Revolver locking notches

A

Aguila Blanca

Staff
I have a replacement cylinder for an Italian SAA clone that was begun for me by an older machine shop owner who was a Vietnam veteran. Unfortunately, he became ill and had to close down his shop before the cylinder was completed. What's left to do is deepening the notches for the bolt to lock into.

I can't find any information on the correct dimensions for these notches (radius of cutter, and depth and width of cut). Even if I had the information, I doubt it's anything I could accomplish at home. But with the information I might be able to find a local shop that could handle it. Does anyone have this information?

Second question: can anyone recommend a revolver gunsmith who is qualified to do this?
 
It would seem to me that you could measure the width and length of your bolt, add a few thousandths and that will work for the slot. You could measure bolt protrusion from the frame to get a approximate depth. To me laying them out would be the most difficult task.

I found this drawing on the web that gives frame dimensions for various manufacturers. You don't say who made yours. I am assuming it is an older piece made by a company that is no longer in business?

ColtUbertiComparsions1-vi.jpg
 
Tidewater Kid said:
It would seem to me that you could measure the width and length of your bolt, add a few thousandths and that will work for the slot. You could measure bolt protrusion from the frame to get a approximate depth. To me laying them out would be the most difficult task.
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I have laid them out. I measured a completed cylinder from a different SAA optically, using an optical comparator, and came out with a notch length of .375" and a width of .155" + ,005 (or so). Depth seems to measure approximately .040".

After entering all that into AutoCAD, what I come up with is that I need a Woodruff Key cutter with a diameter of 7/8" and a face width of 5/32". Does that sound about right?

If so, I'll buy the cutter, and then I'll have to figure out how to make the cuts -- reliably located. They're started, but they need to be cut a bit more than twice as deep as they are now.
 
"I'd use a key cutter too and a milling machine with a rotary index head. "

Use a key cutter as mentioned or grind an oversize one to your exact dimension. I'd rather build a fixture with a pin to locate on each cylinder bore so if there is any variance hole to hole the cut will be in the right place in relationship to the bore.
 
That would work too but are the notches center of the cylinder hole? Even if not, the jig could be made such that the key cutter would be in the right spot.

A jig would be good for production but I don't see any advantage over an index head. Once that is set up, you leave it alone. Make the cut, back off. Rotate the head whatever # of times to the next position and then move it back to the cutter.

Can you post a sketch of the jig you have in mind?
 
I would guess he's got bolt/latch offset from the cylinder pin holes that he can measure in the frame. An important point is that a six shooter has one chamber at 6:00 when another is at 12:00, and that offset often doesn't move the notch very far from the 6:00 chamber wall. It's a weak point on all six guns (the reason a lot of the heavy caliber modern revolvers are five-shot rather than six). On the other hand, the pressure of the 45 Colt is pretty low. Bottom line, though, is that I would try to err on the side of shallow and err on the side of as much offset as you think will still function.
 
The notches are already there (and they are offset), but they're not deep enough. Locating them isn't the problem -- cutting them is the problem.
 
Bought the Woodruff Key cutter with a diameter of 7/8" and a face width of 5/32" from MSC Supply. Worked very well. Cost me a grand total of $22.58, and I have a cutter that may have other applications down the road.
 
May I suggest you measure the offset of the edge of the notch closest to a chamber wall and then figure out how thin it will make the metal between itself and the chamber. A hoop stress analyses will then show how much pressure the resulting chamber can take. Jack Belk pointed out a Chiappa Rhino 44 Mag that had .045" between the chamber and the bottom of the locking bolt slot that blew out and cut off a trigger finger. So, even though you will be at much lower pressures, it's not a consideration to simply ignore without checking. That's why you were wise to ask for a drawing to see what the original value was. I'm just sorry nobody had one of the cylinder to offer.
 
Unclenick said:
May I suggest you measure the offset of the edge of the notch closest to a chamber wall and then figure out how thin it will make the metal between itself and the chamber. A hoop stress analyses will then show how much pressure the resulting chamber can take. Jack Belk pointed out a Chiappa Rhino 44 Mag that had .045" between the chamber and the bottom of the locking bolt slot that blew out and cut off a trigger finger. So, even though you will be at much lower pressures, it's not a consideration to simply ignore without checking. That's why you were wise to ask for a drawing to see what the original value was. I'm just sorry nobody had one of the cylinder to offer.
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That's a sobering consideration. Using the dimensions I have for the cylinder and standard SAAMI dimensions for the chamber diameter, I modeled it in AutoCAD and I come up with a wall thickness of .0345" between the deepest part of the notch and the chamber wall. That's using a .44 Magnum chamber, which is what this cylinder began life as. But it was cut down to the diameter of a .45 SAA cylinder. Notches in a SAA cylinder measure .040" deep and I cut mine only .030", so a .45 cylinder would leave much less wall -- about .0205". I'm loading very light, just for plinking, so I think I'll be okay.

That said, perhaps I'll use a wire pull rod to activate the trigger for my first dozen or so rounds.

Thanks for helping me not be overly close-focused.

[Edit to add] I ran my bullet through the Hodgdon reloading web site. I use Winchester 231/HP-38, so that gets me right on the mark. The test load I first ran through the modified cylinder (before recutting the notches) was the starting load of 4.5 grains. There was essentially NO felt recoil (about like shooting a .22), and the brass was black -- clearly not obturating. So I'll bump it to 4.7 grains, which is still well below the max load of 5.3 grains. The max load is shown as producing 9,400 CUP, and the starting load just 7,000 CUP. At 4.7 grains, I'll be maybe at 7,500.

.44 Magnum loads for the same powder run in the 38,000 CUP range, or five times what I'll be shooting. I think I will use a pull rod for the first few shots, but I don't think I'm into dangerous territory.
 
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Sounds like a plan! I also read about a gun that, rather than blowing apart, merely pushed the floor of the notch to bulge outward over time. Can't recall the gun or the chambering, but I'm thinking you could take a light and a probe like a dental pick with you and make sure you don't see or feel any depression forming inside the chamber nearest to the notch. Check it from time to time for cumulative effect. At the pressures you are are running, the brass will spring back to shape pretty completely, so I don't know if you can count on it to mirror such an indentation, but monitor it over time anyway.
 
Hmmmm, if I were having a dedicated cylinder made for a S.A. of any caliber, I would have a Ruger type notch (thin, which extends the offset) and fit the bolt head for the new locking notch. That way, you build in strength/safety. (It's always best to fit the bolt to the notch, not the notch to the bolt.)


As far as the Rhino, I knew there was a reason I didn't like the "firing" chamber being that close to my hand!!

Mike
www.goonsgunworks.com
Follow me on Instagram @ goonsgunworks
 
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