I decided to share what I learned in case others have similar questions.
Before we begin, a quick disclaimer (from Ruger)
I'll start this off with a couple of "exploded pictures" of an old model and a New Model (I capitalize New Model as it was part of the actual model name, where as old model or "three screw" is just a reference to the older guns).
This is a picture of my old model (1972 vintage) Blackhawk before I refinished it.
My primary source for information regarding parts was Ruger. You can download pdf copies of their manuals for every gun they have manufactured.
Really the only reason to seek this information is if you have an old model and need parts, as the New Model parts are readily available.
So that will be our focus.
Ruger left us some clues as to what parts would interchange. See the part numbers below, you can see Ruger updated the part numbers but basically left the "core" number the same:
Part description OM part # NM Part #
Pawl Spring Plunger R-51 XR05100
Main Spring R-4 XR0400
Base Pin Latch bolt R-27 XR02700
I'll list the part numbers under each parts section.
I will go through the old model parts, one at a time and explain what parts from a New Model or other firearm will fit.
It is important to understand that the old model .357 Blackhawk was built on a medium sized frame (nearly identical to the Colt SAA) where as the New Model .357 Blackhawks are built on the larger .44 Magnum sized frame.
Grip Frame:
This is one of the most complicated of the parts, they do interchange but you will need to modify or buy a new trigger to make the New Model grip frame work on an old model gun, see my complete write up here
Grip Frame Screws:
The screws on the old model and New Model grip frames (as well as the Old Army model) do interchange with one caveat: One of the bottom screws on the New Model is different, it has the extra length to act as a pivot lock (hammer pivot pin lock). You could probably cut the extra material off and make this screw work in an old model frame or just order two of the New Model XR01900 screws.
Part OM Part # NM part #
Front R-17 XR01700
Rear R-18 XR01800
Bottom R-19 XR01900 + XR01901 (pivot lock)
The old model part # for the 6.5" barrel is MR-6, while the new part # is KMR20602. This only tells part of the story. Both the new and old model guns have the same size and thread pitch (24tpi), the New Model barrels will fit in an old model gun with some machining (a shoulder needs to be removed and the barrel clocked)
The old model barrels and the barrels from the New Vaquero and 50th Anniversary (mid size frame) Blackhawks will completely interchange
Cylinders:
The cylinders between the .357 old and New Model will probably not interchange (different size frames). The om .357 cylinder measures approx. 1.672" in diameter while the NM cylinder measures approx. 1.73".
However the .44 old model Super Blackhawk and New Model Super Blackhawk may interchange. I would have to do more research to be certain.
Cylinder Base Pin:
The old model part # for the .357 is MR-29 and measures just under 1/4" in diameter, the New Model part # is MR02900D, but that doesn't tell the whole story. The New Model base pins have a plunger at the hammer end, where the old models were just rounded. The plunger puts positive pressure on the transfer bar safety so it will clear the firing pin.
Here is a collection of base pins that I purchased as a lot on ebay:
The question as to whether a new model pin will fit in an old model frame: They will, but you would need to make sure you have the correct length. Remember the New Model .357 Blackhawk is built on a larger (and longer) frame, so the New Model 357 pin will not work on an old model 357 because of the length. You may be able to shorten a New Model pin, remove the plunger and round the edges.
With the smaller 50th Anniversary Blackhawk, the frame is the same size as the old model Blackhawks, however I am not sure if the plunger would interfere with the operation of the old model hammer.
Cylinder Base Pin Latch:
The cylinder base pin latch bolt, spring and nut are completely interchangeable between old Model and New Model
Part OM Part # NM part #
Bolt R-27 XR02700
Nut R-28 XR02800
Spring R-47 XR04700
Loading Gate:
The loading gates will not interchange, not only are they physically different in size, the detents/mechanism that hold the gate shut are also different.
Ejector Rod Housings:
The Ejector Rod Housings (ERH) are still a bit of a mystery, as described in my post for my old model Blackhawk project.
I have been told, and have tested the theory that with few exceptions (.357 Maximum, Vaquero Montado etc..) the ERHs will interchange between old model Blackhawks and Single Sixes and their NM brethren. There is a difference with the screw hole size, the Super Blackhawk, Bisley and some other models came with a larger screw hole. I made this mistake when buying a new steel ERH for my NM Super Blackhawk, to make it fit I simply drilled the 3/16" hole to the larger 1/4" size.
Keep in mind some of the New Models may have steel ejector rod housings (most still have aluminum) whereas all of the old models were aluminum.
Part OM Part # NM part #
ERH R-22 XR12200
Ejector Rods:
Despite having a different appearance, the ejector rods from old model to new model will interchange (with a few exceptions like the longer .357 Maximum, or the shorter Vaquero Montado etc..). The om part # is MR-23A, the NM part # is XR-55, but don't let that scare you, I have personally swapped rods between old and New Models.
Part OM Part # NM part #
ER MR-23A XR55
Ejector Rod Spring:
As above with a few exceptions the ejector rod spring is identical.
Part OM Part # NM part #
ERSp R-44 XR04400
Ejector Rod Housing Screw:
This can get tricky, some early Blackhawks came with a threaded stud in the barrel, which was later changed to a shouldered bolt. The shouldered bolt came in two sizes: 3/16" and 1/4" (I believe the threads are the same: 8x40). The larger ones being used on the Super Blackhawk, Bisley and some other models.
