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Stove bolts are said to have been rendered obsolete when the tapping screw was introduced in 1917. Despite this hindrance, they continue to live on although their popularity has waned and are seldom, if ever, used with ductwork today. Metal to metal applications, though, are still common along with wood to metal and wood to wood. They are machine screws with either a slotted flat countersunk head or slotted round head and are supplied with nuts—hex nuts are now more prevalent than square nuts. The three smaller sizes of stove bolts are specified in terms of their approximate fractional inch diameter rather than a number size as is customary with machine screws. Otherwise, mate a nut with a slotted flat or round head coarse threaded machine screw and you've created a stove bolt.
Threads are standard right-hand and Unified inch coarse series (UNC, Unified National Coarse). Short lengths are fully threaded while very long lengths may be partially threaded. The point is usually a plain sheared end.
Typical stove bolt sizes are 1/8" (which is a 6-32 screw size), 5/32" (8-32), 3/16" (10-24), 1/4-20, 5/16-18 and 3/8-16. Lengths range from about 3/8" to 6". Measure the length of a flat head screw from the top of the head to the threaded end of the bolt; measure round heads from under the head.
Drive style is slotted and head style is flat countersunk and round head. The head angle (conical bearing surface) of flat heads is 82°. Table 1 lists head diameter and height of flat heads and round heads are listed in Table 2.
Steel, zinc plated, is the most common material and finish (long ago, unplated steel may have been an option).
When installing stove bolts in wood, use a large diameter washer under the nut and round heads to distribute clamping force over a larger area and minimize compression of the wood. Use an 82° countersink to create a countersunk recess (a beveled opening) for flat heads. The body diameter of the countersink should be equal to or greater than the head diameter to ensure that a uniform bearing surface is created.
Refer to American Society of Mechanical Engineers Standard ASME B18.6.3, Machine Screws and Machine Screw Nuts, for specifications relating to stove bolts.
Stove Bolt Dimensions for Flat Heads
Stove Bolt Dimensions for Round Heads