5/8"-18 Locknuts | McMaster-Carr

Filter by

System of Measurement

Nut Type

Fastener Strength Grade/Class

Finish

Cadmium Yellow-Chromate Plated

Phosphate Coated

Zinc Flake Coated

Zinc Plated

Zinc Yellow-Chromate Plated

Material

Locking Type

Hex Nut Profile

Height

Thread Spacing

Thread Type

Width

Thread Direction

Performance

Fastener Strength Rating

Drive Style

Thread Fit

Unified Standard Class 2B

Unified Standard Class 3B

Export Control Classification Number (ECCN)

EAR99

U.S.–Mexico–Canada Agreement (USMCA) Qualifying

REACH

RoHS

DFARS Specialty Metals

Sold As

Insert Maximum Temperature

185° F

220° F

250° F

Certificate Type

Material Certificate with Traceable Lot Number

Flange Diameter

45 Products

Nylon-Insert Locknuts

Versatile yet economical, these are the most commonly used locknuts. The nylon insert grips the screw to prevent loosening from moderate vibration without damaging threads. However, the insert may become brittle if exposed to high temperatures. These locknuts are reusable a handful of times, but the holding power decreases with each use.

High-Strength Steel

Rated Grade 8, Grade C, or Class 10, these locknuts are about 25% stronger than medium-strength steel nuts. To avoid stripped threads during installation, make sure your screw has a comparable strength rating.

Zinc-Yellow-Chromate-Plated Steel—With a more durable finish than clear zinc plating, these nuts are suitable for most wet and outdoor environments. However, they won't hold up to the harsh chemicals that stainless steel can.

Black-Oxide Steel—Typically chosen for their appearance, these nuts have a dark, matte coating to create a finished look. They offer minimal corrosion resistance, so they're best for dry environments.

Steel—Best for dry environments since moisture will cause these nuts to rust.

	Wd.	Ht.	Fastener Strength Grade/Class	Insert Max. Temp., ° F	Pkg. Qty.		Pkg.
Zinc-Yellow-Chromate-Plated Steel



	5/8"-18	15/16"	3/4"	SAE Grade 8	185	5	97135A275	00000


	Black-Oxide Steel



			5/8"-18	15/16"	3/4"	SAE Grade 8	185	5	97135A428	0000


			Steel



					5/8"-18	15/16"	3/4"	SAE Grade 8	185	5	90630A165	0000

Medium-Strength Steel

These Grade 5 or Class 8 nuts are your go-to for fastening most machinery and equipment. To avoid stripped threads during installation, make sure your screw has a comparable strength rating.

Zinc-Plated Steel—A step up from plain steel, the zinc plating withstands occasional exposure to moisture.

	Wd.	Ht.	Fastener Strength Grade/Class	Insert Max. Temp., ° F	Pkg. Qty.		Pkg.
Zinc-Plated Steel



	5/8"-18	15/16"	3/4"	SAE Grade 5	185	25	95615A260	000000


	Black-Oxide Steel



			5/8"-18	15/16"	3/4"	SAE Grade 5	185	25	95615A113	00000

Low-Strength Steel

Rated Grade 2, Class 6, or lower, these locknuts are about half as strong as medium-strength steel nuts. Use them with screws that have a similar strength rating to avoid stripping threads during installation.

Zinc-Plated Steel—A step up from plain steel, the zinc plating withstands occasional exposure to moisture.

	Wd.	Ht.	Insert Max. Temp., ° F	Pkg. Qty.		Pkg.
Zinc-Plated Steel



	5/8"-18	15/16"	3/4"	185	25	90640A170	000000

Corrosion-Resistant Stainless Steel

18-8 Stainless Steel—The choice for wet and outdoor environments, these locknuts resist rust. However, they won't hold up to the harsh chemicals that 316 stainless steel can.

316 Stainless Steel—Our most corrosion-resistant nuts, these hold up in just about any environment. They won't degrade from exposure to weather, salt water, and most chemicals.

	Wd.	Ht.	Insert Max. Temp., ° F	Pkg. Qty.		Pkg.
18-8 Stainless Steel



	5/8"-18	15/16"	3/4"	185	5	91831A145	00000


	316 Stainless Steel



			5/8"-18	15/16"	3/4"	185	1	90715A187	0000

Thin-Profile Nylon-Insert Locknuts

About half the height of standard-profile locknuts, these fit in spaces with low clearance. You can also use them as jam nuts by threading them against a hex nut to hold it in place. The nylon insert grips the screw to prevent loosening from moderate vibration without damaging threads. However, the insert may become brittle if exposed to high temperatures. These locknuts are reusable a handful of times, but the holding power decreases with each use.

