I don't know why people are protecting these "secrets". Do you really think you are going to lose jobs if you tell the forum?
James has described his method, which can be done on a mill without DRO but requires a rotary table. I did my first few without a rotary table on a mill with DRO, using X-Y coordinates. See below for a set of coordinates. This method is as accurate as the rotary table method but not quite as fast. I would add that with either method it's best to make one run through the pattern with a rigid spotting drill bit, and a second run with the "F" tap drill bit.
I am now using a template made from a badly scored V-8 flywheel. I put it on the mill to add an extra 5/16" hole to use as a tap guide. The center plug is a press fit in the template bearing bore and a tight slip fit in the flywheel bearing bore. This method could result in a thousandth or so inaccuracy on any given hole, but is still much more accurate than the stamped holes in a pressure plate. Photo below. This could be done on a large drill press, but I prefer to use the mill.
Anyone whose method is to place a pressure plate on a flywheel and use it as a template is kidding themselves. There is no surface on a pressure plate that is a reliable indication of center, including the hole pattern itself. The holes are often far enough from the pattern to cause trouble starting bolts--sometimes so far off that some bolts cannot be started, as has been reported here more than once.
Joe
X= -1.4299, Y= 4.9865
X= 1.4299, Y= 4.9865
X= 5.0334, Y= -1.2550
X= 3.6035, Y= -3.7316
X= -3.6035, Y= -3.7316
X= -5.0334, Y= -1.2550
