Guide
What O-ring groove dimensions should a machined seal gland use?
O-ring groove dimensions are the diameter, width, depth, corner radii and surface finish of the machined cavity (the gland) that holds an O-ring and squeezes it against a mating surface. Together with the bore, rod or flange the ring seals against, they decide three numbers that make or break the seal: how much the ring is squeezed, how full the groove is, and how far the ring is stretched on assembly.
By Saola Manufacturing engineering team · Published · Updated
This guide is for designers who are drawing a seal groove on a turned or milled part and need it to work the first time it is pressurized. It explains how squeeze, fill and stretch interact, why static and dynamic glands are sized differently, what finish each surface needs and how to tolerance the groove so that the worst-case stack still seals. Ring sizes follow the SAE AS568 dash-number system; gland dimensions come from SAE AS4716 (static and dynamic), SAE AS5857 (static only) or, for metric hardware, ISO 3601-2. Only the public scopes of those documents are quoted here. Starting values are labeled as typical industry guidance, and the governing standard or the seal maker’s catalog should be the final word on the drawing.
On this page
How much squeeze should an O-ring gland give, static or dynamic?
Squeeze is the percentage by which the gland compresses the ring’s cross-section, and it is the first number to check on any groove. Static glands use more squeeze than dynamic ones, because a ring that moves needs lower friction and less heat buildup.
Squeeze is calculated from the gland depth, meaning the radial distance from the groove bottom to the mating surface, not the depth of the groove alone. For a piston groove, the gland depth is half of the bore diameter minus the groove diameter; for a rod groove, it is half of the groove diameter minus the rod diameter. Industry guidance puts typical static radial squeeze at roughly 15 to 30 percent of the cross-section, face seals typically a little higher, and reciprocating dynamic seals typically at roughly 10 to 20 percent. Pneumatic and rotary seals usually run at the low end of the dynamic range, because friction and heat dominate their failure modes.
The aerospace standards draw the same line. SAE AS4716 (scope of revision B) covers glands for static and dynamic service and sizes them to give enough squeeze to seal while limiting it for dynamic use. SAE AS5857 is the static-only companion; its scope says the glands give more squeeze than AS4716 for better sealing at low temperatures and low seal swell, and that they are not recommended for dynamic use. So the first decision on the drawing is which family the gland belongs to. Do not copy a static groove from an old part onto a piston that strokes, and do not use a dynamic groove on a cold static joint without checking the minimum squeeze.
Always calculate squeeze at both extremes of the stack: largest ring cross-section in the shallowest gland, and smallest cross-section in the deepest gland with the shaft pushed fully to one side. A groove that looks right at nominal can drop below a workable squeeze once tolerances and eccentricity are added, which is the most common reason a new seal weeps at low pressure.
Where do O-ring groove dimensions come from for each AS568 cross-section?
Start from the ring cross-section, then take gland depth and groove width from AS4716, AS5857 or ISO 3601-2 rather than deriving them from scratch. The table below gives typical starting values for a first layout, so a designer can see the size of the numbers before opening the standard.
The SAE AS568 standard specifies inside diameters, cross-sections, tolerances and dash numbers for O-rings, and the first digit of a -0xx to -4xx dash number identifies the cross-section family. Metric hardware is covered by ISO 3601-2, which gives housing dimensions for class A rings in general industrial service and class B rings on selected metric cylinder bores and piston rods. Note that ISO 3601 calls the groove plus its mating surface the housing, the same thing the SAE documents call the gland.
| Dash series | Nominal cross-section, in (mm) | Static radial gland depth, in | Reciprocating gland depth, in | Groove width, no backup, in |
|---|---|---|---|---|
| -0xx | 0.070 (1.78) | 0.050 to 0.052 | 0.055 to 0.057 | 0.093 to 0.098 |
| -1xx | 0.103 (2.62) | 0.081 to 0.083 | 0.088 to 0.090 | 0.140 to 0.145 |
| -2xx | 0.139 (3.53) | 0.111 to 0.113 | 0.121 to 0.123 | 0.187 to 0.192 |
| -3xx | 0.210 (5.33) | 0.170 to 0.173 | 0.185 to 0.188 | 0.281 to 0.286 |
| -4xx | 0.275 (6.99) | 0.226 to 0.229 | 0.237 to 0.240 | 0.375 to 0.380 |
Two patterns are worth noticing. The reciprocating column is deeper than the static column for every family, which is how lower dynamic squeeze is achieved. The groove width is the same for both, because width is set by fill and by room for the ring to move, not by squeeze. When one or two backup rings are added the groove gets wider, and the gland standards tabulate separate widths for zero, one and two backups. The published scopes of SAE AS4716B and SAE AS5857A describe glands without backup rings below 1500 psi (10.3 MPa) and with one or two backup rings above it.
