A2 Stainless Steel Cup Washers are supplied to our own published dimensions — this is a trade-series article rather than one covered by a dimensional standard, so confirm the figures against the part you are replacing. The grade provides good general corrosion resistance and is largely non-magnetic; the practical service range is approx. -200 °C to +400 °C, subject to the property class and the joint.
Why Choose A2 Stainless Steel for Cup Washers
The most common stainless group for indoor and general outdoor fasteners. For cup washers, this matters when good general corrosion resistance is the primary requirement.
Mechanical & Physical Properties
| Grade designation | ISO 3506 group A2 (304 / 1.4301), commonly supplied A2-70 |
|---|---|
| Magnetic response | Largely non-magnetic |
| Service temperature | approx. -200 °C to +400 °C, subject to the property class and the joint |
| Corrosion characteristic | Good general corrosion resistance |
Galvanic & Galling Considerations
For stainless cup washers the compatibility question is galvanic rather than galling: joined to aluminum or galvanized steel in wet service, stainless drives corrosion of the less-noble metal. Isolate with a coating, sleeve or non-conductive washer where the joint sees moisture. Note also that stainless is softer than hardened carbon steel, so where the part carries a wear or shear duty, check the load against the grade before substituting.
Comparable Grades
For Cup Washers, the alternative stainless grade is A4 (316). The practical dividing line is chlorides: A2 (304) is fine until salt is involved, and A4 (316) exists for the cases where it is — coastal, marine and road-salt service. Neither reaches the strength of a plated class 8.8 or 10.9 fastener.
Using Stainless Fasteners in Aluminum
Expect the aluminum to corrode, not the fastener — stainless sits higher in the galvanic series, so the aluminum is the one that gives up material. A small fastener in a thick section spreads the loss over a large area and is usually fine; a large fastener in thin sheet concentrates it and is not. Keep water out of the joint, or isolate the two metals with a nylon or coated washer and a sealant.
Sizes of Cup Washers Available in A2 Stainless Steel
| Screw size | Inner diameter d1 (mm) | Outer diameter d2 (mm) | Thickness s (mm) | Cup depth (mm) | Seat angle (deg) | Reference standard |
|---|---|---|---|---|---|---|
| No.6 (3.5 mm) | 3.7 | 10.0 | 0.6 | 1.6 | 90 | Industry standard |
| No.8 (4.0 mm) | 4.2 | 11.5 | 0.7 | 1.8 | 90 | Industry standard |
| No.10 (4.8 mm) | 5.0 | 13.0 | 0.8 | 2.0 | 90 | Industry standard |
| No.12 (5.5 mm) | 5.7 | 15.0 | 0.9 | 2.3 | 90 | Industry standard |
| M4 | 4.3 | 11.5 | 0.7 | 1.8 | 90 | Industry standard |
| M5 | 5.3 | 13.0 | 0.8 | 2.0 | 90 | Industry standard |
| M6 | 6.4 | 15.0 | 1.0 | 2.4 | 90 | Industry standard |
Dimensions are independent of the material: every size above is available in A2 Stainless Steel.
Custom manufacturing · Made to drawing
Need this washer in a grade that is not listed?
A2 Stainless Steel is one of the grades the catalog describes. Bore and outside diameter are set by the joint rather than by the bolt, which is why an off-table size is so often the right answer. Made by stamping or punching.
Minimum order from 2,000 pieces; samples within 7 days of drawing approval.
