Fender Washers
Washers & Retaining Rings · 10 specs
Direct Answer
Fender washers feature an oversized outer diameter approximately three times larger than their central hole (e.g. 1/4" ID × 1.0" to 1.25" OD), designed to bridge enlarged holes and distribute clamping force across thin sheet metal, automotive panels, and wood.
Source: ASME B18.22.1 / Automotive Fastener Standards · Last reviewed Aug 28, 2026
Specs
| Governing Standards | ASME B18.22.1 (Large Diameter Flat Washers) / SAE J485 |
| Defining Characteristic | Outer diameter is ~3× to 4× the inner bolt diameter |
| 3/16" × 1.0" Fender Washer | ID: 0.219 in (5.6 mm) | OD: 1.00 in (25.4 mm) | Thickness: 0.050 in (1.27 mm) |
| 1/4" × 1.0" Fender Washer | ID: 0.281 in (7.1 mm) | OD: 1.00 in (25.4 mm) | Thickness: 0.062 in (1.57 mm) |
| 1/4" × 1.25" Fender Washer | ID: 0.281 in (7.1 mm) | OD: 1.25 in (31.8 mm) | Thickness: 0.062 in (1.57 mm) |
| 1/4" × 1.50" Fender Washer | ID: 0.281 in (7.1 mm) | OD: 1.50 in (38.1 mm) | Thickness: 0.062 in (1.57 mm) |
| 5/16" × 1.25" Fender Washer | ID: 0.344 in (8.7 mm) | OD: 1.25 in (31.8 mm) | Thickness: 0.062 in (1.57 mm) |
| 5/16" × 1.50" Fender Washer | ID: 0.344 in (8.7 mm) | OD: 1.50 in (38.1 mm) | Thickness: 0.062 in (1.57 mm) |
| 3/8" × 1.50" Fender Washer | ID: 0.406 in (10.3 mm) | OD: 1.50 in (38.1 mm) | Thickness: 0.062 in (1.57 mm) |
| 1/2" × 2.00" Fender Washer | ID: 0.531 in (13.5 mm) | OD: 2.00 in (50.8 mm) | Thickness: 0.078 in (1.98 mm) |
Standard Fender Washer Sizing Chart
| Bolt Size | Inner Diameter (ID) | Outer Diameter (OD) | Thickness (t) | Bearing Area (A = pi(R²–r²)) | Standard Flat Washer Area Comparison |
|---|---|---|---|---|---|
| 3/16" (#10) | 0.219" (5.6 mm) | 1.000" (25.4 mm) | 0.050" (1.3 mm) | 0.748 sq in | 0.160 sq in (+367% area) |
| 1/4" | 0.281" (7.1 mm) | 1.000" (25.4 mm) | 0.062" (1.6 mm) | 0.723 sq in | 0.245 sq in (+195% area) |
| 1/4" | 0.281" (7.1 mm) | 1.250" (31.8 mm) | 0.062" (1.6 mm) | 1.165 sq in | 0.245 sq in (+375% area) |
| 1/4" | 0.281" (7.1 mm) | 1.500" (38.1 mm) | 0.062" (1.6 mm) | 1.705 sq in | 0.245 sq in (+595% area) |
| 5/16" | 0.344" (8.7 mm) | 1.250" (31.8 mm) | 0.062" (1.6 mm) | 1.134 sq in | 0.279 sq in (+306% area) |
| 5/16" | 0.344" (8.7 mm) | 1.500" (38.1 mm) | 0.062" (1.6 mm) | 1.674 sq in | 0.279 sq in (+500% area) |
| 3/8" | 0.406" (10.3 mm) | 1.500" (38.1 mm) | 0.062" (1.6 mm) | 1.638 sq in | 0.388 sq in (+322% area) |
| 1/2" | 0.531" (13.5 mm) | 2.000" (50.8 mm) | 0.078" (2.0 mm) | 2.920 sq in | 0.665 sq in (+339% area) |
Why Fender Washers Were Invented for Automobiles
Fender washers originated in early automotive assembly to mount stamped sheet-metal fenders to vehicle chassis frames:
- Tear-Out Prevention in Thin Metal: Thin 20-gauge (0.036 in) sheet metal quickly deforms or tears under the concentrated point load of a standard small flat washer. Expanding the bearing area to 1.25 or 1.5 inches reduces local surface pressure (P = F / A) by over 75%.
- Slot & Oversized Hole Bridging: Stamped automotive frames feature elongated slots for body alignment adjustments; fender washers span these oversized slots without pulling through.