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Plow Bolts

Plow bolts date back to about 1850 and are used to attach the plow share (blade) to the frog (frame) and allow earth to pass over their heads without obstruction to the moldboard. They are also used to fasten the blade to bulldozers and motor graders. Plow bolts have a small round countersunk head and a square neck—the width of the square (measured across the flats) is the same size as the nominal diameter of the bolt. The top of the head can be flat (for plows) or dome (convex) shaped (for dozers/graders); the conical (tapered) bearing surface of a plow bolt is 80°.

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

Typical plow bolt sizes range from 3/8" to 3/4" in diameter for flat heads and 5/8" to 1" for dome heads. Depending on size, lengths span from about 1" to 4" for flat heads and 1 1/2" to 7" for dome heads. For flat heads, length is measured from the top of the head to the threaded end of the bolt; for dome heads, measure length from the largest diameter point of the head (where the dome top meets the conical bearing surface) to the extreme end of the bolt.

The size of the head varies with the size of the bolt. Table 1 lists head diameter, depth of square and head, square neck width and minimum threaded length of #3 flat head plow bolts.

Grade 5, steel, zinc plated, and Grade 8, steel, yellow zinc plated, are common grades, material and finishes.

Refer to American Society of Mechanical Engineers Standard ASME B18.9, Plow Bolts, for specifications relating to plow bolts.

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