4-40 Locknuts | McMaster-Carr

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36 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.

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



	4-40	1/4"	9/64"	220	100	000000000	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.

Black-Oxide 18-8 Stainless Steel—Typically chosen for their appearance, these nuts have a dark, matte coating to create a finished look. They resist rust in wet and outdoor environments, but 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



	4-40	1/4"	9/64"	220	100	000000000	00000


	Black-Oxide 18-8 Stainless Steel



			4-40	1/4"	9/64"	220	100	000000000	00000


			316 Stainless Steel



					4-40	1/4"	9/64"	220	100	000000000	0000

High-Strength Lightweight Titanium

Grade 5 Titanium—The best of both worlds, these locknuts are lightweight like aluminum and as corrosion-resistant as 316 stainless steel. They're ideal for use in chemical-processing and water-treatment facilities since they stand up to strong acids and other harsh chemicals. Grade 5 is the strongest of all the titanium alloys due to its higher aluminum and vanadium content.

	Wd.	Ht.	Insert Max. Temp.	Pkg. Qty.		Pkg.
Grade 5 Titanium



	4-40	1/4"	9/64"	Not Rated	1	000000000	00000

Brass

Brass—Conductive and nonmagnetic, brass locknuts are typically used to secure electrical equipment, such as circuit board assemblies and mounting brackets inside enclosures. They're also softer than most metals, so they won't damage other fastening components.

	Wd.	Ht.	Insert Max. Temp., ° F	Pkg. Qty.		Pkg.
Brass



	4-40	1/4"	9/64"	220	50	000000000	00000

Mil. Spec. Distorted-Thread Flange Nuts

Not only do these have the strongest hold of all the flange nuts we offer, they are also manufactured and tested to strict U.S. military standards for material and construction. They have irregularly shaped threads that grip the screw to resist loosening in high-vibration applications. The threads prevent the nut from backing out, 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.

Low-Strength Steel

Image of Product. Front orientation. Flange Nuts. Mil. Spec. Distorted-Thread Flange Nuts.

About half the strength of medium-strength steel nuts. Use them with screws that have a similar strength rating to avoid stripping threads during installation.

Cadmium-Plated Steel—The plating reduces friction during installation, so these nuts thread on smoother than others. It also creates a barrier against rust that's comparable to zinc yellow-chromate plating.

Cadmium Yellow-Chromate-Plated Steel—Better corrosion resistance than plain cadmium. These nuts withstand salt water and are suitable for most wet and outdoor environments.

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

				Flange
	Thread Size	Wd.	Ht.	Dia.	Thk.	Mil. Spec.	Pkg. Qty.		Pkg.
Cadmium-Plated Steel



		4-40	5/32"	1/8"	0.206"	0.03"	MS21042L04	5	000000000	00000


		Cadmium Yellow-Chromate-Plated Steel



				4-40	5/32"	1/8"	0.206"	0.03"	MS21042-04	5	000000000	0000

Corrosion-Resistant Stainless Steel

High-Temperature A286 Stainless Steel—Stronger than 316 stainless steel and withstand high temperatures up to 1500° F. These nuts resist corrosion from chemicals, such as bleach and chlorine, but they won't hold up to salt water as well as 316 stainless steel.

High-Temperature Silver-Plated A286 Stainless Steel—The silver plating reduces friction during installation, so these nuts thread on smoother than others. They are stronger than 316 stainless steel and withstand temperatures up to 800° F. Nuts resist corrosion from chemicals, such as bleach and chlorine, but won't hold up to salt water as well as 316 stainless steel.

				Flange
	Thread Size	Wd.	Ht.	Dia.	Thk.	Mil. Spec.	Specs. Met	Pkg. Qty.		Pkg.
High-Temperature A286 Stainless Steel



		4-40	5/32"	1/8"	0.206"	0.01"	—	NAS1291C04M	1	000000000	00000


		High-Temperature Silver-Plated A286 Stainless Steel



				4-40	5/32"	1/8"	0.206"	0.03"	MS21043-04	—	5	000000000	00000

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.

