Key Takeaways
- Function separates them, not shape. All three can be flat rings of the same diameter. What differs is the job each one does inside the assembly.
- Shims correct dimension. A shim fills a measured gap or adjusts alignment, so its thickness tolerance is the defining specification. Precision shims are often held to .0005 inch.
- Washers manage load. A washer distributes clamping force, protects a surface, or seals a joint under a fastener. Its bearing area matters more than its exact thickness.
- Spacers establish distance. A spacer sets and holds a designed separation between two components. It is a structural dimension, not a correction.
- Specifying the wrong one costs time. Ordering a washer when the assembly needs a shim usually produces a part that fits but doesn’t perform. Naming the function on the print prevents that.
Three Parts That Look Identical and Are Not
Put a precision shim, a flat washer, and a spacer of the same outside diameter on a bench, and most people cannot tell them apart. All three may be flat, round, and drilled through the center. The confusion is understandable, and it causes real purchasing problems.
The distinction lies in what the part is asked to do. That purpose drives which dimension carries the tight tolerance, which material makes sense, and how the part should be inspected before it ships.
We have manufactured all three for more than a century, and the explanation below is what we give engineers deciding what to call the part on their drawing.
What a Shim Does
A shim exists to take up space that should not be there, or to move one component relative to another by a known amount. It is a corrective part, sized to a measurement taken from the assembly rather than from a table. Because the entire purpose is dimensional, thickness is the controlled feature, and precision shims are routinely specified to within .0005 inch.
Shims frequently ship in graduated sets so a technician can build up to a required dimension during assembly. Alignment shims under a bearing housing, laminated shims that peel to thickness, and thin flat shims that correct stack-up tolerance all serve the same corrective role.
Because a shim is measured from the assembly rather than the drawing, the same part number may need several thicknesses on hand. Ordering a graduated range up front costs far less than stopping an assembly to wait on a .002 inch part.
When to Specify a Shim
The situations below all call for a shim rather than a washer or spacer, because each one involves correcting a dimension that came out other than intended.
- Closing a measured gap: The assembly came out slightly oversized and needs a known thickness added.
- Correcting alignment: Shimming under one mounting foot brings a shaft or housing back into position.
- Setting preload or clearance: Bearing and gear assemblies use shim thickness to establish running clearance.
- Compensating for stack-up: Several parts at the edge of tolerance combine into an error a shim absorbs.
What a Washer Does
A washer sits under a fastener and manages the relationship between that fastener and the surface beneath it. It spreads clamping load across a wider area, protects a soft or finished surface from galling as the fastener turns, and in some designs provides a seal or maintains electrical contact.
Because the job is load distribution rather than dimensional correction, bearing area and material properties usually matter more than a tight thickness tolerance. That said, many applications call for both, particularly in electronics and instrumentation, which is why we hold many of the flat washers we produce to the same tolerances as our shims.
Material choice carries most of the design intent here. Copper for conductivity and sealing, phosphor bronze where a degree of spring is useful, stainless for corrosion resistance, and plastics where isolation is required. The right answer depends on the part’s environment, which is worth reviewing against the industries and applications we serve.
What a Spacer Does
A spacer establishes and maintains a designed distance between two components. Unlike a shim, it is not correcting an error; the separation it creates was intended from the start and appears on the drawing as a nominal dimension.
That difference changes how the part is specified. A spacer usually carries a nominal thickness with a symmetric tolerance, while a shim is ordered to an exact measured value. Spacers also tend to be thicker and may need compressive strength that a thin shim never sees, and they are often made from plastics such as Nylon, Nylatron, or PTFE when weight matters.
Spacers also carry a load path that shims and washers often do not. A spacer holding two plates apart may see compression, vibration, and thermal cycling for the life of the product, so material selection deserves more attention than the flat profile suggests.
Choosing the Right Part for Your Assembly
Working through these five steps in order will usually resolve the naming question on its own, and it produces a print a manufacturer can quote without guessing.
Washer thickness matters in one common situation worth noting here. When a washer is stacked with other components in a controlled assembly height, its thickness becomes part of the stack-up, and at that point it is doing a shim’s job as well as its own.
- State the function first: Decide whether the part corrects a dimension, distributes a load, or sets a distance.
- Identify the controlled feature: Thickness for shims, bearing area and material for washers, nominal separation for spacers.
- Set tolerance to purpose: Do not apply shim tolerances to a washer, since tighter than necessary raises cost without benefit.
- Match material to environment: Consider temperature, corrosion, conductivity, and whether isolation is required.
- Note surface and edge condition: Burr direction and flatness matter more than most prints acknowledge, especially in thin stock.
Cost rises more steeply with tolerance than most buyers expect. Tightening a dimension from .005 inch to .0005 inch can change the inspection method, scrap rate, and run time, so apply that tolerance only where the function requires it.
Where Buyers Get It Wrong
The mistakes below come up regularly in prints we receive. None of them is difficult to avoid once you know to look for it.
- Calling everything a washer: A generic term on the purchase order invites a supplier to quote the wrong tolerance entirely.
- Over-specifying tolerance: Requiring .0005 inch on a part that only distributes load adds cost for no functional gain.
- Ignoring flatness: A part can measure correctly for thickness and still fail because it is not flat.
- Forgetting burr direction: In thin material, the burr side affects how the assembly seats.
- Assuming catalog sizes fit: Standard tables cover common fastener sizes, not the dimension your assembly actually needs.
Specifying a Custom Part
Once you know which part you need, a few habits make the quoting process faster and the first article more likely to be correct.
- Give the dimension that matters most: Tell your supplier which single feature governs whether the part works.
- Describe the application briefly: Knowing how a part is used lets a manufacturer flag a problem before production.
- Confirm material availability: Ask what is in stock, since an equivalent alloy on the shelf can shorten lead time considerably.
- Order a sample lot first: Verify fit and function in the real assembly before committing to a production quantity.
For reference on standardized flat washer dimensions, ASTM publishes F844 for plain unhardened washers. Parts outside those tables are exactly what we build, in thicknesses from .0005 inch, using the materials we stock.
Let Brewster Washers Build the Right Part
Naming the part correctly on your drawing removes most of the ambiguity from the quoting process and prevents the kind of mismatch that only appears during assembly. When the terminology is unclear, describe the function and let the manufacturer help you name it.
We make all three parts in the same facility, from the same stock, on the same equipment, so we can also tell you when the part you asked for isn’t quite the part you need. We cover that conversation during quoting, not after delivery.
Send us your blueprint, and we will confirm the specification and return pricing within 24 to 48 hours. Request a quote from Brewster Washers to begin.