Dash Number 251 O-Rings | McMaster-Carr

Made of Buna-N, these O-rings resist grease, hydraulic oil, and motor oil—plus mild chemicals and water. However, Buna-N is not for use with brake fluid. These O-rings are commonly used to create multidirectional seals between two parts in power transmission assemblies, hydraulic cylinders, and pneumatic cylinders. They resist compression and tearing to keep their shape over time. All have shelf life, a cure date, and a traceable lot number printed on the package.

Inch

		Fractional										Pkg.
	Dash No.	ID	OD	ID	OD	Hardness	Specs. Met	Temp. Range, ° F	Color	Pkg. Qty.		1-9	10-49	50-Up
1/8 Fractional Width (0.139" Wide)



		251	5 1/8	5 3/8	5.109"	5.387"	Durometer 70A (Medium)	ASTM D2000, SAE AS568, SAE J200	-30 to 212	Black	10	9452K199	00000	00000	00000

Square-Profile Oil-Resistant Buna-N O-Rings

With flat edges on all four sides, these O-rings make contact with a larger surface area than round-profile O-rings. The increased contact creates a better seal in static applications, such as hose, pipe, and faucet connections. However, these O-rings aren’t as good of a choice in dynamic applications, where the flat edges cause higher friction that leads to increased wear. Made from Buna-N, they resist tearing and stand up to hydraulic and motor oils.

Inch

		Fractional										Pkg.
	Dash No.	ID	OD	ID	OD	Hardness	Specs. Met	Temp. Range, ° F	Color	Pkg. Qty.		1-9	10-Up
1/8 Fractional Width (0.134" Wide)



		251	5 1/8	5 3/8	5.109"	5.377"	Durometer 70A (Medium)	ASTM D2000, SAE AS568, SAE J200	-30 to 212	Black	25	4061T273	000000	000000

Ultra-Chemical-Resistant FEP-Encapsulated Viton® Fluoroelastomer O-Rings

An FEP casing protects these O-rings from moisture and harsh solvents such as acetone, methanol, and boric acid. If the casing wears away, the Viton® fluoroelastomer core will continue to ward off chemicals. These O-rings are more flexible than PTFE O-rings and resist chemicals better than standard Viton® O-rings. They’re often used as seals in centrifugal pumps and gas venting valves. All come with shelf life, a cure date, and a traceable lot number on the bag.

		Fractional										Each
	Dash No.	ID	OD	ID	OD	Hardness	Core Hardness	Specs. Met	Temp. Range, ° F	Color		1-9	10-Up
1/8 Fractional Width (0.139" Wide)



		251	5 1/8	5 3/8	5.109"	5.387"	Durometer 55D (Hard)	Durometer 75A (Medium)	ASTM D2000, ASTM D2116, SAE AS568, SAE J200	-15 to 400	Clear with Black Core	93445K541	000000	000000

Water- and Steam-Resistant Parker E0603 EPDM O-Rings

Trusted for their quality and consistency, these E0603 EPDM O-rings resist water, steam, and glycol-based fluids, including brake fluid. They’re often used to seal plumbing and refrigeration systems, since they won’t swell or break down in moist environments. All have a cure date and traceable lot number printed on the bag.

		Fractional										Pkg.
	Dash No.	ID	OD	ID	OD	Hardness	Specs. Met	Temp. Range, ° F	Color	Pkg. Qty.		1-9	10-49	50-Up
1/8 Fractional Width (0.139" Wide)



		251	5 1/8	5 3/8	5.109"	5.387"	Durometer 70A (Medium)	ASTM D2000, SAE AS568, SAE J200	-70 to 250	Black	10	9557K251	000000	000000	000000

X-Profile Oil-Resistant Buna-N O-Rings

With four points of contact for a strong seal, these X-profile O-rings are often used in dynamic applications, such as on pistons and rotating shafts. Also known as Quattro seals and X-rings, they resist rolling and twisting better than round- and square-profile O-rings. They also require less compression to seal, which reduces friction and extends their service life. These O-rings are made of Buna-N, which is resilient and tear resistant. They resist hydraulic and motor oil.

