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Dowel pins are often used as precise locating devices in machinery. Steel dowel pins are machined to tight tolerances, as are the corresponding holes, which are typically reamed. A dowel pin may have a smaller diameter than its hole so that it freely slips in, or a larger diameter so that it must be pressed into its hole (an interference fit). When designing mechanical components, mechanical engineers typically use dowel holes as reference points to control positioning variations and attain repeatable assembly quality. If no dowels are used for alignment (e.g., components are mated by bolts only), there can be significant variation, or "play", in component alignment.

Typical drilling and milling operations, as well as manufacturing practices for bolt threads, introduce mechanical play proportional to the size of the fasteners. For example, bolts up to 10 mm (0.394 in) in diameter typically have play on the order of 0.2 mm (0.008 inches).[citation needed] When dowels are used in addition to bolts, however, the tighter dimensional tolerances of dowels and their mating holes—typically 0.01 mm (0.0004 inches)—result in significantly less play, on the order of 0.02 mm (0.0008 inches).[citation needed] Manufacturing costs are inversely proportional to mechanical tolerances and, as a result, engineers must balance the need for mechanical precision against cost as well as other factors such as manufacturability and serviceability.

There are a variety of specifications, military, ISO, DIN, ASME that pins may be made to. And size can even vary by dowel pin material. Metric dowel pins are often found in two size. In DIN 6325 standard the dowel pins are slightly larger than the nominal value. For example a 3 mm dowel pin will range from 3.002 to 3.008. In the ISO 2338 standard the dowel pins are slightly smaller - 3mm nominal range is 2.986 to 3.000. The terminology (e.g. "oversized", "standard") is not entirely consistent across suppliers. In inch pins "oversized" refers to pins that are more significantly oversized for worn out dowel pin holes. The most common inch sized pins are slightly oversized, and "Undersized" versions are also available.

In automobiles, dowels are used when precise mating alignment is required, such as in differential gear casings, engines, and transmissions.

Bolts in a bolted joint often have an important function as a dowel, resisting shear forces. For this reason, many bolts have a plain unthreaded section to their shank. This gives a closer fit to the hole and also avoids some problems with fretting wear when a screw thread bears against an unthreaded component.



Grease fittings are permanently installed by a threaded connection, leaving a nipple connection that a grease gun attaches to. The pressure supplied by the grease gun forces a small captive bearing ball in the fitting to move back against the force of its retaining spring. The arrangement is thus essentially a valve that opens under pressure to allow lubricant to pass through a channel and be forced into the voids of the bearing. When the pressure ceases, the ball returns to its closed position. The ball excludes dirt and functions as a check valve to prevent grease escaping back out of the fitting. The ball is almost flush with the surface of the fitting so that it can be wiped clean to reduce the amount of debris carried with the grease into the bearing. The convex shape of the fitting allows the concave tip of the grease gun to seal against the fitting easily from many angles, yet with a sufficiently tight seal to force the pressured grease to move the ball and enter the fitting, rather than simply oozing past this temporary annular (ring-shaped) seal. Grease fittings are commonly made from zinc-plated steel, stainless steel, or brass.


Threaded pipes can provide an effective seal for pipes transporting liquids, gases, steam, and hydraulic fluid. These threads are now used in materials other than steel and brass, including PTFE,[2] PVC, nylon, bronze, and cast iron.

The taper on NPT threads allows them to form a seal when torqued as the flanks of the threads compress against each other, as opposed to parallel/straight thread fittings or compression fittings in which the threads merely hold the pieces together and do not provide the seal. As the thread body is tapered (0.75 in/ft or 62.5 mm/m) a larger diameter keeps compressing into a smaller diameter and finally forms a seal (no clearance remains between the crests and roots of the threads because of the taper). This means that NPT fittings should be burr-free and lubricated using a lubricating material like lubricating paste or tape. The use of tape also helps to limit corrosion on the threads, which otherwise can make future disassembly nearly impossible.

Commonly used sizes are 1/8, 1/4, 3/8, 1/2, 3/4, 1, 1 1/4, 1 1/2, and 2 inch, appearing on pipes and fittings by most U.S. suppliers. Sizes smaller than 1⁄8 inch are occasionally used for compressed air, while sizes larger than 2 inches are uncommon, due to the use of alternative methods of joining that are used with these larger sizes.