What is a Bolt Circle?
A bolt circle (also called a Pitch Circle Diameter or PCD) is a circular pattern of holes used to mount flanges, wheels, and other components with bolted connections. The bolt circle diameter is the diameter of the imaginary circle that passes through the center of each bolt hole. Accurately laying out bolt circles is essential in construction, fabrication, and mechanical assembly.
Bolt circles are specified by the number of holes and the circle diameter. For example, "6 holes on a 4.5-inch bolt circle" means 6 equally spaced holes centered on a 4.5-inch diameter circle. The X and Y coordinates of each hole can be calculated using trigonometry.
Bolt Circle Formulas
Common Bolt Circle Patterns
| Application | Holes | Typical PCD |
|---|---|---|
| 4-lug wheel | 4 | 4" - 4.5" |
| 5-lug wheel | 5 | 4.5" - 5" |
| Pipe flange (4") | 8 | 7.88" |
| Pipe flange (6") | 8 | 9.5" |
| Pipe flange (8") | 8 | 11.75" |
Layout Methods
- Divider method: Set dividers to the chord length and step around the circle.
- DRO/CNC: Use calculated X,Y coordinates directly.
- Rotary table: Index by the angle between holes.
- Template: Print a full-scale template and center-punch through it.
Frequently Asked Questions
How do I measure an existing bolt circle?
For an even number of holes, measure from the center of one hole to the center of the hole directly opposite. For an odd number (like 5), measure from one hole center to the far edge of the most distant hole, then subtract half a hole diameter.
What is the minimum edge distance for bolt holes?
The minimum edge distance from the bolt hole center to the edge of the material is typically 1.5 times the bolt diameter for structural connections (per AISC). For non-structural applications, 1.25 times the bolt diameter is often acceptable.
Can I use an unequal bolt circle?
Yes, holes don't have to be equally spaced. You can specify different angles for each hole. However, unequally spaced patterns create uneven load distribution and are used only when physical constraints require it.