		Fractional										Pkg.
	Dash No.	ID	OD	ID	OD	Hardness	Specs. Met	Temp. Range, ° F	Color	Pkg. Qty.		1-9	10-Up
1/8 Fractional Width (0.139" Wide)



		251	5 1/8	5 3/8	5.109"	5.387"	Durometer 70A (Medium)	ASTM D2000, SAE AS568, SAE J200	-30 to 225	Black	5	90025K615	000000	000000

High-Temperature Soft Silicone O-Rings

Easier to compress than standard silicone O-rings, these O-rings create a better seal in enclosures, reservoirs, and other low-pressure equipment. They can withstand temperatures up to 400° F. All have shelf life, a cure date, and a traceable lot number printed on the bag.

Inch

		Fractional										Pkg.
	Dash No.	ID	OD	ID	OD	Hardness	Specs. Met	Temp. Range, ° F	Color	Pkg. Qty.		1-9	10-Up
1/8 Fractional Width (0.139" Wide)



		251	5 1/8	5 3/8	5.109"	5.387"	Durometer 50A (Soft)	ASTM D2000, SAE AS568	-85 to 400	Red	1	1173N491	00000	00000

Oil-Resistant Hard Buna-N O-Rings

Harder than standard Buna-N O-rings, these 90A durometer O-rings are more durable and wear resistant. They resist petroleum and grease, so they’re often used in hydraulic and pneumatic systems. All have shelf life, cure date, and a traceable lot number printed on the bag.

Inch

		Fractional										Pkg.
	Dash No.	ID	OD	ID	OD	Hardness	Specs. Met	Temp. Range, ° F	Color	Pkg. Qty.		1-9	10-Up
1/8 Fractional Width (0.139" Wide)



		251	5 1/8	5 3/8	5.109"	5.387"	Durometer 90A (Hard)	ASTM D2000, SAE AS568, SAE J200	-30 to 250	Black	10	5308T596	00000	00000

Ultra-Chemical-Resistant Rigid PTFE O-Rings

Made from PTFE, these O-rings withstand extreme temperatures and harsh chemicals such as acetone, fluorinated solvents, and ethanol. They’re also rigid, which makes them more difficult to install than flexible O-rings. These O-rings are often used to seal pump shafts, mixing tanks, and other equipment that processes chemicals or pharmaceuticals. They’re also FDA compliant for direct contact with food. All have a cure date and a traceable lot number printed on the bag.

Inch

		Fractional											Pkg.
	Dash No.	ID	OD	ID	OD	Hardness	Specs. Met	Food Industry Std.	Temp. Range, ° F	Color	Pkg. Qty.		1-9	10-Up
1/8 Fractional Width (0.139" Wide)



		251	5 1/8	5 3/8	5.109"	5.387"	Durometer 55D (Hard)	SAE AS568	FDA Compliant 21 CFR 177.1550	-300 to 500	White	1	9559K236	000000	000000

Low-Temperature Chemical-Resistant FEP-Encapsulated Silicone O-Rings

Seal lines and valves in HVAC systems, freezers, and other cold applications exposed to chemicals. Made with an FEP casing, these O-rings are protected against chemicals such as methanol and boric acid. They’re a step up from Viton® fluoroelastomer in chemical resistance but less robust than PTFE. Their silicone core stays compressible even in temperatures as low as -75° F. All have a shelf life, cure date, and traceable lot number printed on the bag.

		Fractional											Each
	Dash No.	ID	OD	ID	OD	Hardness	Core Hardness	Specs. Met	Food Industry Std.	Temp. Range, ° F	Color		1-9	10-Up
1/8 Fractional Width (0.139" Wide)



		251	5 1/8	5 3/8	5.109"	5.387"	Durometer 55D (Hard)	Durometer 70A (Medium)	ASTM D2000, SAE AS568, SAE J200	FDA Compliant 21 CFR 177.1550, FDA Compliant 21 CFR 177.2600	-75 to 400	Clear with Red Core	9319K267	000000	000000

Oil-Resistant Soft Buna-N O-Rings

For a better seal in low-pressure applications, such as hydraulic return lines and oil drain plugs, these O-rings are softer than standard Buna-N O-rings. They offer the same level of oil resistance and are often used with hydraulic oil and motor oil.

