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Replacing a Uberti 1872 Open-top Firing Pin
   After shooting a Uberti 1872 Navy open-top .38 revolver about a hundred times this past summer, the hammer became increasingly difficult retract from the forward position and cock. It turns out that the firing pin of this Colt replica had eventually bent itself so that it curled upwards and was interfering with the frame itself.

   Although this seems to be a common problem for these guns, I have not been able to find good information on the dimensions/size, material, or hardness of the firing pin itself.

   This page is a collection of documentation regarding the machining, heat treatment, and testing of a new firing pin.






Some Notes on Existing Literature

   My impression so far has been that this Colt revolver style firing pin is not common enough to have much literature in web circulation. An interesting forum conversation at Practical Machinist is found here and is in agreement with my findings: "Firing Pin Material and Hardness." To summarize, a firing pin ought to be neither soft nor brittle (too hard). This implies an alloy or high carbon steel without heat treatment, or the alternative: heat treatment with a near or full temper.




Removing the Old Pin

   The firing pin is retained by a solid steel pin passing through both the hammer and the pin itself. This must be pressed or punched out. I first measured the diameter using a pocket-sized optical comparator loupe. Some pictures are posted below of the 'interface' seen when placing this loupe over the feature of interest. I measured the pin diameter at being 0.085" +- 0.005".



   This picture below is for reference. Due to lighting, the pin is hidden in darkness.



   A brass punch was made (brass in order to prevent damaging the steel hammer/finish). It's tip diameter was 0.07" in order to pass into the hole as needed. Three small knocks and the pin fell through.






   The detent pin is so small and short it is difficult to measure taper along its length. However, I punched it out from the left hand side and it seems to only want to re-enter from the right and in a certain orientation. Measurements on the pin are shown in a sketch below. Wether or not the pin is actually tapered would be open to debate.



   The firing pin was snugly seated and was reluctant to leave its hole. A yank with a pair of pliers drew it out. As can be seen, the detent hole had egged slightly and the neck of the pin was bent. This makes it very difficult to ascertain the desired form.





Measuring the Firing Pin Size

   In order to make a new firing pin, we must measure the old one and produce a design to work from. The outside diameter, length (despite deformation), and basic form are apparent. However, because of the tiny scale, the hole position and size is hard to determine using relatively gargantuan hand-held instruments. The optical comparator is the ideal measurement tool for this case. The firing pin is placed on a backlit table and can be measured via its position on the screen to a resolution of ten-thousandths of an inch (albeit subjectively). By this method I determined the size of the firing pin's hole (0.081") as well as its center distance from the fat end of the pin (0.109").






   Here is my drawing of the existing Colt Uberti firing pin. It represents the original deformed part. I made an educated guess on the intended length of the original pin to be 0.515" and thus my replacement pin is 0.034" longer than depicted in the drawing. All other dimensions of my firing pin were representative of the original part. Especially the 0.109" dimension. My impression is that if this dimension were to grow (perhaps by an estimated 0.015" or more, it would be impossible to get the detent pin through the hole. The hole in which the firing pin is seated is very shallow, in other words.



   Trying to figure out why the pin deformed in the first place, its hardness was tested. Our only indentor is for the Rockwell C scale. The firing pin hardly even registered on the scale. This means that it was not hardened, and is likely a lower grade, softer, or unhardened steel.





Machining a New Firing Pin

   For my firing pin I chose some of the best tool steel we had: A2 (air hardening tool steel). For practicality, S2 steel (shock resistant tool steel) would have been more practical but it wasn't available. This steel must be hardened after machining, but is extremely tough stuff. Of course, according to what I've read in my limited research, people are using anything between spring steel (hi carbon hardened), chromoly, and other alloys (i.e.5xxx)

   First it was turned in a lathe, then transferred to the mill (Bridgeport) for drilling. Then it was sawn off and put back in the lathe to have its rear end cleaned up.











   Test reinstall and clearance check. Note: I'm not sure how far the pin must extend in order to reach the cap, but this seems to reach at least that far. At absolute most, my firing pin is 0.02" longer than the original, so it couldn't be too bad. (Note: since writing this, its become apparent that a .38 rim fills nearly the entire gap between the cylinder and the frame, thus, the firing pin probably only needs to reach halfway into said gap.)




A2 Heat Treatment

    To acquire their intended properties, tool steels require heat treatment. A2 must be brought to 1750 degF before quenching and for this I used a propane torch (burns at 1950 degF). You are supposed to hold the part at critical temperature for an hour or so, however I only did for about a minute, as I don't have access a furnace. This temperature, I've read is identified as the part travels between salmon orange and bright orange. I presume I overshot that temperature by at most 200 deg.F







   'Quenching' occurs as the part chills in open air, hardening its grain structure. The firing pin has reached its maximum hardness (mid sixties HRc). It is very brittle and prone to cracking. Tempering is carried out in a toaster oven at 450 degF (according to the toaster dial), for 2 hr. This will trade hardness for strength and allow the stresses in the grain structure to mostly be relieved. It is wrapped in foil to reduce 'scale' growing on its surfaces.



   A follow-up hardness test gives a Rockwell C-scale Hardness of 30 for the firing pin. This is significantly less than the expected 55 HRc. Softer is not a bad thing though, as cracking would be catastrophic, and it is still much much harder than the original pin.

Installation

   The new pin was dropped into the hole and the detent pin was knocked back in from the direction it came. Everything went together very nicely. I would stress again that the 0.109" dimension mustn't be allowed to get much larger.








Testing

   I had to wait a while before testing results were available. A recent trip to the Black Rock Playa, CA gave to a small shooting session in which the improved Uberti was unveiled. Unlike recent past experiences, wrought with stuck hammers and struggles to cycle the cylinder, a few flawless firings were quickly attributed to exceptional performance of the gun as a whole. An inspection of the pin after a dozen shots revealed no discernable wear (of course not), but most importantly, lent peace of mind to fears of a bending firing pin, or a chipping point. The hardness given to the pin has thus far fallen in the desirable range -- at least it seems for now. Another few hundred shots may tell a different story, but at least I am sure that the arrangement will last significantly longer than the previous.

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