SCREWS vs BOLTS What's the difference between a screw and a bolt,? Well, a screw differs from a bolt in that it mates with an internal thread into which it is tightened or released by turning its head with the appropriate driver. A little nomenclature caveat is in order here. Lots of folks call any threaded fastener larger than about ¼" in diameter a "bolt." While that's not technically correct, I won't split hairs here about the absolute technical definition; instead, from this point on, let's just assume bolts can sometimes be inserted directly into threaded holes and screws can be used with nuts - even though that's not the usual scheme of things, I think you get the idea here, right? Screws vary widely in design, size and purpose. Hex cap screws are frequently found throughout automobile construction and are frequently considered the same as bolts (in actuality, cylinder head bolts are really hex cap screws). Screws are found with hex socket caps, Torx socket caps and even slotted and Phillips heads. There are wood screws, sheet-metal screws, self-tapping screws and many, many others. Other than those holding engines, transmissions and drive gear together, the most commonly found ones on automobiles are self-tapping and trim screws. Machine screws - these are used for the assembly of metal parts, so virtually every one of these on a car and most other metal machinery is a machine screw. Studs - these are rods that are threaded on either one or both ends, and the thread type is often different on opposing ends. The most common studs found on automobile and other engines are on the intake and exhaust manifolds. WASHERS What would you call a bolt or screw without a washer? Loose is a good epithet, for starters! Most fasteners are installed where vibration and temperature changes occur. Mechanical motion, over time, causes fasteners to back off and loosen, hence the need for something to help prevent that effect: the washer. The primary function of a washer is to provide a surface against which the head of the fastener or surface of a nut can bear. Flat washers do this very well and spread loads, but normally don't help to keep the fastener tight. Lock washers are designed to keep fasteners from loosening. They come in many forms, the two most common being the split (helical) ring washer and the toothed washer. Split washers function in much the same way that helical springs do; when the bolt is tightened sufficiently, the ends of the washer come together under compression, resisting movements of the bolt by creating a certain degree of friction against which the bolt would have to overcome. Toothed washers work very well because their many teeth bite into the surface against which the head or nut bears, creating large amounts of friction. They come in external, internal and internal-external tooth forms. It's interesting to note that the softer the surface under the teeth, the better the locking washer works.
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