What a bolt circle is
A bolt circle, also called a pitch circle diameter (PCD) or bolt pattern, is the circle on which a set of equally spaced holes sits: flanges, wheel hubs, pipe fittings, motor mounts, turntables. To drill one on a mill or with a drill press, you need the position of each hole relative to the center.
The formulas
For N holes on a circle of diameter D, with the first hole at angle θ₀ from the X axis:
- angle between holes = 360° ÷ N;
- angle of hole k: θk = θ₀ + (k − 1) × 360° ÷ N;
- X = Xc + (D ÷ 2) × cos θk;
- Y = Yc + (D ÷ 2) × sin θk;
- distance between neighboring holes (chord) = D × sin(180° ÷ N).
Angles are measured counterclockwise from the positive X axis, the usual convention on machine tools and in CAD.
Example
Six holes on a 4-inch circle, the first at 0°:
- holes are 60° apart;
- hole 2 is at X = 2 × cos 60° = 1.000 in, Y = 2 × sin 60° ≈ 1.732 in;
- neighboring holes are 4 × sin 30° = 2.000 in apart.
With six holes, the chord always equals the radius, a handy check.
Common patterns
| Pattern | Chord between neighboring holes |
|---|---|
| 4 × 100 mm (wheels) | 70.71 mm |
| 5 × 4.5 in (114.3 mm) | 2.645 in (67.18 mm) |
| 5 × 120 mm | 70.53 mm |
| 8 holes on 6 in | 2.296 in |
The chord lets you check a pattern with calipers. For five-hole wheel patterns, which have no opposite holes, measuring from the center of one hole to the far edge of a non-adjacent hole is another quick method.
Drilling tips
- Set the center precisely and zero your X and Y readouts there.
- Center-drill or spot each hole before drilling to stop the bit from wandering.
- For a single part, a printed paper template or a CAD drawing taped to the work can be enough.
- For many parts, a drilling jig saves time and keeps every pattern identical.
Rotated patterns
If the pattern must straddle an axis, for example with two holes symmetrical about the vertical, set the start angle to half the spacing: 30° for six holes, 45° for four.