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



	4-40	1/4"	7/64"	220	100	000000000	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.

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

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



	4-40	1/4"	7/64"	220	50	000000000	00000


	Black-Oxide 18-8 Stainless Steel



			4-40	1/4"	1/8"	220	100	000000000	00000

Locknuts with External-Tooth Lock Washer

To save on assembly time, these locknuts combine a hex nut with a free-spinning lock washer, so you can twist them into place by hand. Once the nut is against your material surface, tighten it down with a wrench or socket to get the teeth to bite. Although these nuts install more quickly than nylon-insert locknuts, they don't hold up as well against vibration and can mar your material. As long as the washer stays intact, you can reuse them. Also known as Keps and K-lock nuts.

Low-Strength Steel

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

Black-Zinc-Plated Steel—The dark zinc plating withstands occasional exposure to moisture while creating a finished look.

	Wd.	Ht.	Washer OD	Pkg. Qty.		Pkg.
Zinc-Plated Steel



	4-40	1/4"	7/64"	0.281"	100	000000000	00000


	Black-Zinc-Plated Steel



			4-40	1/4"	1/8"	0.281"	100	000000000	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.	Washer OD	Pkg. Qty.		Pkg.
18-8 Stainless Steel



	4-40	1/4"	1/8"	0.281"	100	000000000	00000

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.

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



		4-40	1/4"	5/32"	25/64"	0.02"	25	000000000	00000

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.

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



	4-40	1/4"	5/32"	MS21044-N04	250	10	000000000	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.	Mil. Spec.	Insert Max. Temp., ° F	Specs. Met	Pkg. Qty.		Pkg.
18-8 Stainless Steel



	4-40	1/4"	5/32"	MS21044C04	250	NASM21044	10	000000000	000000

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.

Low-Strength Steel

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

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



	4-40	1/4"	1/8"	MS21083N04	250	NASM21083	10	000000000	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.	Mil. Spec.	Insert Max. Temp., ° F	Specs. Met	Pkg. Qty.		Pkg.
18-8 Stainless Steel



	4-40	1/4"	1/8"	MS21083C04	250	NASM21083	10	000000000	000000

Nylon-Insert Locknut Assortments

With a range of sizes on hand, you can use these assortments to fasten a variety of parts. 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.

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

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.

	Locknuts Included	Container Type		Each
Zinc-Plated Steel



	550	4-40 (100 Pieces) 6-32 (100 Pieces) 8-32 (100 Pieces) 10-24 (100 Pieces) 10-32 (100 Pieces) 1/4"-20 (50 Pieces)	Plastic Box	000000000	000000


	18-8 Stainless Steel



			550	4-40 (100 Pieces) 6-32 (100 Pieces) 8-32 (100 Pieces) 10-24 (100 Pieces) 10-32 (100 Pieces) 1/4"-20 (50 Pieces)	Plastic Box	000000000	00000

Thread-Forming Locknuts

Form threads into these nuts for a tight fit that won't loosen under vibration. They are partially threaded, so when you fasten them onto metal screws, the tightening action will cut the rest of the threads. Nuts are not reusable.

Plastic

Nylon 6/6—Slippery and abrasion resistant, these nuts thread on smoothly and stand up to daily wear and tear. Nylon will swell and weaken when exposed to moisture, so it's best for dry environments.

	Wd.	Ht.	Pkg. Qty.		Pkg.
White Nylon 6/6



	4-40	1/4"	3/32"	100	000000000	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.

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.

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.

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



	4-40	1/4"	5/32"	MS21045-04	5	000000000	000000

Corrosion-Resistant Stainless Steel

Silver-Plated 18-8 Stainless Steel—The choice for wet and outdoor environments, these locknuts won't rust. The silver plating adds lubricity to help them thread on smoothly during installation.

	Wd.	Ht.	Mil. Spec.	Specs. Met	Pkg. Qty.		Pkg.
Silver-Plated 18-8 Stainless Steel



	4-40	1/4"	9/64"	MS21046C04	NASM21046	1	000000000	000000

Distorted-Thread Cap Nuts

For a stronger hold than nylon-insert cap nuts, these have irregularly shaped threads that grip the screw to resist loosening. The threads prevent the nut from backing out in high-vibration applications, 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. They have a smooth, rounded head that conceals and protects protruding screw threads while creating a finished look. Nuts are not reusable.

