AS568-257 B70 (NBR) Buna-N Nitrile 70 Duro O-Ring - Mil-Spec MS28775 - Freudenberg N888 Black

Dimensions: 5-7/8" ID x 6-1/8" OD x 1/8" CS

The AS568-257 is a B70 (NBR) Buna-N Nitrile O-Ring molded from Freudenberg N888, a specialized aerospace-grade nitrile compound, in a 70 Shore A durometer and black color. It measures 5-7/8" ID x 6-1/8" OD x 1/8" CS. This size is built to Mil-Spec MS28775 and cross-references AS28775 and MS28784. It is stocked and shipped same day by The O-Ring Store LLC, an ISO 9001:2015 certified distributor.

Seal aerospace and hydraulic systems that a standard O-Ring cannot handle. The AS568-257 Freudenberg N888 O-Ring holds a tight seal from -65°F all the way to +250°F and stands up to petroleum hydraulic fluids day after day. That makes it a dependable pick for aircraft hydraulics, shock absorbers, struts and heavy-duty industrial circuits, and a solid replacement when a lesser seal swells, cracks or takes a set.

Operating Temperature: -65°F to +250°F (-54°C to +121°C)

Material and Performance Details

Freudenberg N888 is a specialized NBR (Nitrile Butadiene Rubber) developed for the aerospace industry. It is designed for hydraulic systems that meet MIL-G-5514 and SAE AS5440, and it brings the tensile strength, abrasion resistance and low-temperature flexibility those jobs demand. The 70 Shore A durometer gives good squeeze and a firm seal without going brittle in the cold. Every one of these O-Rings is black nitrile built to the same aerospace standard, so you get the same part every time.

• Excellent resistance to: petroleum-based hydraulic fluids including MIL-PRF-5606, MIL-PRF-87257 and MIL-PRF-83282, plus petroleum oils, fuels and greases
• Standard nitrile cautions: keep away from ozone, sunlight and long weathering, ketones such as MEK and acetone, and glycol brake fluids

Compliance and Mil-Spec

This is an aerospace-grade seal, built and documented to the standards the defense and aviation world runs on. It is compliant with AS28775, MS28775, MS28784 and is designed for hydraulic systems meeting MIL-G-5514 and SAE AS5440. When your print calls out an MS28775 packing, this is the part that fills it.

Fluid Compatibility

Freudenberg N888 is built to seal petroleum-based hydraulic fluids, including MIL-PRF-5606, MIL-PRF-87257 and MIL-PRF-83282. That is exactly the service you find in aircraft and industrial hydraulic systems, so this O-Ring is right at home in harsh hydraulic environments.

Sizes and Measurements

• Nominal: 5-7/8" ID x 6-1/8" OD x 1/8" CS
• Actual: 5.859" ID x 6.137" OD x 0.139" CS
• Metric: 148.82mm ID x 155.88mm OD x 3.53mm CS
• Part Number: B70257-N888
• AS568 Dash Number: -257
• Cross-reference: Parker 2-257 (3.53mm CS / 0.139" CS)

Applications and Use Cases

The AS568-257 Freudenberg N888 O-Ring earns its keep in aerospace and heavy hydraulic work. You will find this compound in aircraft hydraulic systems run to military standards, in shock absorbers and struts where the seal sees high pressure and a lot of movement, and in industrial hydraulic systems that demand strong resistance to hydraulic fluid. It also holds up in automotive and marine service on fuel systems, oil seals and hydraulic circuits.

• Aerospace hydraulic systems built to military standards
• Aircraft shock absorbers and landing gear struts
• High-pressure, high-movement dynamic seals
• Industrial and heavy-duty hydraulic systems
• Automotive fuel systems and oil seals
• Marine hydraulic circuits and fittings
• MS28775 packing replacements on existing equipment

Key Specifications

• Material: Buna-N (Nitrile), Freudenberg N888
• Durometer: 70 Shore A
• Color: Black
• Compliance: AS28775, MS28775, MS28784
• Hydraulic Standards: MIL-G-5514 and SAE AS5440
• Fluid Compatibility: MIL-PRF-5606, MIL-PRF-87257 and MIL-PRF-83282
• Temperature Range: -65°F to +250°F (-54°C to +121°C)
• AS568 Dash Number: -257
• Size (nominal): 5-7/8" ID x 6-1/8" OD x 1/8" CS

Why Buy from The O-Ring Store LLC

The O-Ring Store LLC is an ISO 9001:2015 certified distributor with aerospace experience and USA stock for fast delivery. Most in-stock items ship the same day. We handle custom sizes and bulk orders, and we can send full documentation and lot traceability when you need it. Freudenberg is a trusted name in aerospace-grade elastomers, and every order gets checked before it leaves the building. Fair pricing, real people and no runaround. We make getting O-Rings easy!

Frequently Asked Questions

Q: What are the exact dimensions of the AS568-257 O-Ring?
A: The AS568-257 has an actual inside diameter of 5.859 inches (148.82mm), an outside diameter of 6.137 inches (155.88mm) and a cross-section of 0.139 inches (3.53mm).

Q: What hydraulic fluids does Freudenberg N888 handle?
A: N888 is built for petroleum-based hydraulic fluids, including MIL-PRF-5606, MIL-PRF-87257 and MIL-PRF-83282, which is why it is used in aerospace and industrial hydraulic systems.

Q: Is this O-Ring Mil-Spec?
A: Yes. It is an aerospace-grade B70 nitrile built to AS28775, MS28775, MS28784 and designed for hydraulic systems meeting MIL-G-5514 and SAE AS5440.

Q: What is the Parker equivalent of the AS568-257?
A: The Parker cross-reference is the Parker 2-257, which shares the same AS568 dash size.