Quick answer
Choose a flat washer whose inner diameter (ID) matches your screw or bolt size, outer diameter (OD) and thickness suit the load, and material finish resists the environment. For #6 fasteners, a .156" ID washer like the #6 x 3/8" OD steel trivalent zinc washer is a reliable starting point.
Why washer size matters more than you think
The washer's ID must be slightly larger than the screw or bolt diameter to fit without binding, but not so large that the fastener head pulls through. The OD should be wide enough to spread the clamping load over the surface without overlapping adjacent hardware. Thickness affects load distribution: thicker washers handle higher torque without deforming, but may not fit in tight clearances.
For #6 screws (nominal diameter .138"), the standard ID is .156" — that's the ID of the #6 flat washer. A 3/8" OD provides good load spread for most light-to-medium duty applications. The .030" thickness is a common general-purpose choice, balancing strength and stack height.
Material and finish trade-offs
Steel washers with a trivalent zinc finish offer a cost-effective balance of strength and moderate corrosion resistance. Trivalent zinc is an environmentally friendlier alternative to hexavalent zinc, and it protects against rust in indoor or sheltered outdoor use. For wet or marine environments, stainless steel or nylon washers would be more appropriate, though they cost more.
The steel trivalent zinc washer is a good default for general construction, automotive, and equipment assembly where the washer is not directly exposed to salt spray or chemicals.
Decision rules for flat washer selection
- Match ID to screw size: The washer's inner diameter must be slightly larger than the screw or bolt diameter. For #6 fasteners (0.138"), use a #6 washer with .156" ID. For metric, choose ID 1-2 mm larger than the bolt diameter. Check the screw's nominal diameter and pick the washer ID from a sizing chart.
- Choose OD for load and clearance: A larger OD spreads the clamping force over a wider area, reducing surface damage. But ensure the OD does not overlap adjacent fasteners or interfere with other parts. For #6 screws, 3/8" OD is a common general-purpose size.
- Pick thickness for torque and stack height: Thicker washers (e.g., .030" or more) resist deformation under higher torque. Thinner washers save space but may crush or embed in soft materials. For most #6 applications, .030" thickness works well. If you are tightening to a high torque or using soft materials, consider a thicker washer.
- Select material and finish for the environment: Steel trivalent zinc is suitable for indoor and sheltered outdoor use. For wet, salty, or chemical environments, upgrade to stainless steel or nylon. For high temperatures, use brass or copper. Match the finish to the worst expected exposure — not just the assembly environment.
Product facts
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#6 x 3/8" OD Flat Washers | .030" Thick | Steel Trivalent Zinc (5000 / Pkg.): These flat washers are manufactured from Steel with a trivalent zinc finish. They are well suited to distributing load and protecting surfaces under fasteners. The steel core provides strength for clamping, while the trivalent zinc coating resists corrosion in most indoor and sheltered outdoor conditions without the environmental concerns of older finishes.
#6 x 3/8" OD Flat Washers | .030" Thick | Steel Trivalent Zinc (5000 / Pkg.)
Who should use this washer?
Best for
- General assembly and maintenance with #6 screws in dry or sheltered conditions
- Applications needing a cost-effective, corrosion-resistant washer for light-to-medium loads
Not for
- Marine or chemical-exposure environments where stainless steel or nylon is required
- High-torque or heavy-load applications requiring thicker or larger OD washers