Bolt (fastener)

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A bolt, in the most common usage, is a threaded fastener with a nut-like head intended to be turned with a wrench. If the threaded shaft does not have some other means of being held in the hole, such as threads cut into the hole, the shaft end will be fastened with a nut. Quite often, two wrenches are used simultaneously to tighten a bolt.

One example of a bolt that does not need a wrench to tighten the head is a carriage bolt. Such a bolt has a round head for neat appearance, but, below it, are square faces that will jam into the top of the drilled hole, keeping the bolt from turning. In wood, the carriage bolt may even be hammered down to get the faces to lock in place. A nut does fasten the shaft end.

The line is blurred between machine screws and bolts. Both can be fastened with a nut, or be screwed into threads in the hole. The only real difference is size.