High-Strength Steel

Zinc-Plated Steel—A step up from plain steel, the zinc plating withstands occasional exposure to moisture.

	Wd.	Ht.	Fastener Strength Grade/Class	Insert Max. Temp., ° F	Pkg. Qty.		Pkg.
Zinc-Yellow-Chromate-Plated Steel



	5/8"-18	15/16"	25/64"	SAE Grade 8	250	5	94945A122	00000


	Zinc-Plated Steel



			5/8"-18	15/16"	25/64"	SAE Grade 8	220	5	94945A230	0000


			Black-Oxide Steel



					5/8"-18	15/16"	25/64"	SAE Grade 8	250	5	94945A113	00000

Medium-Strength Steel

These Grade 5 or Class 8 nuts are your go-to for fastening most machinery and equipment. To avoid stripped threads during installation, make sure your screw has a comparable strength rating.

Zinc-Plated Steel—A step up from plain steel, the zinc plating withstands occasional exposure to moisture.

	Wd.	Ht.	Fastener Strength Grade/Class	Insert Max. Temp., ° F	Pkg. Qty.		Pkg.
Zinc-Plated Steel



	5/8"-18	15/16"	25/64"	SAE Grade 5	220	5	91342A220	000000

Low-Strength Steel

Zinc-Plated Steel—A step up from plain steel, the zinc plating withstands occasional exposure to moisture.

	Wd.	Ht.	Insert Max. Temp., ° F	Pkg. Qty.		Pkg.
Zinc-Plated Steel



	5/8"-18	15/16"	25/64"	220	50	90566A235	000000

Corrosion-Resistant Stainless Steel

18-8 Stainless Steel—The choice for wet and outdoor environments, these locknuts resist rust. However, they won't hold up to the harsh chemicals that 316 stainless steel can.

	Wd.	Ht.	Insert Max. Temp., ° F	Pkg. Qty.		Pkg.
18-8 Stainless Steel



	5/8"-18	15/16"	25/64"	220	1	90101A260	00000

Top-Lock Distorted-Thread Locknuts

Irregularly shaped threads at the top of these locknuts grip the screw for a stronger hold than nylon-insert locknuts. Although the threads prevent the nut from backing out due to vibration, they also increase the effort to install and remove. To get these nuts off, you may need special tools, such as an impact wrench or propane torch. Nuts are not reusable.

High-Strength Steel

Zinc-Plated Steel—A step up from plain steel, the zinc plating withstands occasional exposure to moisture.

Zinc-Flake-Coated Steel—The most durable zinc coating, this multilayer finish resists wear for long-term rust protection. However, similar to zinc-yellow-chromate plating, it won't hold up to harsh chemicals like stainless steel can.

Cadmium-Plated Steel—These locknuts have good lubricity to thread on smoothly and ease installation. The plating also creates a barrier against rust that's comparable to zinc-yellow-chromate plating.

Steel—Best for dry environments since moisture will cause these nuts to rust.

	Wd.	Ht.	Fastener Strength Grade/Class	Pkg. Qty.		Pkg.
Zinc-Yellow-Chromate-Plated Steel



	5/8"-18	15/16"	35/64"	ASTM Grade C	5	99291A280	00000


	Zinc-Plated Steel



			5/8"-18	15/16"	35/64"	ASTM Grade C	25	92501A534	00000


			Black-Oxide Steel



					5/8"-18	15/16"	35/64"	ASTM Grade C	5	99291A510	00000


					Zinc-Flake-Coated Steel



							5/8"-18	15/16"	35/64"	ASTM Grade C	1	99291A730	0000


							Cadmium-Plated Steel



									5/8"-18	15/16"	35/64"	ASTM Grade C	25	92501A450	00000


									Steel



											5/8"-18	15/16"	35/64"	ASTM Grade C	25	92501A514	00000

Medium-Strength Steel

These Grade 5 or Class 8 nuts are your go-to for fastening most machinery and equipment. To avoid stripped threads during installation, make sure your screw has a comparable strength rating.

