SAE flat washers are machined to a tighter specification and have a smaller outside diameter. Flat Washer, Fits Bolt Size M10 ,18-8 Stainless Steel 20 PK.USS flat washers have a large, wider body.
. A carriage bolt has a domed or countersunk head, and the shank is topped by a short square section under the head.The square section grips into the part being fixed (typically wood), preventing the bolt from turning when the nut is tightened.