastomer itself. We compare the eight commonly used O-ring compounds — FFKM, ETP, TFE/P, FKM, EPDM, NBR, VMQ, FVMQ — across temperature range, physical properties, and chemical compatibility, with guidance on which to reach for first.

Continue with Part 6 — Elastomer Materials for O-Rings.

Series Navigation

1. What Is an O-Ring? A Practical Introduction to Static Sealing
2. Gland Dimension Calculations
3. ID Stretch and OD Interference
4. Compression Squeeze, Compression Ratio, and Gland Fill
5. Extrusion Gap and Other Groove Details
6. Elastomer Materials for O-Rings
7. Six Sigma Quality and Process Capability