Stainless Steel

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

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



	4-40	5/32"	1/4"	1/4"	10	000000000	000000

Assortments of Locknuts with External-Tooth Lock Washer

With a range of sizes on hand, you can use these assortments to fasten a variety of parts. These locknuts combine a hex nut with a free-spinning lock washer, so you can twist them into place by hand and save on assembly time. Once the nut is against your material surface, tighten it down with a wrench or socket to get the teeth to bite. Although these nuts install more quickly than nylon-insert locknuts, they don't hold up as well against vibration and can mar your material. As long as the washer stays intact, you can reuse them. Also known as Keps and K-lock nuts.

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

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.

	Locknuts Included	Container Type		Each
Zinc-Plated Steel



	600	4-40 (100 Pieces) 6-32 (100 Pieces) 8-32 (100 Pieces) 10-24 (100 Pieces) 10-32 (100 Pieces) 1/4"-20 (100 Pieces)	Plastic Box	000000000	000000


	18-8 Stainless Steel



			600	4-40 (100 Pieces) 6-32 (100 Pieces) 8-32 (100 Pieces) 10-24 (100 Pieces) 10-32 (100 Pieces) 1/4"-20 (100 Pieces)	Plastic Box	000000000	00000

Locknuts with Spring-Lock Washer

The curved, spring-like washer grips surfaces more tightly than tooth-lock washers for better vibration resistance. These locknuts also install just as quickly since you can twist them into place by hand. Once the nut is against your material, tighten it down with a wrench or socket. The washer's smooth edges won't mar the surface like teeth can. As long as the washer stays intact, you can reuse these locknuts.

Low-Strength Steel

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

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.

	Wd.	Ht.	Washer OD	Pkg. Qty.		Pkg.
Zinc-Plated Steel



	4-40	1/4"	9/64"	0.328"	25	000000000	000000


	Black-Oxide Steel



			4-40	1/4"	9/64"	0.328"	50	000000000	00000

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.
Cadmium-Plated Steel



	4-40	1/4"	5/32"	10	000000000	000000

Press-Fit Locknuts for Sheet Metal

The nylon insert inside these nuts grips screws so they won't back out or loosen. Use them to keep thin parts fastened on engines and other vibrating machinery.

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

18-8 Stainless Steel—The choice for wet and outdoor environments, these nuts resist rust and mild chemicals.

	For Maximum Panel Hardness	Head Ht.	Flange Dia.	Pkg. Qty.		Pkg.
Nylon-Insert Locking







	Steel

	4-40	5/32"	0.05"	Brinell 205, Rockwell B95	0.09"	0.2"	10	000000000	00000




			18-8 Stainless Steel

			4-40	5/32"	0.05"	Brinell 205, Rockwell B95	0.09"	0.2"	10	000000000	0000

Self-Aligning Press-Fit Locknuts for Sheet Metal

Fasten together unaligned parts and keep screws in place. The threaded hole within these press-fit nuts shifts up to .01" in any direction, so you can fasten together thin panels with off-center holes. The irregularly shaped threads tightly grip screws to prevent them from loosening or backing out under vibration.

18-8 Stainless Steel—The choice for wet and outdoor environments, these nuts resist rust and mild chemicals.

					Head		Barrel
	Thread Size	Drill Bit Size	For Min. Panel Thk.	For Maximum Panel Hardness	OD	Ht.	Dia.	Ht.	PEM® Part No.	Pkg. Qty.		Pkg.
Distorted-Thread Locking







				18-8 Stainless Steel

				4-40	L	0.038"	Brinell 125, Rockwell B70	0.36"	0.038"	9/64"	3/16"	LAC-440-1MD	5	000000000	00000


				4-40	L	0.054"	Brinell 125, Rockwell B70	0.36"	0.054"	9/64"	3/16"	LAC-440-2MD	5	000000000	0000