Inch

		Fractional										Pkg.
	Dash No.	ID	OD	ID	OD	Hardness	Specs. Met	Temp. Range, ° F	Color	Pkg. Qty.		1-9	10-Up
1/8 Fractional Width (0.139" Wide)



		251	5 1/8	5 3/8	5.109"	5.387"	Durometer 50A (Soft)	ASTM D2000, SAE AS568, SAE J200	-30 to 225	Black	3	2418T415	00000	00000

Chemical-Resistant Hard Viton® Fluoroelastomer O-Rings

With a 90A durometer, these O-rings are harder and more wear-resistant than standard Viton® fluoroelastomer O-rings. They hold up to the high pressures in fuel injectors and heavy duty hydraulic cylinders. They resist a wide range of chemicals, including benzene, butyl alcohol, and ethylene glycol. All have shelf life, a cure date, and a traceable lot number printed on the bag.

Certificates with a traceable lot number are available for these products. Download certificates from ORDER HISTORY after your order ships.

		Fractional										Pkg.
	Dash No.	ID	OD	ID	OD	Hardness	Specs. Met	Temp. Range, ° F	Color	Pkg. Qty.		1-9	10-Up
1/8 Fractional Width (0.139" Wide)



		251	5 1/8	5 3/8	5.109"	5.387"	Durometer 90A (Hard)	ASTM D2000, SAE AS568, SAE J200	0 to 400	Black	1	8297T546	00000	00000

Oil-Resistant Mil. Spec. Buna-N O-Rings

Choose these O-rings for their adherence to strict military standards for material and construction. Made of Buna-N, they stand up to a variety of oils and fuels as well as ammonia, ethylene glycol, and isopropyl alcohol. They’re also resilient and tear resistant. These O-rings are made in the U.S.

MIL-I-45208—O-rings that meet MIL-I-45208 have undergone rigorous inspection for quality control.

MIL-P-5315—O-rings that meet MIL-P-5315 are the best choice for use with hydrocarbon fuels. They’re often used in engine fuel systems in airplanes and other vehicles.

MIL-P-83461—O-rings that meet MIL-P-83461 have been tested in conditions that simulate long-term, compressed use at high heat. Choose them when you need O-rings that will stand up to extended direct contact with high-temperature hydraulic oil.

		Fractional											Pkg.
	Dash No.	ID	OD	ID	OD	Hardness	Specs. Met	Mil. Spec.	Temp. Range, ° F	Color	Pkg. Qty.		1-9	10-Up
1/8 Fractional Width (0.139" Wide)



		251	5 1/8	5 3/8	5.109"	5.387"	Durometer 65A (Medium)	ASTM D2000, SAE AS568, SAE AS3578, SAE J200	MIL-I-45208	-20 to 250	Black	1	5018T597	000000	00000


		251	5 1/8	5 3/8	5.109"	5.387"	Durometer 65A (Medium)	ASTM D2000, SAE AS568, SAE J200	MIL-P-5315	-65 to 155	Black	5	4464T288	00000	00000
		251	5 1/8	5 3/8	5.109"	5.387"	Durometer 70A (Medium)	ASTM D2000, SAE AS568, SAE J200	MIL-P-83461	-65 to 275	Black	5	4679T288	00000	00000

High-Temperature Parker S1138 Silicone O-Rings

Use these O-rings in temperatures up to 400° F. Made of Parker S1138 silicone, they’re pure enough for food or pharmaceutical applications—they’re USP VI certified. These O-rings are also FDA compliant for direct contact with food. All come with a cure date and a traceable lot number printed on the bag.

		Fractional												Pkg.
	Dash No.	ID	OD	ID	OD	Hardness	Specs. Met	Food Industry Std.	USP Class	Temp. Range, ° F	Color	Pkg. Qty.		1-9	10-49	50-Up
1/8 Fractional Width (0.139" Wide)



		251	5 1/8	5 3/8	5.109"	5.387"	Durometer 70A (Medium)	ASTM D2000, SAE AS568, SAE J200	FDA Compliant 21 CFR 177.2600	VI	-60 to 400	Red	5	9396K242	000000	000000	000000

Water- and Steam-Resistant EPDM O-Rings

Made of EPDM, these O-rings have excellent resistance to water, steam, and glycol-based fluids, including brake fluid. They’re often used to seal plumbing and refrigeration systems, since they won’t swell or break down in moist environments. All have shelf life, a cure date, and a traceable lot number printed on the bag.

