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⭕ Aircraft O-Ring / Seal Reference
Quick reference for AS568 dash sizes, common aerospace seal materials, and installation notes. Use it to identify standard O-ring sizes fast, then verify the exact material and part number in the approved IPC, AMM, or parts catalog.
Use This Page
This page is a general quick-reference aid only. It does not reproduce or replace approved maintenance data. Dash number and size alone are not enough for airworthy substitution. Match size, material, hardness, fluid compatibility, and temperature rating before installation.
Dash-Series Cross Section
Most standard inch-series O-rings follow AS568 grouping by cross section.
- -0xx / -1xx = 0.070 in cross section
- -2xx = 0.103 in cross section
- -3xx = 0.139 in cross section
- -4xx = 0.210 in cross section
Typical Aerospace Specs
Common designations you may see in aircraft parts lists.
- AS568 dimensional dash-number standard
- MS28775 legacy dash-numbered O-ring family used in many IPCs
- AS3209 fluorocarbon O-rings for fuel and oil system service in many applications
- Backup rings may be required in high-pressure dynamic or gland-clearance applications
Before You Install
A good seal can still fail if the gland or handling is wrong.
- Inspect gland for scratches, corrosion, and contamination
- Confirm lubricant is approved for the seal material and fluid system
- Do not roll or twist the ring during installation
- Replace any seal with cuts, flat spots, swelling, or age cracking
Common AS568 Dash Size Reference
| Dash # | ID (in) | CS (in) | OD (in) | Series | Typical Aviation Use |
|---|---|---|---|---|---|
| -006 | 0.114 | 0.070 | 0.254 | 0xx | Small fittings, caps, low-flow pneumatic or instrument connections |
| -007 | 0.239 | 0.070 | 0.379 | 0xx | Drain fittings, small plugs, gauge adapters |
| -008 | 0.364 | 0.070 | 0.504 | 0xx | Hydraulic test fittings, fuel sampling plugs, caps |
| -010 | 0.489 | 0.070 | 0.629 | 0xx | Quick-drain style components, small valve bodies |
| -011 | 0.614 | 0.070 | 0.754 | 0xx | Sensor ports, line fittings, filter bowl caps |
| -012 | 0.739 | 0.070 | 0.879 | 0xx | Common fitting size for service caps and plugs |
| -014 | 0.864 | 0.070 | 1.004 | 0xx | Small actuator or servo covers, cap seals |
| -016 | 0.989 | 0.070 | 1.129 | 0xx | Filter housings, static seals in light accessory assemblies |
| -111 | 0.424 | 0.070 | 0.564 | 1xx | Common gland size in pumps, regulators, and small manifolds |
| -116 | 0.799 | 0.070 | 0.939 | 1xx | Fuel and oil system caps, quick-disconnect couplings |
| -214 | 0.859 | 0.103 | 1.065 | 2xx | Larger boss fittings, hydraulic manifolds, valve covers |
| -215 | 0.984 | 0.103 | 1.190 | 2xx | Pressure port covers and static hydraulic seals |
| -218 | 1.359 | 0.103 | 1.565 | 2xx | Reservoir caps, filter heads, medium gland diameters |
| -222 | 1.859 | 0.103 | 2.065 | 2xx | Large caps, pump covers, access plugs |
| -325 | 1.734 | 0.139 | 2.012 | 3xx | Heavier static seals on larger housings and covers |
| -327 | 1.984 | 0.139 | 2.262 | 3xx | Common large-body static seal size in accessory housings |
| -326 | 1.859 | 0.139 | 2.137 | 3xx | Pump end covers, sumps, and larger static joints |
| -345 | 4.109 | 0.139 | 4.387 | 3xx | Large covers, inspection plates, and tank sender flanges |
| -428 | 3.734 | 0.210 | 4.154 | 4xx | Large low-speed static seals and heavy cover plates |
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Material / Fluid Compatibility Guide
| Material | Common Spec Family | Good With | Avoid / Watch | Typical Use Notes |
|---|---|---|---|---|
| Nitrile / Buna-N | General-purpose fuel/oil elastomer | Petroleum oils, grease, many hydraulic fluids | High heat, ozone, phosphate ester fluids unless specifically approved | Common on legacy systems and shop assortments. Good low-cost service material when allowed by the IPC. |
| FKM / Viton | AS3209 family in many aircraft listings | Jet fuel, avgas, engine oil, many synthetic lubricants, elevated temperatures | Very low-temperature service can reduce elasticity | Widely used in fuel and oil systems where heat resistance matters. |
| Fluorosilicone | Fuel-resistant low-temp elastomer | Fuel exposure with better cold flexibility than FKM | Lower tear and abrasion resistance than FKM | Used where fuel resistance and cold start flexibility are both required. |
| Silicone | High/low temperature seal family | Air systems, water systems, high-temp static applications | Petroleum fuels and dynamic abrasion-heavy applications | Good temperature range, but not a universal aircraft fuel-system replacement. |
| EPDM | Skydrol-compatible family | Phosphate ester hydraulic fluids, water, steam | Petroleum oils and hydrocarbon fuels | Frequently used in Skydrol systems. Do not mix with fuel/oil system stock. |
| PTFE Backup Ring | Anti-extrusion support ring | High pressure glands and wider clearances | Incorrect orientation or omission where required | Not an O-ring by itself. Supports the elastomer seal in pressure-loaded applications. |
Seal Inspection Rejects
- Cuts, nicks, or spiral damage from removal tools
- Flat spotting from compression set
- Swelling, softening, or tackiness from chemical attack
- Brittleness, cracking, or chalking from age and heat
- Embedded metal chips, lint, or hardened grease
Installation Reminders
- Use plastic picks or approved seal tools where possible
- Deburr threads and sharp edges before passing a seal over them
- Lubricate lightly with the system-approved lubricant only
- Install backup rings exactly as shown in the component manual
- After assembly, check for twist and proper gland seating