Flat washers are one of the most widely used components in industrial fastening. They are also among the most frequently overlooked components during the specification process. Select the right washer and it quietly does its job for the life of the assembly. Select the wrong one and you may face loosening fasteners, damaged surfaces, accelerated wear, or failed joints.
This guide covers the main types of flat washers stocked and manufactured by Superior Washer: round flat washers, standard washers, shims, and military-specification washers. Each section includes sizing and specification notes, material availability, and more.
Editor’s Note: The information provided in this article is intended for general informational purposes only and should not be interpreted as engineering, design, material selection, or application-specific guidance. Material requirements and specifications vary by application and should be evaluated according to individual project requirements.
What Is a Flat Washer?
A flat washer is a thin, flat disc with a centrally located hole, used in conjunction with a threaded fastener such as a bolt, screw, or nut. Simple in form, but important in function. In any bolted assembly, the fastener head or nut bears down against the mating surface as it is tightened. Without a washer, that clamping force is concentrated on a small contact area, which can damage the surface, deform softer materials, or allow the fastener to pull through entirely. A flat washer spreads that load across a larger area, protecting the mating surface and improving the overall integrity of the joint.
Flat washers also play a role in preventing loosening. By providing a smooth, consistent bearing surface, they reduce the friction variation that can cause fasteners to back out over time under vibration or dynamic loading. In assemblies where precise clamping force matters, the consistency of the washer surface is part of the design. Browse the flat washers collection or contact us.
Types of Flat Washers
Not all flat washers are the same. The type you need depends on the geometry of your assembly, the fastener system specified, and whether commercial catalog sizes meet your requirements. Here is an overview of the main types Superior Washer stocks and manufactures.
Round Flat Washers
Round flat washers are the foundation of the flat washer product line. Available in more than 25 materials, they are the standard choice for general-purpose load distribution, surface protection, and fastener bearing in industrial assemblies across virtually every sector.
Superior Washer stocks round flat washers in a wide range of materials including stainless steel, aluminum, brass, copper, nylon, carbon steel, and specialty alloys. Standard sizes are available in stock, with additional dimensions available on request.
Standard Flat Washers
Standard flat washers are the most common type: round, flat discs with a central hole, available in a broad range of sizes and materials. They are used across general-purpose industrial fastening wherever load distribution and surface protection are required.
Two sizing conventions are widely used in the United States:
- SAE washers have a smaller outside diameter relative to the bolt size and tighter dimensional tolerances. They are commonly specified in automotive, precision manufacturing, and applications where a compact bearing surface is appropriate.
- USS washers have a larger outside diameter and more generous tolerances. They are commonly used in general industrial, structural, and construction-related applications where a broader bearing surface is needed.
Superior Washer stocks SAE and USS flat washers in stainless steel, aluminum, brass, copper, nylon, carbon steel, and other materials.
View standard washers.Shims
Shims are thin, flat components used to fill gaps, adjust clearances, and achieve precise spacing in mechanical assemblies. Where a standard washer provides a fixed bearing surface, a shim provides a controlled, variable thickness that allows fine dimensional adjustment.
Shims are available in a wide range of thicknesses and are commonly used in stacks. Multiple shims can be layered together to build up to a required dimension, making them particularly valuable in applications where the exact gap cannot be predicted in advance or where wear over time requires periodic re-shimming.
Common applications include bearing and shaft alignment, gearbox and transmission assembly, tooling and fixture setups, and any situation where components must be positioned to a precise clearance or fit. Shims are available in stainless steel, aluminum, brass, copper, and other metals.
Military Washers
Military-specification (mil-spec) washers are manufactured and certified to AN, NAS, and MS standards established by the U.S. Department of Defense and related aerospace and defense bodies. These standards define not only the dimensional requirements of the washer but also the material, finish, testing, and traceability requirements. These are things that commercial catalog washers are not designed to meet.
The difference matters in defense procurement. A commercial washer and a mil-spec washer may look identical, but the mil-spec part carries documentation: material certifications, lot traceability, and dimensional inspection records. These are records that defense contractors and government programs are required to have on file.
Superior Washer stocks a broad range of military specification washers, including:
- AN series: AN901, AN960, AN961, AN970
- NAS series: NAS1149, NAS620, NAS1515, NAS1587
- MS series: MS15795, MS20002, MS21206, MS27183, MS51412, MS51496, MS51859, and others
Superior Washer holds CAGE codes 1L160 (Hauppauge, NY) and 4WUG7 (Rock Hill, SC) and adheres to a ISO 9001:2018 QMS system. These are credentials that support sourcing through defense supply chains and government contractor programs.
Browse the military washers collection, including AN960, AN961, and NAS1149.
Understanding Flat Washer Sizes
Flat washer dimensions are defined by three primary measurements: inside diameter, outside diameter, and thickness. Understanding how each of these affects performance is the starting point for any washer selection.
| Dimension | Definition | What It Affects |
| Inside Diameter (ID) | The hole diameter; must clear the fastener shank | Fit on the fastener; typically specified slightly larger than the bolt or screw diameter |
| Outside Diameter (OD) | The outer edge diameter of the washer | Bearing surface area and load distribution. A larger OD spreads load over a wider area |
| Thickness | The depth of the washer | Load capacity, assembly stack height, and gap-filling capability |
Two sizing conventions are used for standard flat washers in the United States. SAE washers have a smaller OD relative to the bolt size. They are compact and commonly specified where a tighter bearing surface is appropriate. USS washers have a larger OD and provide a broader bearing surface, making them more common in structural and general industrial fastening.
Have a sizing question? Contact our team or request a quote.
