The Nuts and Bolts of Threaded Fasteners, Part 1

Tom Benford Jul 8, 2008 0 Comment(s)

Threaded fasteners literally keep your Corvette together. Without these little wonders, the engine would just be a pile of metal components, your suspension wouldn't stay on, and so on. There's so much to discuss that this edition of Garage Talk is Part 1 of 2 parts so we don't gloss over any of the important stuff. That being said, let's not screw around and get right down to the nuts and bolts of it all.

The most common threaded fasteners are bolts, screws, and studs. Technically, bolts are externally threaded fasteners designed for insertion through holes in assembled parts. Specifically, a bolt is normally tightened and released by turning a mating nut.

The nomenclature of bolts can be somewhat confusing at first, so here's an explanation of what's what:

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Corp_0804_01_s Stainless_steel_hex_head_cap_screws Fas_kit_stainless_screw_kit 3/7

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Stove bolts are very common and named for their original purpose, which was the assembly of cast-iron stoves. They are available in diameters from 5.32 inch to 1/2 inch and in lengths from 3.8 inch to 6 inches. They have slotted heads for screwdrivers (some more modern versions can have Phillips heads); the heads themselves are available in round (half a sphere), flat (flat on top, tapered to threads), or pan (wide, rounded head).

Carriage bolts are a class of bolts that generally have round heads with small square sections underneath. They usually fit into square holes, thereby "locking" themselves in place so that a nut can be turned tight without spinning the bolt itself. Sometimes carriage bolts have ribbed necks rather than square ones.

Step bolts are carriage bolts that have enlarged heads that "step down" to the square neck. The flanged heads spread loads over a very large area. Bumper bolts are typically step bolts so the chromed round head gives a better appearance.

Joint bolts are tapered at the tip to act as a "self-aligning" device to center itself in a captive nut and start threading straight. They are found in many areas on automobiles and other vehicles, as well as some machinery.

T-head bolts are made with a T-shaped head instead of the usual hex or square heads. They are used in situations where there is no easy way to put a wrench on the head. Engine blocks and transmission cases frequently utilize these bolts.

Countersunk bolts have heads that are flat on top and tapered to the shaft. They usually have hex sockets (Allen), but sometimes have large Phillips sockets. They are used in applications where there is no space for the head to protrude.

Screws differ from bolts because they mate with an internal thread into which they are tightened or released by turning with the appropriate driver.

A little nomenclature caveat is in order here. Lots of folks call any threaded fastener larger than about 1/4-inch 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.

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, sheetmetal screws, self-tapping screws, and many, many others. Other than those used to hold engines, transmissions, and drive gear together, the most commonly found screws on automobiles are selftapping and trim screws.

Machine screws 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 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.




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