How much gland fill and ring stretch is safe?
Keep the ring’s volume well below the groove volume so it can swell and expand with heat without being forced into the clearance gap. Keep installed stretch on the ring’s inside diameter small, because stretch thins the cross-section and quietly reduces squeeze.
Gland fill is the ring’s cross-sectional area divided by the area of the groove. Typical industry guidance is to stay at or below about 85 percent fill at worst case, meaning the largest ring in the smallest groove, with nominal designs typically around 70 to 80 percent. A groove that is overfilled at room temperature has nowhere to go when the ring swells in fluid or grows with temperature, so the rubber is extruded into the gap and nibbled away. This is why widening a groove is usually a better fix for a tight assembly than making it shallower.
Stretch matters most on piston and male face grooves, where the ring is rolled over a diameter larger than its own ID. Typical guidance keeps installed stretch in the range of about 1 to 5 percent; above that, the cross-section thins enough to cut squeeze noticeably and the ring ages faster. For rod glands and female face grooves the ring sits in compression on its outside diameter instead, and a little circumferential compression is acceptable as long as the ring does not buckle in the groove.
For face seals, choose the groove diameter by the direction of pressure. With internal pressure, size the groove outer diameter to the ring’s outside diameter so pressure pushes the ring against a wall it already touches. With external pressure or vacuum, size the groove inner diameter to the ring’s inside diameter. Getting this backwards lets the ring shuttle across the groove on every pressure cycle.
What surface finish do the groove and the sealing surface need?
The surface the ring slides on or seals against needs a finer finish than the groove itself, and dynamic surfaces need the finest of all. Gas and vacuum service also call for finer finishes than liquid service, because gas finds leak paths that oil would plug.
Roughness is a leak path and an abrasive at the same time. On a static groove the ring does not move, so a moderate finish is enough as long as there are no radial scratches across the sealing band. On a dynamic rod or bore the surface acts like a file on every stroke, so it needs to be fine; yet a mirror surface is not ideal either, since it holds too little lubricant. Lay direction matters too: circumferential lay from plunge-turning a groove bottom is harmless, while a radial tool drag or a spiral feed mark across a face seal land is a straight leak path. The values below are typical industry guidance; confirm them against the seal maker’s data for the compound you use.
| Surface | Liquid service, Ra max | Gas or vacuum service, Ra max |
|---|---|---|
| Static groove bottom and side walls | 63 µin (1.6 µm) | 32 µin (0.8 µm) |
| Static mating surface (bore, rod or flange land) | 32 µin (0.8 µm) | 16 µin (0.4 µm) |
| Dynamic groove bottom and side walls | 32 µin (0.8 µm) | 32 µin (0.8 µm) |
| Dynamic sliding surface (rod or cylinder bore) | 8 to 16 µin (0.2 to 0.4 µm) | 8 to 16 µin (0.2 to 0.4 µm) |
The practical consequence for sourcing is that a plain as-machined finish is often fine for a static groove but rarely fine for a dynamic sliding surface. Fine bore and rod finishes usually need a secondary operation such as honing, grinding, burnishing or roller burnishing, so call out the sliding surface separately from the groove and expect it to be quoted as its own operation. See the surface roughness guide for how Ra, RMS and N-grades relate.
Which tolerances and edge breaks belong on an O-ring groove callout?
Tolerance the groove diameter and the mating bore or rod as a pair, because squeeze depends on both. Then add the bottom corner radius, a small edge break on the groove top and a lead-in chamfer on any surface the ring has to slide past during assembly.
Gland standards express the groove as limits rather than a nominal with a bilateral tolerance, and the drawing should do the same. A width given as limits, such as 0.187 to 0.192 in, is typical industry guidance for a -2xx groove, and it lets the machinist aim for the middle without arguing about which way the tolerance falls. A typical radial callout on a piston groove reads: GROOVE DIA 1.776 TO 1.778, WIDTH 0.187 TO 0.192, BOTTOM RADII 0.005 TO 0.015, BREAK GROOVE EDGES 0.005 MAX, GROOVE SURFACES 32 MICROINCH RA MAX, FOR AS568 -2XX O-RING. Pair it with the bore it seals in, for example a bore of 2.000 to 2.002 in, and check that the resulting gland depth falls within the static or dynamic column of the governing table.
Common mistakes are predictable. Sharp groove edges cut the ring during assembly; a bottom radius that is too large reduces the effective gland volume; a threaded or cross-drilled port that crosses the sealing band shaves the ring. A bore or rod end that the ring has to slide past needs a lead-in chamfer, typically about 15 to 20 degrees per industry guidance and long enough to compress the ring before it reaches the edge; leaving it off is a common way to cut a ring on assembly. Coatings are easy to forget: anodizing and plating add thickness to groove walls and mating bores, so state whether the groove limits apply before or after finish, or mask the gland (see the anodizing dimensional change guide).