Industrial Applications for Flat Washers
Flat washers are used across virtually every industrial sector, but the type, specification, and material requirements vary significantly depending on the application. Here is an overview of the industries and environments where flat washers from Superior Washer are commonly found.
Military and Defense
Defense and aerospace procurement requires washers that meet documented military specifications, not just commercially equivalent parts. Mil-spec washers in the AN, NAS, and MS series are used across military aircraft, rotorcraft, ground vehicles, naval vessels, and defense OEM assemblies. The distinction from commercial washers is not just dimensional: it includes material traceability, lot certification, and compliance with the specific standard called out in the program's engineering documentation.
Standard commercial washers are typically not acceptable substitutes in defense applications, even when the dimensions appear to match. Superior Washer's CAGE codes (1L160 and 4WUG7) and adherence to a ISO 9001:2018 QMS system are the relevant credentials for defense procurement sourcing.
Explore the military washers collection.Aerospace
Beyond military applications, commercial aerospace assemblies also rely heavily on flat washers and shims. Airframe structures, engine mounts, avionics enclosures, and interior components all use washers, often to NAS or AN specifications even in non-military programs. Shims are commonly used in aerospace for achieving precise fits in structural joints and accommodating manufacturing tolerances in assembled structures.
Material selection in aerospace typically emphasizes lightweight options and high-performance alloys.
Electronics and Electrical Equipment
Electronic and electrical assemblies use washers for more than mechanical fastening. Non-metal washers such as nylon, PTFE, and fiber are commonly used to provide electrical insulation and prevent galvanic contact between dissimilar metals. Brass and copper washers are used in grounding applications and power distribution connections where conductivity matters.
Automotive and Heavy Equipment
SAE flat washers are among the most widely used fastener components in automotive OEM manufacturing and maintenance, repair, and operations (MRO). From engine assemblies to body panels to underbody structural components, flat washers are a constant across the vehicle.
Shims are also commonly found in drivetrain and suspension assembly. Bearing preload adjustment, shaft alignment in transmissions, and clearance control in differential and gearbox assemblies all rely on precise shimming to achieve the correct fit.
Industrial Manufacturing and OEM
General industrial manufacturing, process equipment, and high-volume OEM assembly represent the broadest use case for flat washers. Standard washers in a range of materials are used across machinery, conveyors, pumps, frames, and enclosures.
How to Choose the Right Flat Washer
Selecting the right flat washer involves working through a short set of decisions in sequence. The brief guidance below is a starting point. Always verify against the applicable design specification, engineering drawing, or industry standard for your application.
| Decision | Options | Determined By |
| Type | Round Flat, Standard, Shims, Military | Fastener geometry, hole pattern, specification requirements, and whether a catalog part meets the need |
| Size | ID, OD, Thickness | Fastener diameter (ID), required bearing area (OD), and assembly stack-up (thickness) |
| Standard | SAE, USS, AN, NAS, MS | The design specification or industry standard called out on the engineering drawing |
| Material | Stainless, aluminum, brass, nylon, specialty alloys, and more | Operating environment, corrosion exposure, temperature, conductivity, and weight |
The Right Washer Type Makes All the Difference
The right flat washer supports more than a secure fit. It helps protect mating surfaces, maintain alignment, meet specification requirements, and improve the reliability of the finished assembly.
Whether your project calls for a standard catalog washer or a specialty configuration, Superior Washer can support your sourcing needs with U.S.-made washers produced in Hauppauge, New York, and Rock Hill, South Carolina. For help identifying the right washer for your requirements, contact our team today.
Frequently Asked Questions
Below are answers to some of the most common questions related to flat washers.
What does a flat washer do?
A flat washer distributes the clamping load of a fastener over a larger surface area, protecting the mating surface from damage and deformation. It also provides a consistent bearing surface that helps maintain clamp force and resist loosening under vibration.
What is the difference between SAE and USS washers?
SAE washers have a smaller outside diameter relative to the bolt size and tighter dimensional tolerances. USS washers have a larger outside diameter, providing a broader bearing surface. The choice between them depends on the load distribution requirement and the standard specified in the design.
What types of flat washers does Superior Washer stock?
Superior Washer stocks and manufactures round flat washers (available in a wide range of materials), standard flat washers (SAE and USS), shims, and military-specification washers (AN, NAS, and MS series).
What is a shim and when is it used?
A shim is a thin flat component used to fill gaps, adjust clearances, and achieve precise dimensional spacing in mechanical assemblies. Shims are commonly used in bearing and shaft alignment, gearbox and transmission assembly, and tooling setups where fine dimensional adjustment is required.
What are odd shaped washers used for?
Odd shaped washers are used in assemblies where a standard round washer cannot accommodate the geometry of the shaft, fastener, or mating component. Common applications include keyed and D-profile shafts, non-round hole patterns, and assemblies requiring locking, slotted installation, or identification tabs.
What are military-specification (mil-spec) washers?
Military-specification washers are manufactured and certified to AN, NAS, or MS standards established by the U.S. Department of Defense and related aerospace and defense bodies. These standards define dimensional requirements, material specifications, finish, and traceability requirements that commercial washers are not designed to meet.
What is the difference between AN960 and NAS1149 washers?
AN960 and NAS1149 are two common military washer specifications. AN960 is a general-purpose flat washer used across a wide range of aerospace and defense fastening applications. NAS1149 covers a specific range of flat washers with defined material and dimensional requirements commonly found in aerospace structural applications. The applicable specification is always determined by the engineering drawing or program documentation.
Does Superior Washer stock metric flat washers?
No. Superior Washer's standard flat washer stock follows SAE and USS sizing conventions rather than metric. For metric fastener systems, contact our team to discuss sourcing options.
Where can I find material and grade information for flat washers?
For detailed material information, contact our team.