When the keyframe has been bedded on the bench to have the same shape it had in the piano, our strike samples will work. We took each of these with the weight of a WNG Dip Tool at the front of the key. We first blocked the sample hammers against their strings, then "let them off" with a slight hesitation to refine the location. Now, working the inverse on the bench, we adjust the "strings" (templates) to produce the same symptoms. When done, their bottom edges will be at strike relative to the action.
Now that the action is positioned on the bench, shim the backrail to be stable. Do this with front punchings, placed every twelve to fourteen inches. These will support backrail and frontrail the way glidebolts support the balancerail. Share the weight evenly between these points of contact. And when done, draw placement markings on rail, punchings, and bench so that if the keyframe is moved, when it is returned, the shims can be placed back in their original locations.