NAS 1638 & SAE AS4059 Cleanliness Codes
Table of Contents
What is NAS 1638?
NAS 1638 is a fluid cleanliness classification standard originally developed in the 1960s by the National Aerospace Standards Committee for the U.S. aerospace industry. It became the de facto standard across American industrial and hydraulic components. NAS 1638 classifies contamination by counting particles across five size ranges, 5–15 µm, 15–25 µm, 25–50 µm, 50–100 µm, and >100 µm, and assigning a class number from 00 (cleanest) through 12 (dirtiest) based on the worst-performing size range in the sample.
Why NAS 1638 Still Matters (Even Though it’s “Obsolete”)
NAS 1638 was formally marked “inactive for new designs” after May 2001 and was replaced by SAE AS4059. In practice, it is still widely specified in legacy equipment documentation, older OEM manuals, and by maintenance teams and regions that never fully transitioned. If your equipment specification sheet, service manual, or client’s maintenance program references a NAS class, you need to be able to work with it, even while newer systems use SAE AS4059 or ISO 4406.
To learn more about the International Organization for Standardization (ISO), visit ISO 4406 Cleanliness Code Explained.
Officially retired in 2001 — still written into thousands of maintenance manuals
How NAS 1638 Classes are Structured
Each NAS 1638 class permits double the particle count of the class below it, per size range, per 100 milliliters of fluid. Classes run from 00 (extremely clean, aerospace-grade) to 12 (heavily contaminated). Most industrial hydraulic systems target somewhere in the Class 6–9 range, depending on component sensitivity; servo valves and high-pressure pumps require much cleaner fluid than general-purpose hydraulics.
NAS 1638 Contamination Classes
| NAS 1638 Contamination Classification System | ||||||
| Size Range | 5–15 µm | 15–25 µm | 25–50 µm | 50–100 µm | >100 µm | |
| NAS 1638 Classes (based on maximum contamination limits, particles per 100 mL in specified size range (µm) | 00 | 125 | 22 | 4 | 1 | 0 |
| 0 | 250 | 44 | 8 | 2 | 0 | |
| 1 | 500 | 89 | 16 | 3 | 1 | |
| 2 | 1,000 | 178 | 32 | 6 | 1 | |
| 3 | 2,000 | 356 | 63 | 11 | 2 | |
| 4 | 4,000 | 712 | 126 | 22 | 4 | |
| 5 | 8,000 | 1,425 | 253 | 45 | 8 | |
| 6 | 16,000 | 2,850 | 506 | 90 | 16 | |
| 7 | 32,000 | 5,700 | 1,012 | 180 | 32 | |
| 8 | 64,000 | 11,400 | 2,025 | 360 | 64 | |
| 9 | 128,000 | 22,800 | 4,050 | 720 | 128 | |
| 10 | 256,000 | 45,600 | 8,100 | 1,440 | 256 | |
| 11 | 512,000 | 91,200 | 16,200 | 2,880 | 512 | |
| 12 | 1,024,000 | 182,400 | 32,400 | 5,760 | 1,024 |
What Replaced NAS 1638: SAE AS4059
The legacy NAS 1638 fluid cleanliness standard was officially declared inactive for new designs in May 2001 and was replaced by SAE AS4059. NAS 1638 became outdated due to changes in modern particle counting technology.
SAE AS4059 was developed by the SAE A6 Aerospace panel specifically to correct NAS 1638’s shortcomings, most notably its reliance on the discontinued ACFTD (Air Cleaner Fine Test Dust) calibration standard and its lack of guidance on practical application. AS4059 uses the modern ISO Medium Test Dust (ISO-MTD) calibration and six particle size ranges instead of five.
The standard that fixed NAS 1638’s biggest blind spot
How SAE AS4059 Classes Work
SAE AS4059 replaces the obsolete NAS 1638 standard by incorporating modern automatic particle counters (APCs) calibrated to ISO 11171 using ISO Medium Test Dust (ISO MTD), while still supporting legacy optical microscopy methods. It measures and grades particulate contamination levels in hydraulic fluids using differential counts, cumulative counts, and size categories (A-F).
When using automatic particle counters (ISO 11171 calibrated with ISO MTD), particle size ranges are designated by letters A through F (representing particles ≥ 4 µm(c), ≥ 6 µm(c), ≥ 14 µm(c), ≥ 21 µm(c), ≥ 38 µm(c), and ≥ 70 µm(c)). Cumulative APC cleanliness classes range from 000 through 12, while differential APC cleanliness classes range from 00 through 12. Optical microscopy uses its own specified particle size ranges and corresponding cleanliness classes defined separately in SAE AS4059.
The standard offers two reporting methods. The Specific Cleanliness Code reports the cleanliness class individually for each size range analyzed (for example, 00A/0B/1C/2D/2E/3F), providing reliability engineers visibility into particle size distribution across the full spectrum. The Maximum Cleanliness Class, by contrast, simplifies reporting by assigning a single class number corresponding to the worst (highest) class measured across all required size ranges, trading some granularity for a quicker, single-number read on overall fluid cleanliness.
Knowing your target class is easy. Hitting it is the hard part!
Achieving Your Target Oil Filtration Classification
Knowing your required NAS or AS4059 class is only half the equation; the other half is choosing an oil filtration solution that can actually hit and hold it under real operating conditions. This comes down to three approaches, covered in full detail in our oil filtration guide:
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- Full-flow Filtration: Every drop of oil passes through the filter before reaching the component; the standard baseline for most hydraulic oil and lube oil systems.
- Offline / Kidney-loop Filtration: A secondary, continuous-recirculation loop that polishes the fluid independent of the main oil system, often the most effective way to reach tighter classes (low NAS or SAE AS4059 numbers) on large reservoirs. If you only need to remove solid contaminants, upgrade your hydraulic system with an IOW Group kidney loop centrifugal filter.
- Centrifugal Separation: Uses centrifugal force rather than filter media to remove particulate contamination and free water; particularly effective for high-viscosity fluids and applications that generate fine soot or sludge.
Ready to Find the Right Filtration Solution? Generate Your RFQ Today.
Estimate your annual savings and return on investment with the Separator Spares & Equipment Oil Filtration ROI Calculator. Input your operating parameters below to see how extending oil change intervals can directly lower your equipment’s total cost of ownership.
Which approach, or combination, makes sense depends on your hydraulic fluids or lube oil system’s contamination sources, fluid volume, and how tight a class your most sensitive components (servo valves, piston pumps) require. If you are not sure which hydraulic oil filtration or lube oil filtration setup will meet your target oil cleanliness class, contact the Separator Spares & Equipment Oil Filtration team to help you spec the right system, or browse our oil filtration equipment and spare parts directly.
Matching Oil Filtration Methods to Target Class