Part OM Part # NM part #
ERSc R-33 XR03300
Trigger:
There is no interchangeability here, the New Model Blackhawks employ a transfer bar safety that connects to the trigger (lower hole), in addition the trigger return spring is different, requiring a boss for the spring to pry against. See the picture below, for more detail see my post on the grip frame differences here
The good news is that several styles of old model replacement triggers are available from Clements Custom Guns
in addition looking at the part and the part number, the Ruger Old Army trigger may fit, :
Part Old Model Part # Old Army part #
Trigger R-39 CB03900
Hammer:
While the part numbers are similar MR-40 vs MR04000B the hammers are miles apart with their construction. The old models had three sear notches (with 4 clicks), while the New Models have one (2 clicks). In addition the NM Blackhawks have a transfer bar safety which means a portion of the hammer is missing, which must be replaced by the transfer bar (held in place when the trigger is pulled back) in order for it to strike the firing pin. Even if you could get a NM hammer to fit the old model frame, it would never hit the firing pin. This one of the parts replaced when Ruger performs the safety upgrade (installing a retro-fit transfer bar safety system).
The good news is that replacement old model hammers with some options are available from Clements Custom Guns
Hammer Plunger, Spring & Pin:
This is the small plunger that sticks out of the bottom of the hammer. These parts interchange
Part OM Part # NM part #
Plunger R41 XR04100
Spring R-43 XR04300
Cross pin R-42 XR04200
Hammer Strut:
The hammer struts are identical and fully interchangeable, unless you have a newer Blackhawk with the safety lock.
Part OM Part # NM part #
Hammer Strut R-4 XR00400
Hammer / Main Spring:
These are also fully interchangeable, unless you have a newer Blackhawk with the safety lock.
Part OM Part # NM part #
Hammer Spring R-15 XR01500
Hammer / Main Spring Seat:
As above, these are also fully interchangeable, unless you have a newer Blackhawk with the safety lock
Part OM Part # NM part #
Hammer Spring Seat R-5 XR00500
Pawl:
While I have never attempted a swap, these parts are identical from what I can tell, the NM pawl fits onto the old model hammer and will fit in the gun. The pawl may need to be fitted to your gun, especially if your gun has some wear.
Part OM Part # NM part #
Pawl MR-7 KMR00700
Pawl Spring & Plunger:
The pawl spring & plunger are identical and fully interchangeable, they used to be listed as one part, whereas now they have separate part numbers
Part OM Part # NM part #
Pawl Spring R-51 XR05000
Pawl Spring Plunger same XR05100
Cylinder Latch/Bolt Stop:
There is no interchangeability here, they are completely different parts.
However the Ruger Old Army cylinder latch carries a similar part number and looks like a match, so they may interchange
Part Old Model Part # Old Army part #
Cylinder Latch R-45 CB04500
Cylinder Latch/Bolt Stop Spring, Plunger & Screw:
The New Models use a different arrangement, no interchangeability here.
Firing Pin:
Based solely on the part numbers I believe these parts do interchange
Part OM Part # NM part #
Firing Pin MR-21 MR02100
Firing Pin Rebound Spring:
Based solely on the part numbers I believe these parts also interchange
Part OM Part # NM part #
Firing Pin Spring R-48 KE-48
Recoil Plate (Firing pin housing):
Based solely on the part numbers I believe these parts also interchange
Part OM Part # NM part #
Recoil Plate MR-20 MR02000
Recoil Plate Cross Pin:
The recoil plate cross pin goes through the top portion of the rear frame of the gun. Being that the .357 old model has a smaller frame than the New Model guns, it would make sense that these would not interchange. However you may be able to use the longer New Model pin and file it down to fit.
Part OM Part # NM part #
Recoil Plate Cross pin MR-60 XR04900
Hammer, Trigger & Latch Pivots:
These parts are what give the old models the nickname "3 Screw". These are screws that act as pivot points for the hammer, trigger and cylinder latch. The New Models used a different cylinder latch arrangement and replaced the hammer & trigger screws with 2 pins.
There are some other Ruger parts that will fit. The Ruger Old Army Revolver used the same screws. I know this because I used Old Army screws on my Old Model Blackhawk project.
Part Old Model Part # Old Army part #
Trigger Pivot R-34 CB03400
Latch Pivot R-34 CB03400
Hammer Pivot R-16 CB01600
Rear Sights:
The rear sight did change, the flat tops had a different sight, but once Ruger added the squared sight with the "wings" on the top strap (to protect the sights) I believe they stayed the same when the switch to the New Model was made.
There are some exceptions: Super Blackhawk and the Blackhawk Hunter models had different sights.
Part OM Part # NM part #
Rear Sight Assembly MR-35 MR15900
Elevation Screw MR-65 MR05902
Springs MR-58 MR05906
Retaining/pivot pin MR-56 MR05600
Front Sights:
The front sights on the old model were silver soldered on. On some of the New Models they were held on by a screw. They appear to be the same and if you can silver solder or drill and tap you could probably make them work.
That about wraps it up, as always consult Ruger or a qualified gunsmith if you are unsure about parts interchangeability.
Some of the pictures above were found freely on the world wide web and are used under the guidelines of Fair Use, per Title 17 of the U.S. Code. Where possible the source has been credited.
If you own the copyright to any of these images and wish them to be credited or removed, please contact me immediately.