I just came across this geometric solution to laying out bolt hole patterns on a circle. For any diameter circle AD divide the diameter into as many equal part as bolt holes needed. Then draw an arc intersection that has a radius of the diameter of the circle from A and D and labial this E. A,D,E form an equal lateral triangle with A,D as diameter of circle. Now draw a line from E throw the second equal point from A on the diameter to intersect with the far side of arc of the circle and labial this F. Now take dividers and set to length A,F. Use this length to work around circle to divide the circle evenly.