Inch

		Fractional										Pkg.
	Dash No.	ID	OD	ID	OD	Hardness	Specs. Met	Temp. Range, ° F	Color	Pkg. Qty.		1-9	10-49	50-Up
1/8 Fractional Width (0.139" Wide)



		251	5 1/8	5 3/8	5.109"	5.387"	Durometer 70A (Medium)	ASTM D2000, SAE AS568, SAE J200	-65 to 300	Black	10	8785N724	00000	00000	00000

Chemical-Resistant Viton® Fluoroelastomer O-Rings

Made of Viton® fluoroelastomer, these O-rings combine the oil resistance of Buna-N with the heat tolerance of silicone. They also resist a variety of chemicals, including benzene, butyl alcohol, and ethylene glycol. They’re often used to seal hydraulic systems and equipment in crude oil refineries. All have shelf life, a cure date, and a traceable lot number printed on the bag.

Inch

Brown—Brown O-rings help you tell Viton® fluoroelastomer apart from other materials.

		Fractional										Pkg.
	Dash No.	ID	OD	ID	OD	Hardness	Specs. Met	Temp. Range, ° F	Choose a Color	Pkg. Qty.		1-9	10-49	50-Up
1/8 Fractional Width (0.139" Wide)



		251	5 1/8	5 3/8	5.109"	5.387"	Durometer 75A (Medium)	ASTM D2000, SAE AS568, SAE J200	0 to 400	Black, Brown	5	9464K383	000000	000000	000000

FDA-Compliant Chemical-Resistant Soft Viton® Fluoroelastomer O-Rings

Made of Viton® fluoroelastomer, these O-rings won’t degrade when exposed to chemicals such as sulfuric acid, butyl alcohol, and ethylene glycol. They’re also FDA compliant for direct contact with food. Since they’re softer and easier to compress than standard Viton® O-rings, they form a better seal in low-pressure applications. They offer the same oil resistance as Buna-N O-rings and withstand high temperatures, making them ideal for food lines with hot oil. All have shelf life, a cure date, and a traceable lot number printed on the bag.

		Fractional											Pkg.
	Dash No.	ID	OD	ID	OD	Hardness	Specs. Met	Food Industry Std.	Temp. Range, ° F	Color	Pkg. Qty.		1-9	10-Up
1/8 Fractional Width (0.139" Wide)



		251	5 1/8	5 3/8	5.109"	5.387"	Durometer 60A (Soft)	SAE AS568	FDA Compliant 21 CFR 177.2600	-15 to 400	White	1	1284N276	000000	000000

Steam- and Ultra-Chemical-Resistant Aflas O-Rings

Often used in drilling systems, these Aflas O-rings hold up to petroleum fluids. They also withstand steam and chemicals such as ammonia, calcium hydroxide, and boric acid. All have shelf life, a cure date, and a traceable lot number printed on the bag.

		Fractional										Pkg.
	Dash No.	ID	OD	ID	OD	Hardness	Specs. Met	Temp. Range, ° F	Color	Pkg. Qty.		1-9	10-Up
1/8 Fractional Width (0.139" Wide)



		251	5 1/8	5 3/8	5.109"	5.387"	Durometer 80A (Hard)	ASTM D2000, SAE AS568	25 to 450	Black	1	5240T169	000000	000000

FDA-Compliant Oil-Resistant Buna-N O-Rings

FDA compliant for direct contact with food, these Buna-N O-rings create a strong seal in food and beverage lines. They won’t weaken from animal and vegetable oil, citric acid, sodium bicarbonate, and other substances used in food processing. They also resist tearing when used in moving equipment, so you can count on them for durability. All have shelf life, cure date, and a traceable lot number printed on the bag.

Inch

		Fractional											Pkg.
	Dash No.	ID	OD	ID	OD	Hardness	Specs. Met	Food Industry Std.	Temp. Range, ° F	Color	Pkg. Qty.		1-9	10-Up
1/8 Fractional Width (0.139" Wide)



		251	5 1/8	5 3/8	5.109"	5.387"	Durometer 70A (Medium)	ASTM D2000, SAE AS568, SAE J200	FDA Compliant 21 CFR 177.2600	-30 to 250	Black	5	9555K207	00000	00000

High-Temperature Silicone O-Rings

Made of silicone, these O-rings can withstand temperatures up to 400° F or higher. They're FDA compliant for direct contact with food, so they're often used on hot food lines. They're also used to seal HVAC systems. All come with a cure date and a traceable lot number printed on the bag.

