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Machine Bolts

Machine bolts are used to fasten wood to wood, wood to metal and metal to metal. Similar to a hex cap screw, a machine bolt will differ in these four ways: its point will not be chamfered or rounded; it will be available in larger sizes (diameters); it will not have an underhead washer bearing surface; and it will be manufactured to less exacting standards. Depending on head style, machine bolts are also known as: 

hex bolts, square bolts.

 They are used with nuts and internally threaded (tapped) holes.

Threads are standard right-hand and often Unified inch coarse series (UNC, Unified National Coarse). Short lengths are fully threaded while longer lengths follow a standard formula: for bolts 6" and shorter, the minimum threaded length is two times the basic thread diameter plus 1/4", and for lengths greater than 6", two times the diameter plus 1/2".

Typically, machine bolt sizes range from 1/4" to 1 1/4" for square heads and 3/8" to 1 3/4" in diameter for hex heads, and lengths span from about 3/4" to 18" for square and 6 1/2" to 24" for hex heads. Measure length from under the head to the threaded end of the bolt.

Both square and hex heads are available. The size of the head varies with the size of the machine bolt. Since it is externally wrenched, sufficient space around the head must be allowed for wrench clearance. Tables 1 and 2 list head width across flats, which is wrench size, and across corners, head height and minimum threaded length.

Machine bolts are available in Grade 2. Steel is the standard material, and plain finish, which lacks a surface treatment to inhibit rust, is common.

When installing in wood, drill holes no more than 1/16" larger than the bolt size. If you drill into an untreated area of treated lumber, saturate the hole with a preservative to prevent the migration of decay fungi. In addition, always use a large diameter washer under the head and nut to distribute clamping force over a larger area and to minimize compression of the wood. Bolts should be retightened after about a year then checked yearly for tightness.

Machine and lag bolts are the preferred fasteners for fastening heavy wood members in structures. Machine bolts, in comparison to lag bolts, offer increased rigidity and load-carrying capacity but require a through-hole and access to both sides of the joint for wrenching the head and nut. Lag bolts, despite their lower lateral strength, are useful when access to one side of the joint is limited, the appearance of a nut is undesirable or through-bolt length is excessive. When compared to carriage bolts, machine bolts are available in larger sizes and are often preferable because carriage bolts are not designed to accept a washer under their head. However, carriage bolts are available in hot dip galvanized and stainless steel types.

Refer to American Society of Mechanical Engineers Standard ASME B18.2.1, Square and Hex Bolts and Screws, for specifications relating to machine bolts.

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