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BOLTS
Bolts are generally considered to be larger than screws and are used in a wide variety of applications especially where load-carrying capacity is a requirement. Depending on the type, uses include fastening wood to wood, wood to metal, metal to metal, and objects to wood and metal. Some have similar, smaller screw counterparts—lag bolts are heavy-duty wood screws, for example. Others, like carriage and elevator bolts, do not have diminutive equivalents. Common bolts are listed in Table 1, or click on a type above for detailed information.
By definition, "A Bolt is a headed and externally threaded mechanical device designed for insertion through holes in assembled parts to mate with a nut and is normally intended to be tightened or released by turning that nut." There are exceptions, like anchor bolts, which are cast in concrete. Also, stove bolts are often tightened and released by turning their head. So, officially, some bolts are really screws, like lag, shoulder and stove bolts.
Unified inch coarse series (UNC, Unified National Coarse) threads are largely standard; Unified inch fine (UNF, Unified National Fine) may be available too. Coarse threaded fasteners have fewer threads per inch than fine threaded fasteners because coarse threads are farther apart. Consequently, the larger the number of threads per inch, the finer the thread. Also, smaller sizes have more threads per inch than larger sizes—see Table 2 for a comparison between UNC and UNF threads per inch by bolt size. Short lengths are often fully threaded, longer lengths are partially threaded. Some types, though, are always fully threaded and others are always partially threaded. Unless noted otherwise, right-hand threads are standard, so turn the bolt (or nut) clockwise to tighten. Point style varies by bolt type and some types are not pointed, such as carriage bolts. "The point of a fastener is the configuration of the end of the shank of a headed fastener or of each end of a headless fastener."
Size refers to nominal diameter of the bolt while length is just that—how long the bolt is. While stove bolts can be as small as 1/8" in diameter, expect typical bolt sizes to be between about #10 (3/16") and 1 3/4". Lengths can be as short as 3/8", again for stove bolts, but are usually between about 1/2" and 24". The size and length of inch series fasteners is specified in inches, usually fractional rather than decimal.
The bolt should be long enough to allow at least two full threads to extend beyond the nut face after tightening, which ensures full thread engagement with the nut. Conversely, there should be two full threads exposed on the head side of the nut to make sure the nut can be properly tightened. More exposed threads within the grip (the area between the head and nut) will result in a "springier" bolt. It is also recommended that threads not be in the shear plane(s).
Drive styles for externally wrenched bolts include 12 point, hex and square; for internally wrenched, slotted and hex socket are typical. Head styles include cylindrical, flat countersunk, hex, round and square.
Some bolts are not available in different strength grades while certain steel types are offered in SAE (Society of Automotive Engineers) Grade 2, 5 or 8; head markings identify grade. Never replace a graded bolt with a lower grade or lower strength—do not substitute a stainless steel or silicon bronze bolt for a Grade 5 or 8 bolt, or a brass bolt for a Grade 2, 5 or 8 bolt.
Common materials include steel (unplated and plated—see more about finishes below), stainless steel, brass, silicon bronze, and nylon. Steel remains the least costly material followed by stainless steel; the copper alloys (such as brass and silicon bronze) are the most expensive.
Shoulder bolts and some hex bolts are available from domestic and/or Canadian manufacturers.
Common finishes for steel are zinc plating and hot dip galvanizing. Zinc, the most popular and least expensive commercial plating, offers moderate corrosion resistance. Hot dip galvanized is a thick coating of zinc that protects against corrosion in harsh environments. Stainless steel, though, is a better choice when corrosion is of concern. Unplated and uncoated steel bolts—referred to as plain finish—may also be available and are susceptible to rust. Not all types are available in all materials and finishes.
Hot dip galvanized, stainless steel and silicon bronze are usually recommended if the bolts (less than 1/2" in diameter) will be used with pressure preservative treated wood such as "ACQ" (Alkaline Copper Quaternary)—check local building codes and contact your lumber supplier for recommendations. When submerged in salt water without free oxygen, silicon bronze is a preferred material because stainless steels can suffer from severe pitting corrosion (stainless steel needs oxygen to create its self-healing, corrosion-resisting chromium oxide film).
