Head diameter, head and square depth, square neck width and minimum thread length for countersunk square neck plow bolts, 5/16″ to 1″. No. 3 head (regular and repair) and No. 7 head. ASME B18.9.
A plow bolt has a flat, countersunk round head that sits flush in a countersunk hole, so the working face stays smooth — road grader and snowplow blades, dozer edges, scoop shovels. A square neck under the head keeps it from turning while the nut is tightened. Because the head sits in the part, a plow bolt is measured from the top of the head to the end — the head is part of the length.
| Size | Head dia A max / min | Head + square depth S max / min | Square width B max / min | Body E max / min |
|---|---|---|---|---|
| 5/16″ | .605 / .578 | .269 / — | .325 / .313 | .312 / .299 |
| 3/8″ | .708 / .671 | .312 / — | .387 / .375 | .375 / .360 |
| 7/16″ | .826 / .781 | .364 / — | .450 / .438 | .438 / .421 |
| 1/2″ | .945 / .890 | .417 / — | .515 / .500 | .500 / .483 |
| 5/8″ | 1.147 / 1.094 | .506 / — | .640 / .625 | .657 / .606 |
| 3/4″ | 1.303 / 1.250 | .541 / — | .765 / .750 | .782 / .729 |
| 7/8″ | 1.512 / 1.469 | .626 / — | .906 / .875 | .938 / .853 |
| 1″ | 1.700 / 1.656 | .690 / — | 1.031 / 1.000 | 1.062 / .976 |
| Size | Head dia A max / min | Head + square depth S max / min | Square width B max / min | Body E max / min |
|---|---|---|---|---|
| 5/16″ | .556 / .531 | .232 / — | .325 / .313 | .312 / .299 |
| 3/8″ | .659 / .624 | .272 / — | .387 / .375 | .375 / .360 |
| 7/16″ | .779 / .734 | .324 / — | .450 / .438 | .438 / .421 |
| 1/2″ | .898 / .843 | .375 / — | .515 / .500 | .500 / .483 |
| 5/8″ | 1.096 / 1.047 | .458 / — | .640 / — | — / .606 |
| 3/4″ | 1.252 / 1.203 | .493 / — | .765 / — | .782 / .729 |
| 7/8″ | 1.465 / 1.422 | .573 / — | .906 / — | .938 / .853 |
| 1″ | — / 1.609 | .637 / — | 1.031 / — | — / .976 |
| Size | Head dia A max | Head height S max | Key length G max | Key width |
|---|---|---|---|---|
| 5/8″ | 1.121 | .414 | .363 | — |
| 3/4″ | 1.277 | .440 | .390 | — |
| Size | Threaded to the head up to | Minimum thread length | On lengths |
|---|---|---|---|
| 5/16″ | 1″ | 7/8″ | 1-1/4″ to 6″ |
| 3/8″ | 1-1/4″ | 1″ | 1-1/2″ to 6″ |
| 7/16″ | 1-1/2″ | 1-1/8″ | 1-3/4″ to 6″ |
| 1/2″ | 1-1/2″ | 1-1/4″ | 1-3/4″ to 6″ |
| 5/8″ | 2″ | 1-1/2″ | 2-1/4″ to 6″ |
| 3/4″ | 2-1/4″ | 1-3/4″ | 2-1/2″ to 6″ |
| 7/8″ | 2-1/2″ | 2″ | 3″ to 6″ |
| 1″ | 2-1/2″ | 2-1/4″ | 3″ to 6″ |
| Size | Length, plus or minus |
|---|---|
| 1/4″ to 3/8″ | ±.03″ |
| 7/16″ and 1/2″ | ±.06″ |
| 5/8″ to 1″ | ±.12″ |
Good to know
Measure a plow bolt from the top of the head, not from under it — a 2″ plow bolt is 2″ overall. Match the head number and regular or repair head to the part it’s going in: a 5/8″ repair head is 1.096″ across at most, a regular head 1.147″. Thread pitch and tap drill for each size are on the thread pitch chart; carriage bolts (round head, square neck) are on the carriage bolt chart.
Quick answers
- How is a plow bolt measured?
- From the top of the head to the end of the bolt — the countersunk head sits in the part, so it counts in the length (ASME B18.9).
- How big is the square neck on a 1/2 inch plow bolt?
- .500–.515″ across the flats (No. 3 head, ASME B18.9).
- How big is the head on a 5/8 inch plow bolt?
- A No. 3 regular head is 1.094–1.147″ across; the repair head is smaller, 1.096″ maximum.
- How long is the thread on a 3/4 x 3 inch plow bolt?
- At least 1-3/4″ — twice the diameter plus 1/4″.
- What size square neck goes with each plow bolt size?
- 5/16″ – .313″, 3/8″ – .375″, 7/16″ – .438″, 1/2″ – .500″, 5/8″ – .625″, 3/4″ – .750″, 7/8″ – .875″, 1″ – 1.000″ (minimum, No. 3 head).
- What is a repair head plow bolt?
- A plow bolt with a smaller No. 3 head, marked R on top to tell it from the regular head (ASME B18.9). On a 5/8″ bolt the repair head is 1.096″ across at most, the regular head 1.147″.