A bolt that keeps loosening, marks the surface, or pulls into an oversized hole is usually not a bolt problem. It is often a washer problem. If you are deciding which washer for bolt applications in a workshop, plant room, warehouse or fabrication job, the right answer depends on load, material, hole size and whether vibration is part of the job.
Washers look simple, but they do different jobs. Some spread load. Some protect the joint surface. Some help with spacing. Some are intended to resist loosening, although that point needs a bit of care because not every so-called locking washer performs well in every assembly. Choosing the wrong type can lead to damaged panels, loss of clamp force or a joint that works loose long before the bolt itself is at fault.
Which washer for bolt assemblies depends on the job
The first thing to establish is what the washer needs to do. If the joint is straightforward and you simply need to distribute load under the bolt head or nut, a plain flat washer is usually the starting point. This is the standard choice for general fastening across steelwork, machinery guards, brackets and maintenance tasks.
If the hole is oversized, the material is relatively soft, or the surface needs more support, a larger outside diameter washer may be the better option. Penny washers and repair washers are common where extra bearing area matters. They help stop the fastener head or nut from pulling into timber, sheet material, plastics or thin metal.
If vibration is the real issue, a washer alone may not be the complete answer. Spring washers are still widely used, but their effectiveness can vary depending on joint design, bolt tension and surface hardness. In many engineering applications, correct tightening, suitable nut type and proper joint design matter more than relying on a split washer to keep things tight.
The main washer types and where they fit
Flat washers
Flat washers are the default option for most bolted joints. They sit under the bolt head or nut and spread the clamping load over a wider area. That reduces local surface damage and can improve the reliability of the joint, especially on painted surfaces, aluminium, plastics or thinner steel sections.
For general trade use, if someone asks which washer for bolt fixing on a standard bracket or machine assembly, a flat washer is usually the first recommendation. The main point is to match the internal diameter to the bolt size and choose an outside diameter that gives enough support without interfering with adjacent parts.
Penny washers and large OD washers
A penny washer is essentially a flat washer with a much larger outside diameter. These are useful when the base material is softer or the hole is larger than ideal. You will see them used on sheet metal, timber, cladding, plastic panels and repair work where more surface coverage is needed.
The trade-off is access and space. A larger washer is not always practical in confined assemblies, and if the joint face is uneven, simply increasing washer diameter will not correct that issue.
Form A, Form B and other standard flat washers
Depending on the standard and product range, flat washers may be supplied in different forms and thicknesses. A thinner pattern can be fine for light general use, while a heavier pattern gives more support in harder-working assemblies. If the application carries meaningful load, it is worth checking the washer standard rather than treating all flat washers as interchangeable.
Spring washers
Spring washers, often split type, are commonly used with bolts and nuts where some resistance to loosening is desired. They exert a small spring effect and can bite slightly into the bearing surface. In lighter-duty or legacy assemblies they are still common.
That said, they are not a cure-all. On hardened surfaces, in critical joints or where proper preload control matters, a spring washer may add little real security. If a bolted joint is repeatedly loosening, look at tightening method, joint movement, thread condition and nut selection as well as the washer.
Shakeproof and serrated washers
These washers use teeth or serrations to bite into the mating surface and improve resistance to rotation. They can be useful in lighter assemblies, electrical fittings and situations where surface penetration is acceptable.
They are less suitable where the finish must be preserved or where the joint relies on a coated surface for corrosion protection. Once the teeth break the coating, the joint may become more vulnerable to rust.
Sealing washers
If the bolt passes through a panel that must resist water, dust or oil ingress, a sealing washer is the correct choice. These usually combine a metal washer with a bonded elastomer layer. They are common on roofing, enclosures and external equipment.
In this case, asking which washer for bolt use is really about environmental sealing rather than load spreading alone. The washer has to match both the fastener size and the service conditions.
How to match washer size to bolt size
The basic rule is simple. The washer internal diameter must suit the bolt diameter. An M8 bolt takes a washer sized for M8, an M10 bolt takes a washer sized for M10, and so on. In imperial or inch-based applications, the same principle applies.
What catches people out is clearance. The hole in the washer is always slightly larger than the bolt shank, but too much play can lead to poor seating. At the same time, if coatings, plating or manufacturing variation are involved, an undersized washer can bind and sit badly.
Outside diameter matters as much as internal diameter. A small washer on a thin panel may not spread load enough. An oversized washer on a machined assembly may clash with recesses or edges. Good fit is not just about getting it over the bolt.
Material matters more than many buyers expect
Washer material should normally be compatible with the bolt, the nut and the environment. For indoor dry use, bright zinc plated steel washers are common and cost-effective. For external use, damp conditions or washdown areas, stainless steel is often the better choice.
Mixing materials can create problems. A stainless bolt with a mild steel washer may leave the washer as the weak point in corrosion terms. In aggressive environments, the cheapest washer can become the part that fails first.
Hardness also matters. A washer under a high-tensile bolt should not be too soft for the load it carries. If the washer deforms under preload, clamp force can drop and the joint may loosen. In structural or heavily loaded assemblies, check the specification rather than assuming any washer of the right diameter will do.
When a washer is necessary and when it is not
Not every bolted joint needs a washer. Some flange bolts, flange nuts and purpose-designed assemblies are intended to work without one. In other cases, adding a washer can actually reduce effectiveness if it changes the bearing condition the joint was designed around.
But in everyday maintenance and fabrication work, washers are often a sensible inclusion. They reduce marking, help distribute force and can make the assembly more forgiving on painted or uneven surfaces. If you are working on sheet material, slotted holes or softer substrates, leaving the washer out is usually a false economy.
Common mistakes when choosing which washer for bolt fixings
One common mistake is using a spring washer where a plain washer is actually needed under it. If the joint surface is soft or oversized, the spring washer alone does not spread load properly. Another is choosing stainless for appearance but overlooking galling risk in the full fastener assembly.
A further issue is stacking random washers to make up space. If spacing is required, use proper spacers or purpose-made shims where possible. Piling up washers can lead to uneven loading and poor alignment.
There is also the habit of treating all washers as low-value consumables that do not need specification. In reality, the washer is part of the joint. If the bolt size, grade and finish matter, the washer specification matters too.
A practical way to decide
Start with the bolt size. Then ask four questions. What surface is being clamped, how much load is involved, is vibration present, and does the environment demand corrosion resistance or sealing? Those answers usually narrow the choice quickly.
For many general-purpose jobs, a correctly sized flat washer in a suitable material is all that is needed. If the material is soft or the hole is oversized, move to a larger OD washer. If moisture ingress matters, use a sealing washer. If loosening is a recurring problem, look beyond the washer and review the whole fastening method.
For trade buyers and maintenance teams, the best approach is consistency. Standardise common washer types and materials for the fasteners you use most often. It simplifies stockholding, reduces fitting errors and makes repeat purchasing easier, especially when sourcing bolts, nuts and washers from one supplier such as Warehouse Equip UK.
A washer is a small part of the assembly, but it has a direct effect on how well the joint performs. Choose it with the same care as the bolt, and the rest of the job usually goes more smoothly.