I had the block bored for different sized pistons (overbores), and with an old crank in the block, I would put the piston and rod assembly in the cylinder with the crank turned to TDC, then with a collar stop in the stem of the cutter, would actually use a half inch drill to make the fly cuts. The flycutter I used was designed for just such an application. I got involved in this at one time and used a section of head and an engine block. Record the measurements that you get in the area you are wanting to cut. Walk your piston in between the indicator and the punch. Make sure every thing is solid and repeats '0' when cycled. This isnt something you can just chuck into a drill and start cutting. Position the indicator so that the tip touches the tip of the punch. The difference between a normal or cast piston and a forged piston is the manufacturing technique. Unless you have the jigging equipment, I dont recommend doing this in your garage. The use of forged pistons by racing tuners and mechanics is evidence of this particular pistons capability. A good competent machine shop that deals with building performance engines will know what you need and can deal with it accordingly. Push downward using steady pressure and watch the gap between the drill chuck and the valve seal decrease until the chuck contacts the seal. What this means is that the valve stem is that many degrees off of the perpendicular centerline of the piston deck, or block deck. Pull the valve up slightly off the piston and start the drill. You will notice that there are aftermarket head that are called 13 degree, 18 degree etc. 004-005' off the deck of the block your piston protrusion will be at the very ragged high side of the specs. The cracks are all right at the lip of the fly cuts on each piston.

To get the proper flycut angle will require knowing what the angle is in relation to the deck of the piston and the valve itself. If all of the valve train geometry is correct (valve recession, valve height, lifter preload) you should be ok, but all that depends on the competency of the person doing the machine work and assembling the engine. These were fly cut, delipped and coated Mahle pistons.

Flycut on the piston