An aircraft O-ring is a molded elastomer ring that seals fluid or gas between two parts. The FAA's Aviation Maintenance Technician Handbook describes O-ring packings as seals that prevent both internal and external leakage, seal in both directions, and are the most commonly used type of packing [1].
A seal is only as good as its match to the system. The same FAA handbook notes that O-ring compounds are made for specific operating conditions, temperatures, and fluids, and that an O-ring is useless if it is not compatible with the system fluid and operating temperature [1]. That's why they're bought by specification number, not just by size.
WHAT'S IN STOCK
OneMonroe Aerospaces's O-ring selection covers the common hydraulic, fuel, and high-temperature specs described below, plus MS28773, MS28774, and MS28777 backup rings. Mixed assortments come in O-ring kits.
COMMON O-RING SPECS AND WHAT THEY'RE FOR
The table below uses the descriptions published in two industry spec charts. Always confirm the spec your maintenance manual or drawing calls for.
MS28775 - Nitrile, 70 durometer. For MIL-H-5606 hydraulic fluid systems, -65°F to 275°F [2][3]. The FAA handbook calls the MS28775 series the standard for MIL-H-5606 systems in that temperature range [1].
MS28778 - Nitrile, 90 durometer. Boss gaskets (straight-thread fitting seals) for MIL-H-5606 systems, -65°F to 250°F [2][3].
MS29512 and MS29513 - Nitrile. For aircraft fuel systems with jet fuels, -65°F to 200°F. MS29512 covers boss gasket sizes [2][3].
AS3209 and AS3208 - Fluorocarbon (FKM), 75 durometer, made to AMS 7276 [3].
M83248/1 - Fluorocarbon, 75 durometer. High-temperature, fluid- and compression-set-resistant O-rings [2].
M83485/1 - Fluorocarbon (FKM), 75 durometer, made to AMS 7287 [3].
M83461/1 - Nitrile, 70 durometer. Used in MIL-H-5606 hydraulic fluid [2][3].
M25988/1 - Fluorosilicone, 70 durometer. Oil and fuel resistant, for low-pressure applications [2][3].
NAS1611 - Ethylene propylene (EP), 80 durometer, covering all sizes except tube fitting sizes [2].
Older AN numbers still show up on legacy drawings. The FAA handbook explains that hydraulic O-rings were first made under AN6227, AN6230, and AN6290 for MIL-H-5606 fluid at -65°F to 160°F, and that the newer MS28775 compound made the older series obsolete [1]. Don't assume any substitution without checking your approved data.
BOSS GASKETS VS. STANDARD O-RINGS
A boss gasket is an O-ring sized to seal a straight-thread fitting against a port (the "boss"). The FAA handbook notes that a colored stripe around the circumference of an O-ring marks it as a boss gasket seal [1]. MS28778 and MS29512 are both boss gasket specs [2]. If you're working on the fittings themselves, see how AN and MS tube fittings are tightened.
BACKUP RINGS
A backup ring is a stiff ring installed next to an O-ring to keep it from extruding into the clearance gap under pressure.
When they're used: The FAA handbook says backup rings are used with O-rings in installations above 1,500 psi to prevent extrusion [1].
How many: When an O-ring sees pressure from both sides, as in an actuating cylinder, two backup rings are required, one on each side. With pressure on one side only, a single backup ring is generally used [1].
Which side: A single backup ring always goes on the side of the O-ring away from the pressure [1]. Parker's O-Ring Handbook gives the same order: pressure, then O-ring, then backup ring [4].
Monroe carries MS28773 and MS28774 PTFE backup rings (MS28774 is a single-turn PTFE ring) and MS28777 leather backup rings [2].
WHY COLOR CODES AREN'T ENOUGH
Color markings help, but the FAA warns they are not a reliable or complete means of identification [1]. According to the handbook:
- O-rings compatible with MIL-H-5606 always contain blue, but may contain other colors [1].
- Seals for Skydrol always carry a green stripe [1].
- O-rings for hydrocarbon fluids always contain red and never blue [1].
- Color codes don't identify size, age, or temperature limits, and on some rings they aren't permanent [1].
The most reliable way to identify an O-ring is the basic part number on its sealed envelope [1]. Keep each ring in its original packaging until installation.
INSPECTING O-RINGS BEFORE INSTALLATION
Small surface flaws can cause leaks under the changing pressures of aircraft systems. The FAA handbook notes that one manufacturer recommends inspecting each ring with a 4-power magnifier and good lighting, and rolling the ring on an inspection cone or dowel to check the inner surface for cracks, foreign particles, or other defects [1]. Reject any O-ring with flaws that will affect performance [1].
INSTALLATION TIPS
These general practices come from Parker's O-Ring Handbook [4]. Your aircraft maintenance manual takes priority.
- Keep it clean. Dirty glands lead to leaks, and any cleaning material or assembly grease must be compatible with the elastomer [4].
-Lubricate. Parker recommends lubricant as an important installation step, and the lubricant must be compatible with both the O-ring and the system fluid [4].
- Protect from sharp edges. Don't pull an O-ring over sharp edges or threads. Cover threads, splines, and slots, or use fitting sleeves [4].
- Don't overstretch. O-rings and backup rings shouldn't be stretched more than 50% of their inside diameter during assembly [4].
- Avoid rolling or twisting. A ring that rolls during assembly can suffer spiral failure or leak [4].
- Check the size before installing [4].
Not every joint uses a molded ring. Flat gaskets, grommets, and conical seals are covered in a separate category.
Part of the OneMonroe Aerospace Guides library. This guide is one of a growing collection of technical resources on aircraft fasteners and aviation hardware. It covers AN, MS, and NAS specifications, part-number coding, materials, and installation best practices. Browse all Monroe Aerospace Guides to find related topics, or contact our sales engineers for help choosing the right part for your application.
References:
[1] FAA, Aviation Maintenance Technician Handbook - General, Chapter 5 (Seals, O-Ring Packings), via AvStop: http://avstop.com/ac/Aviation_Maintenance_Technician_Handbook_General/5-36.html
[2] Orings USA, Military Specification Chart: https://www.oringsusa.com/MS_specs.pdf
[3] Seal & Design, Mil-Spec O-Rings: https://www.sealanddesign.com/products/o-rings/specialized-compounds/mil-spec-o-rings/
[4] Parker Hannifin, O-Ring Handbook (PTD5705): https://www.parker.com/content/dam/Parker-com/Literature/Praedifa/Catalogs/Catalog_O-Ring-Handbook_PTD5705-EN.pdf