If multiple bolts secure one component of an engine to another, then there should be a pattern used to remove and tighten the bolts. If the bolts are simply tightened by installing each bolt adjacent to each other, using a clockwise pattern as an example, then the metal parts may warp. Other bolt circle pattern problems may involve figuring out how to fabricate a component, requiring that you figure out the bolt hole pattern prior to drilling out the bolts using a tap and dye set.
Draw your bolt pattern on your paper. Although the a circular bolt pattern is generally easy to figure out, there are other patterns for noncircular objects that utilize general rules and principles. If you are working on a car engine, consult the repair manual prior to removing the bolts. If you are ever in doubt, loosen each bolt gradually by rotating them in quarter turns.
Count the number of bolt holes and use a ruler to measure the diameter (in inches) of the bolt circle. If there is an even number of bolt circles, then measure from one bolt hole to the bolt hole on the opposite side of the circle. If there is an odd number of bolt holes, measure from one bolt hole to a spot that is located directly between the two bolt holes opposite from the first bolt hole. This will give you the measurement for the bolt-pattern, which should read as a ratio, first stating the number of bolt holes compared to the diameter of the bolt circle.
Unscrew the bolts by loosening each bolt until it can be unscrewed by hand. Start with the bolt closest to the top of the bolt circle. The second bolt to loosen should be opposite from the first bolt. Continue to move clockwise around the bolt circle until each bolt, along with its opposite pair is loosened. Loosen the bolts the rest of the way, turning them by hand. If there is an odd number of bolts, the loosening pattern will resemble a star, choosing the bolt clockwise and up from the direct opposite of the first bolt.
Calculate the bolt hole pattern for fabricating a new component with a circular bolt pattern by using an online bolt hole pattern calculator, such as the one found at DOOV.com. The measurements needed include the diameter of the bolt circle and the number of bolts. Other variables include the angle degree to the first bolt hole and the X and Y coordinates of the center of the circle. This tool will give you the exact location of the rest of the bolt holes on the circle using X and Y coordinates, rounding to the nearest 1/10,000 of an inch.
