A universally accepted distinction between a screw and a bolt does not exist.
In common usage the term screw refers to smaller (less than 1/4 inch) threaded fasteners, especially threaded fasteners with tapered shafts and the term bolt refers to larger threaded fasteners that do not have tapered shafts. The term machine screw is commonly used to refer to smaller threaded fasteners that do not have a tapered shaft.
snip..More specific terms for threaded fastener types that include the word screw or bolt (such as machine screw or carriage bolt) have more consistent usage and are the common way to specify a particular kind of fastener.
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