Ok,
Here's simple. Remove all parts and mark the center of the arbor. Either with a punch or needle file in a dremel, make a punch mark on your marked center. Chuck the arbor vertically in a good vice. Now would be a good time to check for a loose arbor. Grasp the frame and check for any side to side play. You'd be surprised at how many NEW revolvers have a loose arbor. If it has play, well deal with that later.
With the appropriate drill for a 1/4" set screw (you can by a drill and tap set at a big box store for maybe 5 bucks) drill, slowly, keeping the drill straight by viewing alternately from front and side as you slowly drill. Your only going through less than a 1/4" on the end of the Pietta (Uberti's have more meat) arbor before you get to the wedge slot. When you get to the slot, the drill will stop (or break if you're drilling fast!) I have a burr bit that I've used for years (meaning it will last for almost ever!!) to continue the hole into the wedge slot for maybe a 1/4". You may find a burr bit/ rough rotary file at Harbour Freight or Northern Tools better ones probably at McMaster Carr. Anyway, now chamfer the hole (always clean up drilled holes, no burrs allowed!!). Now using the tap in an appropriate handle (you did get one didn't you?!) tap the hole using plenty of oil. Go slow here as well! Digging a broken tap out is not for the faint of heart . . . . I've been told . . . . .
When finished, I use a section of cut off Allen wrench in a drill and sand the end of a set screw flat (you don't want the cone to cut into the wedge, we are making a bearing surface). I use a 1" belt sander but if you don't have one, you may use some course sandpaper on a flat surface to run the set screw over. I polish the surface with some 320 sand paper with some padding under it to help "soften" the edges of the bearing surface.
Now, degrease the arbor and newly cut threads and install the set screw with a little blue thread locker. When the screw is even with the wedge slot (there can be NONE of the screw protruding from the end of the arbor!!), test fitting can begin.
There you have it, an adjustable bearing surface for the wedge. It's much easier to do than it looks written out. If in doubt, practice on a piece of cheap bar stock from the same box store.
Mike
Food for thought:
The arbor wedge slots on the original pieces I've had the opportunity to inspect had straight cuts front and rear. They all had contact marks (from wedge placement) on the left side (off center) of the front section of the slot (so far, they've all been 1860's).
The Uberti's have an angle cut to supposedly contact the wedge fully across the width of the slot. I view this as counter productive as it presents too big of a "point" of contact and will allow loosening in time. On the other hand, the Piettas seem to cut the slot from both sides, leaving a smaller remnant for the contact point. Smart. From my own observations and my understanding of this "triangulation" setup for a tensioned union, I see no reason that the "adjustable bearing" described above would not give an appropriate surface for the intended purpose. Therefore, that's why I do it. It allows one to position the wedge placement anywhere they choose and gives the "tuner" the ability to fine tune the arbor length for a targeted bbl/cyl clearance (by dressing the arbor end as needed which opens the wedge slot) without the need for oversized wedges/single use (read ooops too small) wedges.