Zinc-Plated Steel—A step up from plain steel, the zinc plating withstands occasional exposure to moisture.

	Wd.	Ht.	Fastener Strength Grade/Class	Pkg. Qty.		Pkg.
Zinc-Plated Steel



	5/8"-18	15/16"	5/8"	SAE Grade 5	10	95065A109	00000


	Black-Oxide Steel



			5/8"-18	15/16"	5/8"	SAE Grade 5	10	90673A116	00000

Corrosion-Resistant Stainless Steel

18-8 Stainless Steel—The choice for wet and outdoor environments, these locknuts resist rust. However, they won't hold up to the harsh chemicals that 316 stainless steel can.

	Wd.	Ht.	Pkg. Qty.		Pkg.
18-8 Stainless Steel



	5/8"-18	15/16"	35/64"	1	90045A475	00000

Distorted-Thread Flange Nuts

The strongest hold of all the flange nuts we offer, these have irregularly shaped threads that grip the screw to resist loosening. The threads prevent the nut from backing out due to high vibration, but they also increase the effort to install and remove. To get these nuts off, you may need special tools, such as an impact wrench or propane torch, so they are not reusable. Nuts have a flange that distributes pressure evenly across the mounting surface without the need for a separate washer. They are ideal for securing screws in oversized or irregularly shaped holes since the flange covers a larger area than standard nuts. Also known as collar nuts.

High-Strength Steel

Image of Product. Bottom orientation. Flange Nuts. Distorted-Thread Flange Nuts.

Rated Grade 8, Grade G, or Class 10, these are about 25% stronger than medium-strength steel nuts. They are commonly used to fasten components in valves, pumps, and motors. To avoid stripped threads during installation, make sure your screw has a comparable strength rating.

Black-Oxide Steel—Typically chosen for their appearance, these nuts have a dark, matte coating that creates a finished look. They offer minimal corrosion resistance, so they're best for dry environments.

Black Phosphate-Coated Steel—The dark coating creates both a finished appearance and a barrier against rust that withstands occasional exposure to moisture.

				Flange
	Thread Size	Wd.	Ht.	Dia.	Thk.	Fastener Strength Grade/Class	Certificate Type	Pkg. Qty.		Pkg.
Zinc-Yellow-Chromate-Plated Steel



		5/8"-18	15/16"	11/16"	1 21/64"	0.1"	ASME Grade G	Material Certificate	10	90949A222	000000


		Black-Oxide Steel



				5/8"-18	15/16"	11/16"	1 21/64"	0.1"	ASME Grade G	—	10	90949A234	00000


				Black Phosphate-Coated Steel



						5/8"-18	15/16"	11/16"	1 21/64"	0.1"	ASME Grade G	—	10	90949A112	0000

Serrated Flange Nuts

Serrations bite into material for mild vibration resistance while the flange eliminates the need for a separate washer. These nuts are ideal for securing screws in oversized or irregularly shaped holes since the flange covers a larger area than standard nuts. You can use them again and again without sacrificing strength. However, the serrations may mar the surface of your material. Also known as collar nuts.

Medium-Strength Steel

Rated Grade 5, Grade F, or Class 8, these nuts are your go-to for fastening most machinery and equipment. To avoid stripped threads during installation, make sure your screw has a comparable strength rating.

Zinc-Plated Steel—A step up from plain steel, the zinc plating withstands occasional exposure to moisture.

				Flange
	Thread Size	Wd.	Ht.	Dia.	Thk.	Fastener Strength Grade/Class	Pkg. Qty.		Pkg.
Zinc-Plated Steel



		5/8"-18	59/64"	35/64"	1 1/4"	0.05"	SAE Grade 5	25	99904A227	000000


		Black-Oxide Steel



				5/8"-18	59/64"	35/64"	1 1/4"	0.05"	SAE Grade 5	25	99904A219	00000

Low-Strength Steel

About half the strength of medium-strength steel nuts, these are suitable for light duty fastening applications, such as securing access panels. Use them with screws that have a similar strength rating to avoid stripping threads during installation.

Zinc-Plated Steel—A step up from plain steel, the zinc plating withstands occasional exposure to moisture.