Concentricity matters on radial glands. If the groove is eccentric to the bore it seals in, squeeze is high on one side and low on the other, so a radial runout or position control between the groove diameter and the mating datum belongs on the drawing whenever the stack is tight. The concentricity, runout and position guide explains which control to use.
Can you machine an O-ring groove into a die-cast housing?
Yes, face seal grooves are routinely machined into cast housings, but the cut can open porosity that lay safely below the cast skin. Keep the sealing band out of thick sections and gate areas, and leak test the finished part.
A face groove sits right in the zone where a die casting is most likely to hide gas or shrinkage voids. A pore that breaks into the sealing band under the ring is a leak path no amount of squeeze will close. Tell the caster where the sealing band is so the die design keeps it away from the last metal to freeze, specify the porosity acceptance level in that zone, and require a leak test on the assembled or machined housing. The porosity after machining guide and the leak testing guide cover both decisions.
What should the drawing and RFQ say for a machined seal groove?
State the gland standard and service, the ring dash number and compound, the groove and mating limits, the finish on each sealing surface and the inspection you expect. Then mark the gland diameters as control dimensions so they are watched through the run, not just at first article.
- Gland family. Static or dynamic, radial or face, and whether the groove follows AS4716, AS5857 or ISO 3601-2.
- Ring. The AS568 dash number, compound and hardness, plus backup rings if used.
- Limits as a pair. Groove diameter and width together with the bore, rod or flange they seal against, with any runout or position control.
- Finish by surface. Separate callouts for groove walls, groove bottom and the sealing or sliding surface, with the lay direction where it matters.
- Coating. Whether limits apply before or after finish, and which gland surfaces stay bare.
Saola Manufacturing cuts grooves and chamfers routinely on the CNC turning lathes at its lead CNC plant, and the tolerance each groove can hold is confirmed in the quote. Bores, datum faces and pockets on die-cast housings are machined on the CNC milling line there as well, and the milling page asks buyers to call out a finer finish only where a gasket, seal or bearing actually lands. The standard as-machined finish listed on the turning and milling pages is coarser than the groove and sealing-surface targets in the table above, so put each of those finishes on the drawing. Our capability pages do not list honing, grinding or burnishing, so if your sliding surface needs a fine dynamic finish, raise it at the RFQ stage and it will be confirmed or declined in the written quote. Every toleranced dimension is measured on the first-article report, and dimensions you mark as process-capability control points are checked through the run (see quality and inspection). If an aluminum part is anodized, mark the gland surfaces that must stay bare. ISO 9001 is in progress, not held. Send the drawing through the quote form, and see the RFQ guide for the rest of the package.
Frequently asked
Can I use the same groove for a static and a dynamic seal?
Usually not. A static groove is shallower to give more squeeze, and that squeeze creates too much friction and wear in a moving seal. AS4716 glands are designed to work in both services; AS5857 glands are static only, per their scope.
Should the groove be on the piston or in the bore?
Where the design allows a choice, put the groove on the external part, because a groove on a piston or shaft can be turned and measured directly, while an internal groove in a bore needs special tooling and is harder to gauge, especially deep in a part. A reciprocating rod seal is the usual exception: the groove goes in the stationary housing so the ring does not travel with the rod.
When do I need a backup ring?
When pressure, clearance gap and ring hardness together would extrude the ring into the gap. The SAE gland standards tie backup rings to higher-pressure service and tabulate wider grooves for one or two backups, so widen the groove in the same revision that adds the backup.
Are dovetail grooves worth the cost?
Only when the ring must stay in place with the joint open, such as on a lid or a door that is opened often. A dovetail needs a form cutter and is harder to inspect than a rectangular groove, so use it where retention is a real requirement rather than by default.
Does an inch AS568 ring fit an ISO 3601 metric housing?
Sometimes the sizes are close, but the housing tables are built around different ring series, so close is not the same as correct. Pick the ring and the housing from the same standard, and check squeeze, fill and stretch if a substitution is unavoidable.
Sources
- SAE AS568F, Aerospace Size Standard for O-Rings (scope), SAE International, revised 2020, reaffirmed 2026
- SAE AS5857A, Gland Design, O-Ring and Other Elastomeric Seals, Static Applications (scope), SAE International, revised 2010, reaffirmed 2022
- SAE AS4716B, Gland Design, O-Ring and Other Elastomeric Seals (scope), SAE International, revised 2011 (historical revision, cited for its published scope)
- SAE AS4716D, Gland Design, O-Ring and Other Seals (scope), SAE International, revised 2021 (current revision)
- ISO 3601-2:2025, Fluid power systems, O-rings, Part 2: Housing dimensions for general applications (scope), ISO, 2025