Skip to content

Square and Octagonal (4 & 8-Hole) Bolt Circle Patterns

The Easy Ones

If you can locate the center of your part and move to four corners of a square, congratulations - you can do 4-hole patterns. The 8-hole is just doing it twice with a 45° rotation. No magic numbers needed here, just basic shop math.

4-Hole Pattern (Square)

The Setup

  • Angular spacing: 90° apart (360° ÷ 4 = 90°)
  • Pattern shape: Square inscribed in your bolt circle

The Dead Simple Method

Starting with your center at (0,0) and radius R:

  1. Hole 1: X = +R, Y = 0 (3 o'clock position)
  2. Hole 2: X = 0, Y = +R (12 o'clock position)
  3. Hole 3: X = -R, Y = 0 (9 o'clock position)
  4. Hole 4: X = 0, Y = -R (6 o'clock position)

That's it. You're literally just moving to the radius distance on each axis.

The 45° Rotated Method (More Common)

Most 4-hole patterns have holes at the "corners" rather than on the axes. For this layout:

All four holes are at the same distance from each axis:

  • Offset = R × 0.7071 (or R × sin(45°))

Position all holes:

  1. Hole 1: X = +offset, Y = +offset
  2. Hole 2: X = -offset, Y = +offset
  3. Hole 3: X = -offset, Y = -offset
  4. Hole 4: X = +offset, Y = -offset

Example: 100mm Bolt Circle

For a 100mm diameter bolt circle (50mm radius):

  • Offset = 50 × 0.7071 = 35.35mm

So each hole is at (±35.35, ±35.35)

8-Hole Pattern (Octagonal)

An 8-hole pattern is just a 4-hole pattern done twice. You make one set of 4 holes, then another set rotated 45°.

The Method

  • Angular spacing: 45° apart (360° ÷ 8 = 45°)
  • Pattern shape: Regular octagon

Coordinate Calculation

Set 1 - On the axes (like the simple 4-hole):

  1. Hole 1: X = +R, Y = 0
  2. Hole 3: X = 0, Y = +R
  3. Hole 5: X = -R, Y = 0
  4. Hole 7: X = 0, Y = -R

Set 2 - At 45° angles (like the rotated 4-hole):

  • Offset = R × 0.7071
  1. Hole 2: X = +offset, Y = +offset
  2. Hole 4: X = -offset, Y = +offset
  3. Hole 6: X = -offset, Y = -offset
  4. Hole 8: X = +offset, Y = -offset

Example: 150mm Bolt Circle

For a 150mm diameter bolt circle (75mm radius):

  • Holes 1,3,5,7: On axes at 75mm from center
  • Holes 2,4,6,8: At 45° angles, 53.03mm from each axis

Quick Reference Table

Pattern Bolt Circle Dia Radius On-Axis Distance 45° Offset
4-hole 100mm (4") 50mm 50mm 35.35mm
4-hole 150mm (6") 75mm 75mm 53.03mm
8-hole 150mm (6") 75mm 75mm 53.03mm
8-hole 200mm (8") 100mm 100mm 70.71mm

Shop Tips

  1. Start Simple: For 4-hole patterns, drill opposite holes first (1 & 3, then 2 & 4). This lets you verify your bolt circle diameter immediately.

  2. 8-Hole Sequence: Drill all 4 on-axis holes first, verify spacing, then add the 45° holes.

  3. Rotary Table Alternative: If you have a rotary table, just index 90° for 4-hole or 45° for 8-hole. Way easier than coordinate math.

  4. The 0.7071 Constant: This is just √2/2 or sin(45°). An old timer taught me to remember it as "point seven-oh-seven" and left it at that.

Verification

For any square pattern, the diagonal distance between opposite holes should be:

  • Diagonal = Bolt circle diameter

This gives you a quick sanity check with your calipers.

When to Use What

  • 4-hole on axes: Easier to indicate and measure, use when appearance doesn't matter
  • 4-hole at 45°: Standard for flanges and covers, looks more balanced
  • 8-hole: When you need more bolts but don't want to calculate weird angles

This is like the training wheels of bolt circles. Start here, and once you've mastered it, the 5-hole and other odd patterns won't seem so intimidating.