Bolts with machine screw threads are used with either through holes and nuts or with internally threaded (tapped) holes. Lag threaded bolts, which includes hanger and lag bolts, require predrilled pilot holes to ensure maximum withdrawal resistance; hole size is based on bolt size and wood density. A countersunk recess (a beveled opening) is used with countersunk styles, like flat head stove bolts. Other types may be installed in a counterbored (flat bottom) recess, such as shoulder bolts. See the specific bolt type for more information.
When selecting a nut for use with a strength graded bolt, it is very important that the nut be the same grade as the bolt; one grade higher is also acceptable as one source states. Use hardened flat washers with bolt Grades 5 and 8. As for lock washers, which are often split (helical spring) type, use regular for Grade 2 and high strength alloy steel for Grades 5 and 8.
Always follow the manufacturer's recommendations regarding bolt reuse, thread lubricants/locking compounds and torque values.
It is advisable to match materials and finishes of bolts, washers and nuts. When using hot dipped galvanized bolts, always use hot dipped galvanized nuts, which are tapped oversize (threaded larger than normal) to accommodate the thick zinc coating on the bolts (using a galvanized nut on a non-galvanized bolt will result in an unacceptably loose fit and using a plain finish or zinc plated nut on a hot dip galvanized bolt will result in a fit that is much too tight).
Stainless steel bolts and nuts used together are susceptible to thread galling and seizing. While it may not be completely preventable, it can be substantially reduced. A thread lubricant is the most effective method. Alternatively, stainless steel alloys having different hardnesses—like a 316 nut and a 304 bolt—have less tendency to gall.
For more information, visit our Tech Data section.
BOLTS |
|
|---|---|
| Anchor Bolts | |
|
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| Carriage Bolts | |
|
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| Draw Bolts | |
|
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| Elevator Bolts | |
|
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| Flange Bolts | |
|
 12 Point  Hex  Serrated Hex |
| Hanger Bolts | |
|
 Fully Threaded  Plain Center |
| Hex Bolts | |
|
 Hex Bolt  Grade 5, Global  Grade 5, USA/Canada  Grade 8, Global  Grade 8, USA/Canada  Grade 9 |
| Lag Bolts | |
|
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| Machine Bolts | |
|
 Hex  Square |
| Plow Bolts | |
|
 #3 Flat  #3 Dome |
| Shoulder Bolts | |
|
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| Step Bolts | |
|
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| Stove Bolts | |
|
 Flat  Round |
| Tap Bolts | |
|
 Grade 2  Grade 5 |
Unified Inch Coarse and Fine Threads |
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|---|---|---|---|
| Bolt Size | Threads per Inch | ||
| Frac. [No.] | Decimal | UNC | UNF |
| 1/8 [#6] | 0.1380 | 32 | 40 |
| 5/32 [#8] | 0.1640 | 32 | 36 |
| 3/16 [#10] | 0.1900 | 24 | 32 |
| 1/4 | 0.2500 | 20 | 28 |
| 5/16 | 0.3125 | 18 | 24 |
| 3/8 | 0.3750 | 16 | 24 |
| 7/16 | 0.4375 | 14 | 20 |
| 1/2 | 0.5000 | 13 | 20 |
| 9/16 | 0.5625 | 12 | 18 |
| 5/8 | 0.6250 | 11 | 18 |
| 3/4 | 0.7500 | 10 | 16 |
| 7/8 | 0.8750 | 9 | 14 |
| 1 | 1.0000 | 8 | 12, 14 |
| 1 1/8 | 1.1250 | 7 | 12 |
| 1 1/4 | 1.2500 | 7 | 12 |
| 1 3/8 | 1.3750 | 6 | 12 |
| 1 1/2 | 1.5000 | 6 | 12 |
| 1 3/4 | 1.7500 | 5 | - |
CI:BLTS v1.1a
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