mounting holes are sketchy, I measured off the booster, but still not perfect. Center hole for booster is just centered on bolt patters, then recessed to leave .300 face thickness. This allows you to tighten the nut via chiesel, and then reinstall the snap ring. On the inside is an alignment pin, i drilled it twice once i decided to flip the booster, simply cut down some .25 steel stock as a dowel.