Q: What is the difference between a general purpose, SAE, USS and oversized washer? Are they all flat washers or different styles. And is one better than the other?
A: General purchase, SAE, USS and oversize are all flat washers and all serve the same purpose of distributing the load across a larger surface area than the bolt head itself. The differences in the washer names just refer to different dimensions of that particular sized washer. So for example for a 1/2″ washer the dimensions are all different depending on the style of flat washer you need.
General Purpose 1/2″ Washer: ID=0.531″ OD=1.25″ THK=0.055″-0.069″
SAE 1/2″ Washer: ID=0.531″ OD=1.062″ THK=0.074″-0.121″
USS 1/2″ Washer: ID=0.562″ OD=1.375″ THK=0.086″-0.132″
Oversized 1/2″ Washer ID= 0.531″ OD=1.25″ THK=0.110″-0.114″
Typically SAE washers have a larger ID (inter diameter) and smaller OD (outer diameter) than the other washers and are often used for expensive, more exotic alloys. Oversized washers have either larger diameters and/or thickness than a general purpose washer. Larger diameter oversized washers are often used for covering bigger holes and are great at distributing the force. Flat washer can also be used as spacers/levelers when their thickness is greater. One style is not better than the other, it just depends on the dimensions that your application requires.