				Flange
	Thread Size	Wd.	Ht.	Dia.	Thk.	Pkg. Qty.		Pkg.
Zinc-Plated Steel



		5/8"-18	15/16"	9/16"	1 9/32"	0.05"	10	94831A730	00000


		Black-Oxide Steel



				5/8"-18	15/16"	9/16"	1 9/32"	0.05"	1	93083A118	0000

Locknuts for use with Cotter Pins

Use these locknuts to secure rotating parts subjected to mild vibration, such as the wheels on carts and hand trucks. They’re not recommended for use on parts exposed to high vibration because the pin can shear under prolonged stress. These nuts are reusable, but you'll need a new cotter pin (not included) with each use.

Medium-Strength Steel

These Grade 5 or Class 8 nuts are your go-to for fastening most machinery and equipment. To avoid stripped threads during installation, make sure your screw has a comparable strength rating.

Zinc-Plated Steel—A step up from plain steel, the zinc plating withstands occasional exposure to moisture.

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

	Wd.	Ht.	Fastener Strength Grade/Class	For Cotter Pin Dia.	Pkg. Qty.		Pkg.
Zinc-Plated Steel



	5/8"-18	15/16"	35/64"	SAE Grade 5	5/32"	10	97190A170	000000

Low-Strength Steel

Zinc-Plated Steel—A step up from plain steel, the zinc plating withstands occasional exposure to moisture.

	Wd.	Ht.	For Cotter Pin Dia.	Pkg. Qty.		Pkg.
Zinc-Plated Steel



	5/8"-18	15/16"	35/64"	5/32"	10	95030A250	00000

Corrosion-Resistant Stainless Steel

18-8 Stainless Steel—The choice for wet and outdoor environments, these locknuts resist rust. However, they won't hold up to the harsh chemicals that 316 stainless steel can.

	Wd.	Ht.	For Cotter Pin Dia.	Pkg. Qty.		Pkg.
18-8 Stainless Steel



	5/8"-18	15/16"	23/32"	5/32"	1	91853A545	00000

Center-Lock Distorted-Thread Locknuts

Also known as reversible locknuts, these thread on from either side, so you don't have to worry about fastening them incorrectly and damaging your screw. The irregularly shaped threads in the middle grip the screw for a stronger hold than nylon-insert locknuts. Although the threads prevent the nut from backing out due to vibration, they also increase the effort to install and remove. To get these nuts off, you may need special tools, such as an impact wrench or propane torch. Nuts are not reusable.

High-Strength Steel

	Wd.	Ht.	Fastener Strength Grade/Class	Pkg. Qty.		Pkg.
Zinc-Yellow-Chromate-Plated Steel



	5/8"-18	15/16"	35/64"	SAE Grade 8	5	90636A074	00000

Low-Strength Steel

Zinc-Plated Steel—A step up from plain steel, the zinc plating withstands occasional exposure to moisture.

	Wd.	Ht.	Fastener Strength Grade/Class	Pkg. Qty.		Pkg.
Zinc-Plated Steel



	5/8"-18	15/16"	35/64"	SAE Grade 2	25	90040A145	000000

Corrosion-Resistant Stainless Steel

18-8 Stainless Steel—The choice for wet and outdoor environments, these locknuts resist rust. However, they won't hold up to the harsh chemicals that 316 stainless steel can.

	Wd.	Ht.	Pkg. Qty.		Pkg.
18-8 Stainless Steel



	5/8"-18	15/16"	35/64"	5	90047A150	000000

Thin-Profile Flex-Top Locknuts for Heavy Vibration

About two-thirds the height of standard flex-top nuts, these fit in spaces with low clearance. You can also use them as jam nuts by threading them against a hex nut to hold it in place. The top section expands to grip the screw on all sides for a stronger hold than both nylon-insert and distorted-thread locknuts. As a result, they're suitable for use in equipment with constant, vigorous vibration, such as engines. These locknuts are reusable a handful of times, but the holding power decreases with each use.

Steel

These locknuts have equivalent strength to Grade 8 and Class 10 steel nuts. To avoid stripped threads during installation, make sure your screw has a comparable strength rating.

Steel—Best for dry environments since moisture will cause these nuts to rust.

	Wd.	Ht.	Pkg. Qty.		Pkg.
Cadmium-Plated Steel



	5/8"-18	15/16"	13/32"	1	94830A565	00000


	Steel



			5/8"-18	15/16"	13/32"	1	94830A137	0000

Corrosion-Resistant Stainless Steel

18-8 Stainless Steel—The choice for wet and outdoor environments, these locknuts resist rust. However, they won't hold up to the harsh chemicals that 316 stainless steel can.

