S/S Flat Washer - 5/16 ID x .657 OD(1)

The S/S Flat Washer - 5/16 ID x .657 OD is a precision fastener component engineered for demanding automotive applications where bolt load distribution and vibration resistance are critical. Whether you're assembling a high-performance engine build, reinforcing suspension geometry, or executing a frame-up restoration, this stainless steel washer serves as the unsung foundation of reliable fastener integrity. ARP's flat washers are purpose-built for use alongside ARP studs and bolts in applications where standard hardware simply won't survive heat cycles, resonance, or extreme clamping pressures.

Manufactured from 300M-grade stainless steel, the S/S Flat Washer features a 5/16-inch inner diameter and .657-inch outer diameter—proportions optimized to spread bolt preload across a larger surface area without introducing excessive thickness that could complicate bolt head seating or thread engagement. The flat profile ensures full contact with component surfaces, preventing the edge-loading and stress concentration that plagued conventional washers in vintage and competition engine work. Stainless construction resists corrosion in marine environments and under long-term engine bay exposure, eliminating the risk of washers galling or fusing to studs during disassembly—a common failure mode in aluminum cylinder heads and iron blocks alike.

Builders and mechanics rely on this washer for cylinder head bolt patterns, intake manifold assembly, and any critical fastening point where vibration isolation and load distribution demand OEM-equivalent or superior hardware. Its compact 0.01-pound weight ensures minimal thermal mass while its precise outer diameter accommodates tight packaging in modern engine bay configurations. Compatible with ARP fastener systems across domestic and import platforms—from LS and small-block Chevy applications to Japanese inline engines—this washer is indispensable inventory for serious engine builders, transmission specialists, and restoration shops that refuse to compromise fastener reliability.