B. Most authentic conversion from a NM to 3 screw Old Model, with four hammer 'clicks' (also with or w/o transfer bar safety), but with the transfer bar there is no interaction with the loading gate or the cyl latch. Loading/unloading is strictly the OM way by ½ cocking the hammer. Therefore chambers always align properly with the loading gate trough.
Here it is step by step:
You'll need the following OM action parts or optional parts: hammer (Ruger or Power Custom's 3 screw hammer replacement) or one of those mentioned below under hammer options*, cyl latch, latch spring, Colt 3rd generation hammer screw .220" x 28. Note: A disassembled old model for reference and measurements is a must.
Note: on stainless conversions: Every internal part won't be stainless; the OM parts, such as the cyl bolt, loading gate detent screw and plunger, trigger return plunger and possibly hammer depending on the option chosen above. But all the springs will be 'cause they always were.
TO START: Gut all internal parts from a NM SA and put them aside except the trigger, cylinder pawl and cylinder base pin.
*Hammer options:
1. A carbon steel OM hammer with a polished or brushed finish (to look like stainless) or Powers Custom 3 screw OM hammer replacement (not stainless either). Hammer nose modification needed on both if the transfer bar safety is retained.
2. Powers Custom stainless half cock NM Montado/Super hammer. Hammer nose modification needed if transfer bar safety is NOT retained. The safety notch is useless anyway but a small shelf can be added at the right location for the "click". Enlarge hammer pin hole in base to fit the OM screw.
Tap hammer pin hole in left side frame to .220" x 24 threads and enlarge hammer pin hole in right side frame for clearance and counter bore for the hammer cap screw.
Trigger options:
The NM trigger/cyl bolt pin hole in frame does not align with either of the OM trigger or bolt screw holes. The trigger/cyl bolt pin hole must be tapped on left frame side and counter-bored on right side for the cap of a machine screw* to use with the NM trigger.
NM trigger is modified by grinding the spring perch web off the back for use with an OM return spring and plunger an Old Army stainless grip frame, (note: it's an XR3-RED size). Any NM grip frame can be used with its standard trigger return spring by not grinding off the trigger spring perch web. Or by making the well known adapter** and OM spring and plunger for the true OM look.
Also cut off the transfer bar boss (unless transfer bar is retained. Then the lower right side of the OM hammer will need to be relieved to match the relief cut in the NM hammer.)
The NM trigger stop at the top area must be removed for clearance of the added OM cyl bolt screw.
And the NM trigger sear must be thinned to fit the OM hammer notches or the Power Custom ½ cock hammer, exactly like the Power Custom trigger they supply for their ½ cock hammer or buy the NM Power Custom trigger with already modified trigger sear.
Trigger return spring options:
1. OM trigger plunger/spring with a trigger spring adapter to use the existing NM grip frame.
2. replace grip frame with Ruger Old Army Stainless 'KXR-3' with wide trigger slot and provision for trigger return spring/plunger.
3. Retain NM trigger spring.
To be completely authentic by using an OM narrow (there's no steel OM grip frames with narrow trigger slot) or SBH wide trigger:
The trigger/cyl bolt pin hole must be plugged with two oversize pins, 'peen blended' and polished over flat. Then a new 8 x36 trigger screw hole drilled and tapped*.
Cylinder bolt:
Locate, drill, counter-bore and thread 8-36* a new cyl bolt screw hole in the old model location. Use an old model cylinder latch (stop bolt) and return spring. Drill small hole inside frame to capture end of the above return spring.
Loading gate:
Use OM gate and polish bright for a SS gun. Or existing NM loading gate can be modified to the OM gate style with two flats for the open and closed detents. Grind off the half round protrusion at the back end of gate shaft to disengage from transfer bar if the bar is retained. In either case, drill and tap hole in the bottom of the frame for an old model style gate plunger, spring and set screw.
Cylinder base pin:
You can use the SS cyl pin with the plunger in the end or remove plunger. Or use OM carbon steel pin polished bright.
* Details for the action screws:
Hammer screw option - Make a fake screw/hammer pin, from a large enough screw to remove threads and end up with a shank the size of the pin .202" and a slotted cap the size of the OM hammer screw cap.
Trigger screw – the existing NM trigger pin hole is perfectly sized to tap on left frame side for a stainless 8x36 machine screw which you can buy. Slight dressing down of the threads will allow it to fit thru the NM trigger hole.
Cyl Bolt screw – same as Use the factory OM screw. Drill and thread hole 8x36 for it.
Turn down the 8x36 screw caps of the trigger and cyl bolt screws a little smaller than the two original OM 8x36 screws and they won't look too close together at first glance. Drill the right side frame holes to fit the screw caps.
Locating and Drilling 3rd screw hole in frame:
I measured with the screws in the holes and the screw slots perpendicular to my line of measurement. It's easier to judge the center of the hole that way, as long as the screw slot is cut in the center of the screw cap!
The cyl latch screw hole ctr is .877" from the ctr of the hammer screw hole. And it's .185" from the ctr of the cyl latch screw to the bottom of the cyl window. Scribe a line for each measurement and center punch where they cross for the ctr of the new hole to be drilled. I always start a precise hole with a 1/16" center drill to enlarge the punch dot. Then follow with a little larger pilot drill all the way thru the work. Then the final tap size hole. I use 'gun taps' (2 flutes). The trickiest part is clamping the tapered frame so the hole goes thru at a right angle to the frame ctr line.
**You can make the simple aluminum adapter below to fit in the NM GF behind the old model trigger or just use a .22 LR case to hold the return spring and plunger in place. Either will need the one NM trigger return spring retaining pin closest to the trigger shown below 2nd photo, to hold your OM trigger return spring/plunger adapter or .22 case in place. File a small notch in the .22 case head to nest on the retaining pin.