Zinc Plated Flat Washers: Sizing and Corrosion Tips
By Mutual Screw & Supply | Published: 2026-10-16
Category: Tips & Care
Practical answers on zinc plated flat washer sizes, thickness, OD/ID matching, corrosion limits, and care so you pick the right washer the first time.
Quick answer
Match the washer's inside diameter to the fastener's nominal diameter, then choose an outside diameter and thickness that spread load without interfering with the joint. For indoor or dry applications, zinc plated steel flat washers are a cost-effective choice; for wet, salty, or chemical exposure, upgrade to stainless or another corrosion-resistant finish. Buy a few thicknesses if you are unsure and test-fit before committing to a full pack.
How to read OD and ID when matching a flat washer to a bolt
The inside diameter (ID) is the hole size. It should be slightly larger than the fastener's nominal diameter so the washer slips on without binding, but not so large that the washer wanders off-center. For a #10 screw, an ID around .187" is typical; for a 1/4" bolt, look for an ID near .250". The outside diameter (OD) controls bearing area. A wider OD spreads load over more surface, which helps on soft materials like aluminum or plastic, but it can also overlap a countersink or interfere with nearby hardware. Check clearance before choosing an oversized washer.
What washer thickness actually does in a joint
Thickness affects stiffness and how much the washer can conform to an uneven surface. A thin washer (around .050") is common for general fastening where space is tight and load is moderate. Thicker washers resist deformation better under high clamp load but take up more grip length, which can matter if your bolt is already near its length limit. If you are replacing a washer, measure the old one with calipers rather than guessing; a few thousandths of an inch can change fit.
When zinc plating is enough and when it is not
Zinc plating protects steel by acting as a sacrificial coating: the zinc corrodes before the steel does. In dry indoor environments, that protection can last a long time. In outdoor, marine, or frequently wet conditions, the coating eventually depletes, especially at scratches and cut edges. If the joint sees salt spray, pool chemicals, or repeated washdowns, stainless steel or a heavier-duty coating is usually the better long-term choice. Zinc plated washers are also not ideal for high-temperature service, where the coating can degrade.
Steel washer care and storage basics
Keep zinc plated washers dry and away from open acids or strong cleaners. In a humid shop, store them in sealed bins or original packaging rather than loose in an open tray. If you handle them with bare hands, a light oil film can help, but avoid mixing wet or dirty washers back into a clean bin. Inspect used washers for white rust or red rust before reuse; white rust is a zinc corrosion product that signals the coating is working, while red rust means the steel is exposed and the washer should be replaced.
Washer selection FAQ
- Can I use a flat washer with a lock washer?
- Yes, but order matters. A common arrangement is a flat washer against the material to spread load, then a lock washer under the nut or bolt head to resist vibration. In some joints, a flat washer alone is enough; adding a lock washer can reduce the flat washer's bearing area if the lock washer is smaller. Test the stack-up: if the lock washer sits fully on the flat washer, the combination works; if it overhangs, use a larger flat washer or skip one.
- How do I know if my washer is too thick?
- If you cannot get the nut or bolt head to seat properly, or if the bolt runs out of thread before reaching the required torque, the washer may be too thick. Thickness also changes the grip length, so a thicker washer can require a longer bolt. Check that at least one full thread is visible past the nut after tightening.
- Will zinc plated washers rust?
- The steel core can rust once the zinc coating is depleted, scratched through, or damaged. Zinc plating slows corrosion but does not stop it indefinitely. In wet or salty conditions, expect the coating to sacrifice itself over time. Inspect periodically; replace washers showing red rust.
- What is the difference between a flat washer and a fender washer?
- A fender washer has a much larger outside diameter relative to its inside diameter. That extra bearing area is useful for soft materials or oversized holes, but it can also interfere with nearby parts. A standard flat washer is a better general-purpose choice when clearance is tight. Choose the smallest OD that still covers the hole and spreads load adequately.
Catalog facts that affect your choice
- #10 x 1/2" OD Flat Washers | .050" Thick | Steel Zinc Plated (2000 / Pkg.): These flat washers are steel with a zinc plated finish, .187" ID x .500" OD x .050" thick, sold 2000 per package. The .187" ID fits #10 fasteners, and the .050" thickness is a general-purpose choice for light-to-moderate load. The 2000-piece pack suits repeat assembly or maintenance stock.
- M3 Medium Lock Washers | Steel Brass Plated (1000 / Pkg.): These M3 lock washers are steel with a brass plated finish, sold 1000 per package. Lock washers are a different function from flat washers: they resist loosening under vibration. If your joint vibrates, a lock washer may be needed in addition to or instead of a flat washer.
- 1/4-20 X 2" Hex Head Cap Screws | USA Made | PFC9 Alloy Steel Yellow Zinc (100 / Pkg.): These 1/4-20 x 2" hex head cap screws are USA-made PFC9 alloy steel with yellow zinc CR+6 finish, sold 100 per package. A 1/4-20 bolt needs a washer with a matching ID around .250". The yellow zinc finish offers corrosion protection similar in principle to clear zinc, so pair it with a compatible washer finish to avoid dissimilar-metal issues.
