Start from the joint, not from the seal
When a joint leaks, the first instinct is to torque the bolts harder or order a softer seal. That rarely works, because the leak is almost always a mismatch between the seal type and the joint it sits in. Three families cover nearly all machined joints, and each one asks something different from the part you are machining.
- An elastomeric O-ring in a machined groove — compact, high pressure, needs a controlled groove
- A compression gasket on a flat face — forgives imperfect surfaces, low to medium pressure, large joints
- A metal face seal on a lapped land — for vacuum, aggressive media and high temperature
O-ring in a machined groove
This is the default for fluid power, valves, manifolds and instrument fittings. It handles high pressure in a small cross-section, it is cheap to replace, and it tolerates repeated assembly. The price is that the groove has to be right: width and depth matched to the cord, a controlled squeeze, a fine finish on the sealing surfaces and no burr anywhere near the groove.
Groove dimensions follow ISO 3601-2 or the AS568 dash-number tables, and the squeeze target depends on the application — 20-30% for a static face seal, 15-25% for a static radial seal, 10-15% where the seal has to move. The full dimensional breakdown is in our guide to O-ring groove dimensions.
Compression gasket against a flat face
Gaskets carry the joints that O-rings cannot: large covers, gearbox lids, tanks, pump housings, HVAC casings and anything with an irregular outline. The gasket conforms to the surfaces and spreads load across many bolts, so it forgives finish and flatness the way an O-ring never will.
What it does not forgive is creep. A gasket relaxes under load, so bolt preload has to be maintained (or the design has to allow re-torque), and the failure mode is a slow weep that appears weeks later rather than an instant leak. The joint face still needs to be flat and burr-free, but it does not need the tight groove tolerance an O-ring gland needs.
Metal face seal on a lapped land
When elastomers are out — UHV vacuum, aggressive chemicals, temperatures that would destroy rubber — the joint becomes metal on metal: a knife edge biting into a copper gasket on a CF flange, a lens ring, or a metal C-ring in a machined groove. Nothing outgasses and nothing degrades with heat.
The machining demand goes up accordingly: flatness in the hundredths of a millimetre, surface roughness in the low single-digit micrometres or better, and often a lapping operation after the finish cut. Bolt patterns and bolt-hole position matter here too, because the load has to be even for the knife edge to bite uniformly.
The three families side by side
| O-ring groove | Compression gasket | Face seal / knife edge | |
|---|---|---|---|
| Pressure | High, small cross-section | Low to medium | High, including vacuum |
| Temperature | Limited by the elastomer | Wide, depends on gasket material | Metal-to-metal, no elastomer limit |
| Surface demand | Controlled groove, fine finish, no burrs | Flat and clean, forgiving | Flatness and Ra in the low µm, often lapped |
| Assembly | Repeatable, easy to replace | Re-torque may be needed as it creeps | Usually single use, high bolt preload |
| Where it wins | Valves, manifolds, ports, fittings | Covers, lids, housings, tanks | Vacuum chambers, CF flanges, hot or aggressive media |
What to put on the drawing
- Name the seal standard and the dash number, not just the cord diameter
- State the groove callout with its standard: ISO 3601-2, AS568 or the seal supplier's gland drawing
- Give the sealing surface finish as Ra, and flatness for face seals
- Add a no-burr requirement at every cross-hole that intersects the groove or the sealing face
- Note whether the joint will be disassembled in the field — that decides whether an O-ring or a gasket is the right answer
- For face seals, specify the bolt pattern and whether the land is lapped after machining
Where machining tolerances actually bite
The diameter tolerance of a groove is rarely what kills a seal. The two things that do are the finish of the groove base and a burr where a cross-hole breaks into the groove — a chatter-marked groove base gives the elastomer nowhere to sit, and one burr nicks the O-ring every time the joint is assembled. On face seals it is the opposite: flatness and surface roughness dominate, and a part that is dimensionally perfect but 0.05 mm out of flat will not hold vacuum.
TruPart Precision is an ISO 9001:2015 shop in Dongguan. We cut O-ring and face seal grooves to ISO 3601-2 and AS568 callouts, hold sealing-face flatness and Ra with finishing passes and deburring, and measure critical features on the CMM with the report supplied per batch.
Frequently Asked Questions
When should I use a gasket instead of an O-ring?
When the joint is large or irregular, the pressure is low to medium, and the surfaces are not going to hold a tight gland tolerance. Gaskets conform and spread load across many bolts. O-rings win when the joint is compact and the pressure is high.
What surface finish does an O-ring groove need?
Typically Ra 0.8-1.6 µm on the groove base and sealing surfaces, finer for dynamic seals. The number matters less than the absence of chatter marks and scratches that run across the sealing direction.
What is the difference between a face seal and a gasket?
A gasket is a compressible sheet that conforms to two imperfect surfaces. A face seal is metal on metal — a knife edge or a machined land biting into a soft metal gasket — and it is used when nothing elastomeric or polymeric can survive the vacuum, the chemistry or the temperature.
Can you machine an O-ring groove to ISO 3601-2 or AS568?
Yes. Give the dash number and the standard, or send the seal supplier's gland drawing with the RFQ. If you only send the cord diameter, the groove depth and width are guesses, and a guess is where leaks come from.
Why does my O-ring leak right after assembly?
Usually a burr at a cross-hole that nicked the ring during assembly, or a groove base with chatter that gives the elastomer nowhere to seat. Check the finish and deburr before you change the seal material.