Inch

		Fractional											Pkg.
	Dash No.	ID	OD	ID	OD	Hardness	Specs. Met	Food Industry Std.	Temp. Range, ° F	Color	Pkg. Qty.		1-9	10-49	50-Up
1/8 Fractional Width (0.139" Wide)



		251	5 1/8	5 3/8	5.109"	5.387"	Durometer 70A (Medium)	ASTM D2000, SAE AS568, SAE J200	FDA Compliant 21 CFR 177.2600	-65 to 400	Red	5	9186N294	00000	00000	00000

Chemical-Resistant Mil. Spec. Viton® Fluoroelastomer O-Rings

Made of Viton® fluoroelastomer, these O-rings meet MIL-R-83248 standards. They stay strong when used with chemicals such as benzene, butyl alcohol, and ethylene glycol. You’ll often find them used in aerospace fuel and hydraulic systems. All have shelf life, a cure date, and a traceable lot number printed on the bag.

		Fractional											Pkg.
	Dash No.	ID	OD	ID	OD	Hardness	Specs. Met	Mil. Spec.	Temp. Range, ° F	Color	Pkg. Qty.		1-9	10-Up
1/8 Fractional Width (0.139" Wide)



		251	5 1/8	5 3/8	5.109"	5.387"	Durometer 75A (Medium)	ASTM D2000, SAE AS568, SAE J200	MIL-R-83248	5 to 400	Black	5	1201T883	000000	000000

Super-Resilient Chemical-Resistant Viton® Fluoroelastomer O-Rings

Made for demanding applications, these O-rings hold up to constant motion, repeated compression, and extreme temperatures. They’re often used for centrifugal pumps and high-pressure hydraulic valves in chemical processing systems. These O-rings have a higher tensile strength than standard Viton® fluoroelastomer O-rings and withstand a wider range of temperatures. They also hold their shape four times better when tested to ASTM D395 standards for compression set. You can use them with boric acid, butyl alcohol, ethylene glycol, and other chemicals. They have shelf life, a cure date, and a traceable lot number printed on the bag.

Inch

		Fractional											Pkg.
	Dash No.	ID	OD	ID	OD	Hardness	Tensile Strength, psi	Specs. Met	Temp. Range, ° F	Color	Pkg. Qty.		1-9	10-Up
1/8 Fractional Width (0.139" Wide)



		251	5 1/8	5 3/8	5.109"	5.387"	Durometer 75A (Medium)	2,030	ASTM D395, ASTM D2000, SAE AS568, SAE J200	-15 to 525	Black	2	1288N443	00000	00000

Square-Profile Chemical-Resistant Viton® Fluoroelastomer O-Rings

With flat edges on all four sides, these Viton® fluoroelastomer square-profile O-rings cover more surface than round-profile O-rings for a better seal in static applications, such as hose, pipe, and faucet connections. They resist a wide range of chemicals, including benzene, butyl alcohol, and ethylene glycol. All have shelf life, a cure date, and a traceable lot number printed on the bag.

		Fractional										Pkg.
	Dash No.	ID	OD	ID	OD	Hardness	Specs. Met	Temp. Range, ° F	Color	Pkg. Qty.		1-9	10-Up
1/8 Fractional Width (0.139" Wide)



		251	5 1/8	5 3/8	5.109"	5.387"	Durometer 75A (Medium)	ASTM D2000, SAE AS568	-10 to 400	Black	1	1170N188	000000	000000

Low-Temperature Chemical-Resistant Mil. Spec. Fluorosilicone O-Rings

Create seals in temperatures as low as -65° F. Made of fluorosilicone, these O-rings have the same chemical resistance as Viton® fluoroelastomer O-rings, but they’re more flexible in the cold. They resist ethanol, ethylene glycol, and isopropyl alcohol. All meet MIL-R-25988 standards for performance and quality. All have shelf life, a cure date, and a traceable lot number printer on the bag.

