SAE AS5202 is the aerospace design standard that defines the geometry of an internal straight-thread port used in aircraft fluid power connections. Published by SAE International under the title “Port or Fitting End, Internal Straight Thread, Design Standard,” the current active revision is AS5202A, issued in January 2013.1SAE International. AS5202 Port or Fitting End, Internal Straight Thread, Design Standard It controls the receptacle side of the connection, specifying the threads, counterbore, seal seat, and chamfer that a compatible fitting end threads into and seals against.
What the Standard Covers
AS5202 governs port geometry only. The standard tells a manufacturer how to machine the hole, the threads, the counterbore, and the seal seat; it does not define the mating fitting. Fitting ends come from separate SAE aerospace standards such as AS930, AS1098, AS1941, AS4320, and AS4395, each of which is designed to mate into an AS5202 boss.2NASA. SSTD-8070-0126 B – Fluid Fittings, Tubes, and Connections
The ports show up wherever aircraft fluid systems need a reliable, reusable threaded connection: hydraulic lines serving flight controls, landing gear actuation, engine-driven pumps, and similar high-pressure plumbing. Operating pressures run from roughly 1,350 psig on the largest ports up to 8,000 psig on the smallest, which is why the port specification is treated as a safety-critical item rather than a general-purpose plumbing detail.2NASA. SSTD-8070-0126 B – Fluid Fittings, Tubes, and Connections
The standard is not freely available. It must be purchased from SAE or an authorized distributor, and anyone machining to it should verify they have the latest revision before programming, since dimensional updates between revisions can be subtle but consequential.
Thread Profile: UNJ, Not UN
AS5202 ports use UNJ threads defined by SAE AS8879. The defining feature of a UNJ thread is a controlled, continuously rounded radius at the root of the external thread, between 0.15011 and 0.18042 times the pitch, blended smoothly into the thread flanks with no sharp transitions. The minor diameters on both internal and external UNJ threads are also increased above standard UN values to accommodate that larger root radius.3SAE International. SAE AS8879B – Screw Threads, UNJ Profile, Inch
The reason is fatigue. Sharp cuts at the minor diameter create stress concentration points where cracks initiate. In cyclic-pressure aerospace service, a thread that passes a one-time pressure test can still fail after thousands of cycles if the root geometry allows crack initiation. UNJ eliminates the flat root and distributes stress more evenly.
Port sizes run from 1/4-28 UNJ at the smallest to 2-1/2-12 UNJ at the largest, covering nominal tube sizes from a quarter inch up to two and a half inches. Mid-range sizes like 3/4-16 and 7/8-14 appear in general hydraulic plumbing; the larger 1-7/8-12 and 2-1/4-12 sizes show up in high-flow return lines and actuator housings.
Seal Seat and Cavity Geometry
Below the threads, the port opens into a counterbore with a 45-degree angled seal seat at the bottom. This angled surface is where the O-ring actually seals. When the fitting is threaded in, the fitting’s geometry presses the O-ring against the seat. If the angle is off, the O-ring compresses unevenly and either extrudes under pressure or fails to seal.
Surface finish matters as much as dimensions. Aerospace hydraulic seal surfaces typically require a roughness average of 32 micro-inches Ra, with some configurations permitting up to 63 micro-inches. Too rough tears the O-ring on assembly or opens leak paths; too smooth and the O-ring will not grip the seat properly. The standard’s data tables give the exact relationship between thread size and cavity dimensions, including counterbore diameter, depth, and the entrance chamfer that protects the O-ring during fitting insertion. Deviations of a few thousandths of an inch in any of these dimensions can cause seal failure under operational pressure.
