Question 1

How do I measure a bolt circle diameter on an existing part?

Accepted Answer

For even number of holes: measure across two diametrically opposite holes (center to center) — this is the BCD. For odd number of holes (e.g., 3 or 5 bolt patterns): measure center-to-center distance between two adjacent holes (chord), then BCD = chord / sin(180°/N). For a 5-bolt pattern with 63.5 mm between adjacent holes: BCD = 63.5 / sin(36°) = 108.0 mm.

Question 2

What start angle should I use for standard flange patterns?

Accepted Answer

ASME B16.5 flanges have bolt holes straddling the centerlines — for a vertical pipe, holes are placed symmetrically about the vertical and horizontal centerlines, not on them. For a 4-bolt pattern, start at 45°; for an 8-bolt pattern, start at 22.5°. This 'straddle' position prevents bolt interference with adjacent piping and allows easier wrench access.