The Inside diameter conforms to SAE Standards. The small ID provides a more precise fir on bolts, studs, etc.
The Outside diameter conforms to USS standards. This provides a washer that is up to 36% larger than standard hardened washers.
USAE Washers are also up to 37% thicker than standard hardened washers for greater strength
Washers are made from mild steel. They are case hardened to 60HRc and have a black oxide finish.
Bolt size is stamped on each washer for easy identification