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Many people think that a screw and a bolt are different primarily in their heads. Screws have slots for screwdrivers. Bolts are said to have polygonal heads and are wrenched tight. None of that is true.

In basic mechanics, the difference between a screw and a bolt is solely the means by which it is secured. A screw is threaded into the object it means to attach to. Wood screws are the archetypical screw, because the threads attach directly to the wood. In metal, if the screw attaches into a threaded hole, it's a screw. In engines, the head bolts aren't really head bolts, they're head screws because the block is tapped and the 'head bolts' attach directly to the block

A bolt is held in place by a nut, that is separate from what the bolt is used to secure. Boltholes are not threaded, and the bolt extends through them to be clamped by a nut the other side.

The short rule is, if it has a nut, it's a bolt. If it doesn't, it's a screw. I hope this clears things up, but expect that I won't change common usage.

Some apparently commonly held notions of the defining characteristics of bolts vs screws are incorrect and inherently ambiguous. One consequence of some confused definitions is that a bolt would suddenly be transformed into a "screw" as soon as its nut is removed. That would be arbitrary and inaccurate. The proper distinction is simply that a bolt is a headed externally threaded fastener that has a standard bolt thread (there are many different standards), and can therefore accept a nut (of the appropriate bolt-thread standard). If the nut is removed from the bolt, the bolt is of course still a bolt. A bolt does not need a nut for it to be a bolt. This means that a cylinder-head bolt is a bolt because it has a bolt thread, and could in principle accept a nut. A screw on the other hand does not have a standard bolt thread, and therefore does not accept a nut. An additional qualifier is that bolts are not tapered, but screws may or may not be tapered.

The proper definition makes no assumptions about size, head style, shank length or application, it simply defines the objects in question.

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