What Is The Difference Between A Flat Washer And A Gasket?

Although they may look similar, a flat washer and gasket serve different purposes on a wide variety of applications. Both are coupled with bolts or screws and shaped like a ring, with an O.D. (or outer diameter) and center hole. But your fastener distributor will be quick to remind you that the functions of each differ, and in most cases these fasteners are not interchangeable.

Flat Washers Vs. Gaskets

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Materials:
Flat washers are made from metal - commonly zinc plated steel or stainless steel. This gives the fastener a rigid quality. Flat washers manufactured from plastics, nylon or other soft materials could easily be confused with gaskets, which are made from a wide variety of pliable materials.
Common gasket materials include rubber, cork, paper, felt, polymer, neoprene or fiberglass. Some are made from metal, although usually only for specified purposes and applications.
Purpose:
Flat washers need to be rigid in order to properly distribute load, which is the primary purpose of using this fastener. Gaskets, on the other hand, create a seal by deforming and filling the space left between joint and fastener head. In short, gaskets stop leaks at the joint.
A rigid gasket would be ineffective. A pliable washer may partially distribute load, but not in the same way that a rigid flat washer does.

Does A Gasket Function As A Washer?

You could argue that because a gasket distributes load while creating a seal, this fastener performs a double function. An experienced fastener distributor will understand the difference between these two components, and can help you determine whether both are needed.
In many cases a sealant must be used with the gasket in order to create an effective seal. This creates a bond with the joint material and may affect load distribution, although sealants often improve gasket performance and longevity.
A flat washer sits between the bolt and joint material, but is not bonded to either. This allows for optimum load distribution, although it increases the potential for leakage in certain applications and conditions.
The difference between a flat washer and a gasket is function, since the former distributes load and the latter acts as a seal. Although certain types of flat washers may perform as a gasket (and vice versa), your fastener distributor will agree that these are two distinct components with unique functions and specifications.