	Wd.	Ht.	Pkg. Qty.		Pkg.
18-8 Stainless Steel



	5/8"-18	15/16"	13/32"	1	94140A162	000000

Mil. Spec. Nylon-Insert Locknuts

These locknuts are manufactured and tested to strict U.S. military standards for material and construction. The nylon insert grips the screw to prevent loosening from moderate vibration without damaging threads. However, the insert may become brittle if exposed to high temperatures. These locknuts are reusable a handful of times, but the holding power decreases with each use.

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

Low-Strength Steel

Cadmium Yellow-Chromate-Plated Steel—These locknuts have good lubricity to thread on smoothly and ease installation. The plating also creates a barrier against rust that's comparable to zinc-yellow-chromate plating.

	Wd.	Ht.	Mil. Spec.	Insert Max. Temp., ° F	Pkg. Qty.		Pkg.
Cadmium Yellow-Chromate-Plated Steel



	5/8"-18	15/16"	3/4"	MS21044-N10	250	1	95154A190	00000

Mil. Spec. Thin-Profile Nylon-Insert Locknuts

These locknuts are manufactured and tested to strict U.S. military standards for material and construction. They are about half the height of standard-profile locknuts to fit in spaces with low clearance. You can also use them as jam nuts by threading them against a hex nut to hold it in place. The nylon insert grips the screw to prevent loosening from moderate vibration without damaging threads. However, the insert may become brittle if exposed to high temperatures. These locknuts are reusable a handful of times, but the holding power decreases with each use.

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

Corrosion-Resistant Stainless Steel

18-8 Stainless Steel—The choice for wet and outdoor environments, these locknuts resist rust. However, they won't hold up to the harsh chemicals that 316 stainless steel can.

	Wd.	Ht.	Mil. Spec.	Insert Max. Temp., ° F	Pkg. Qty.		Pkg.
18-8 Stainless Steel



	5/8"-18	15/16"	13/32"	MS21083C10, NASM21083	250	1	97231A520	000000

Flex-Top Locknuts for Heavy Vibration

The top section of these nuts expands to grip the screw on all sides for a stronger hold than both nylon-insert and distorted-thread locknuts. They’re often used in equipment with constant, vigorous vibration, such as engines. These locknuts are reusable a handful of times, but the holding power decreases with each use.

Steel

These locknuts have equivalent strength to Grade 8 and Class 10 steel nuts. To avoid stripped threads during installation, make sure your screw has a comparable strength rating.

	Wd.	Ht.	Pkg. Qty.		Pkg.
Black-Oxide Steel



	5/8"-18	15/16"	49/64"	1	94820A480	00000


	Cadmium-Plated Steel



			5/8"-18	15/16"	49/64"	1	94820A255	0000

Two-Piece-Clamp Locknuts for Extreme Vibration

Similar to steel-insert locknuts, these two-piece nuts withstand extreme vibration but are easier to install. Since the two pieces spin freely by hand until you tighten them together with a wrench, they're perfect for extra long screws and threaded rods. You can reuse them a handful of times, but their hold will decrease with repeated use. Also known as drake-style locknuts.

Height includes both nuts once assembled.

Steel

These locknuts have equivalent strength to Grade 5 and Class 8 steel nuts. To avoid stripped threads during installation, make sure your screw has a comparable strength rating.

Steel—Best for dry environments since moisture will cause these nuts to rust.

	Wd.	Ht.		Each
Steel



	5/8"-18	15/16"	55/64"	90623A200	000000

Mil. Spec. Flex-Top Locknuts for Heavy Vibration

These locknuts are manufactured and tested to strict U.S. military standards for material and construction. The top section expands to grip the screw on all sides for a stronger hold than both nylon-insert and distorted-thread locknuts. As a result, they're suitable for use in equipment with constant, vigorous vibration, such as engines. These locknuts are reusable a handful of times, but the holding power decreases with each use.

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

Steel

These locknuts have equivalent strength to Grade 8 and Class 10 steel nuts. To avoid stripped threads during installation, make sure your screw has a comparable strength rating.

	Wd.	Ht.	Mil. Spec.	Pkg. Qty.		Pkg.
Cadmium-Plated Steel



	5/8"-18	15/16"	49/64"	MS21045-10	1	99638A800	000000