3 Mounting holes is already drill and tapped on the back plate to match the 5" ER40 zero set adjustment collet chuck. A light skim cut is required on the adapter boss OD and the face of the adapter's contact area. The clearance between adapter boss OD and the chuck counterbore ID should be from 0.0078" to 0.0129". The adapter boss nose should not be in contact with the bottom of the chuck counterbore. The gap between the adapter boss and the bottom of the counterbore should be around 0.0787".