O-Rings and Other Seals
AS5202 ports are designed around O-rings sized to AS568, which defines inside diameters, cross-sections, tolerances, and dash number identification for aerospace O-rings. The 900-series dash numbers in AS568 specifically cover straight thread tube fitting boss gaskets, with the dash number typically indicating the tube size in sixteenths of an inch.4SAE International. AS568B – Aerospace Size Standard for O-Rings
The cavity has to provide a specific volume so the O-ring hits the right squeeze percentage when the fitting is torqued down. Too little cavity volume over-compresses the seal, accelerating wear and risking extrusion past the sealing surfaces. Too much volume leaves the O-ring without enough contact pressure to hold back hydraulic fluid. That balance is why AS5202 specifies the cavity dimensions tightly rather than leaving them to the fitting manufacturer. Some installations substitute alternative seals such as K-Seals in place of standard O-rings, which changes the allowable working pressure for the connection.2NASA. SSTD-8070-0126 B – Fluid Fittings, Tubes, and Connections
Pressure Ratings Depend on Size and Fitting
There is no single pressure rating for an AS5202 port. Maximum allowable working pressure depends on port size, which fitting standard mates into it, and whether the seal is a standard O-ring or a K-Seal equivalent. NASA’s fluid fittings technical standard publishes the detailed pressure tables for these combinations.2NASA. SSTD-8070-0126 B – Fluid Fittings, Tubes, and Connections
A sense of the range: a 1/4-inch port with an AS1098 or AS4395 fitting end and an AS28778 O-ring is rated to 8,000 psig. A 2-inch port using an AS930 fitting with a K-Seal drops to 1,350 psig. A 3/4-inch mid-range port typically falls somewhere between 2,650 and 7,300 psig depending on the configuration.2NASA. SSTD-8070-0126 B – Fluid Fittings, Tubes, and Connections System design has to match port size and fitting combination to the actual operating pressure with a real safety margin.
Relationship to MS33649 and J1926
AS5202 carries the same port geometry that was previously covered by military standard MS33649, and tooling manufacturers treat the two as equivalent. AS5202 port tools have formally replaced MS33649 port tools across the industry, so drawings calling out MS33649 can be worked to current AS5202 dimensions.
The industrial equivalent is SAE J1926-1, often called a straight thread O-ring boss (ORB) port. The two look similar and share the basic concept of a straight thread with an O-ring seal, but they are not interchangeable. AS5202 ports use a different seal seat profile, which changes O-ring compression, and the aerospace version requires Class 3 threads rather than the Class 2 threads found on J1926 fittings. Commercial J1926 fittings can physically thread into an aerospace port, and that is exactly the problem: the parts engage without sealing to the aerospace specification, which can compromise airworthiness in a certified system.
Machining an AS5202 Port
Production starts from the size number in the standard’s data tables, which fix every dimension of the finished cavity. Specialized porting tools, typically carbide-tipped reamers or form cutters ground to the exact profile of the seal cavity, produce the counterbore, seal seat angle, and chamfer in a single operation. The tool has to match AS5202 geometry precisely and be sized for the UNJ minor diameter.
Three-fluted tool designs are preferred over older two-flute configurations because they resist chatter, require less spindle horsepower, and produce better surface finishes. Coated carbide inserts help maintain edge sharpness through production runs on steel and other hard-to-machine port materials. CNC feed rates have to be programmed for the specific material to hit the required Ra 32 or Ra 63 surface finish on the seat without burning it or leaving tool marks in the transition zones. Aerospace-grade port blanks in aluminum, steel, or titanium are expensive, and a miscalculated setup means scrap.
Inspecting the Finished Port
Verification starts with thread gauging. GO and NO-GO plug gauges check whether the internal UNJ threads fall within limits, and those gauges have to carry a “J” designation confirming they comply with AS8879. Standard UN gauges are not valid for UNJ inspection because the minor diameter limits differ. A NO-GO gauge that engages more than three complete turns indicates oversized threads and the part fails.
The seal seat is measured with a profilometer against the specified Ra value. A failed reading usually means manual polishing, or in worse cases full re-machining. Visual inspection under magnification checks for burrs or irregularities in the transition between threads and seal cavity, because even a small burr there can nick an O-ring on fitting installation and leak as soon as the system pressurizes. For higher-volume production, air gauging provides non-contact measurement of bore diameter, taper, roundness, and the squareness of the seal seat to the thread axis, detecting variations as small as 0.003 inches.
Aerospace quality management requires that every measurement be recorded and that each part remain traceable from raw material through final inspection, including who machined it, what tools were used, and what the gauging results showed. Those records follow the port through the aircraft’s service life and become the reference point if a connection ever fails in the field.