		Fractional											Pkg.
	Dash No.	ID	OD	ID	OD	Hardness	Specs. Met	Mil. Spec.	Temp. Range, ° F	Color	Pkg. Qty.		1-9	10-Up
1/8 Fractional Width (0.139" Wide)



		251	5 1/8	5 3/8	5.109"	5.387"	Durometer 70A (Medium)	ASTM D2000, SAE AS568, SAE J200	MIL-R-25988	-65 to 390	Blue	1	8333T288	000000	000000

Oil- and Abrasion-Resistant Polyurethane O-Rings

More abrasion resistant than Buna-N O-rings, these polyurethane O-rings resist mineral oil, motor oil, and ethylene glycol. They have a round profile and are suitable for a wide range of static and dynamic sealing applications. All are sized to the SAE standard AS568.

		Fractional										Pkg.
	Dash No.	ID	OD	ID	OD	Hardness	Specs. Met	Temp. Range, ° F	Color	Pkg. Qty.		1-9	10-Up
1/8 Fractional Width (0.139" Wide)



		251	5 1/8	5 3/8	5.109"	5.387"	Durometer 70A (Medium)	ASTM D2000, SAE AS568, SAE J200	-40 to 200	Black	1	9558K278	000000	000000

Weather- and Oil-Resistant HNBR O-Rings

A step up from Buna-N, these HNBR O-rings contain extra hydrogen to better resist chemicals, weather, and abrasion. They won’t swell from contact with grease and acid, making them a good choice in engine, hydraulic, and HVAC systems. Because they won’t deteriorate from ozone, UV, and oxidation, these O-rings are also good for outdoor use. However, they won’t withstand solvents or strong acids like Viton® fluoroelastomer O-rings do. They’re also known as hydrogenated nitrile and highly saturated nitrile O-rings.

All have shelf life, cure date, and a traceable lot number printed on the bag.

		Fractional										Pkg.
	Dash No.	ID	OD	ID	OD	Hardness	Specs. Met	Temp. Range, ° F	Color	Pkg. Qty.		1-9	10-Up
1/8 Fractional Width (0.139" Wide)



		251	5 1/8	5 3/8	5.109"	5.387"	Durometer 70A (Medium)	ASTM D2000, SAE AS568, SAE J200	-40 to 250	Green	5	8551N159	000000	000000

X-Profile Chemical-Resistant Viton® Fluoroelastomer O-Rings

Also known as Quattro Seals and X-rings, these X-profile O-rings have four points of contact for a better seal in moving parts than round-profile O-rings. Since they require less compression to make a seal, they also last longer. Made of Viton® fluoroelastomer, these O-rings resist a wide range of chemicals, including benzene, butyl alcohol, and ethylene glycol. All have shelf life, a cure date, and a traceable lot number printed on the bag.

		Fractional										Pkg.
	Dash No.	ID	OD	ID	OD	Hardness	Specs. Met	Temp. Range, ° F	Color	Pkg. Qty.		1-9	10-Up
1/8 Fractional Width (0.139" Wide)



		251	5 1/8	5 3/8	5.109"	5.387"	Durometer 75A (Medium)	ASTM D2000, SAE AS568, SAE J200	-15 to 400	Black	1	6540K333	000000	00000

Oil-Resistant Buna-N Backup Rings


O-Ring Sealing Properly with Backup Ring	O-Ring Deforms Without a Backup Ring

Image of Attribute. Front orientation. CrossSection view. Contains Annotated. Oil-Resistant Buna-N Backup Rings.

Extend the life of O-rings in oily, high-pressure systems that operate at up to 2,500 psi. These backup rings support O-rings and prevent them from deforming under pressure and causing pressure loss. Made of Buna-N, these rings won’t swell when exposed to grease, petroleum fluids, and motor oil. However, they will degrade when exposed to mild or harsh chemicals. They’re often used in hydraulic, automotive, and industrial systems. All have a shelf life, cure date, and a traceable lot number printed on the bag.

										Pkg.
	For Dash No.	Thk.	ID	OD	Hardness	Temp. Range, ° F	Color	Pkg. Qty.		1-9	10-Up
0.118" Wide



		251	0.05"	5.143"	5.379"	Durometer 90A (Hard)	-30 to 250	Black	10	5288T294	00000	00000