Posted: 11/7/2024 8:37:41 PM EDT
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I just installed a new recoil spring in my K9. I've had it since the late 1990's and have the original recoil spring in it. I asked Kahr about the replacement interval on the K9 and was told 800 rounds or so, OR 2 years. They told me that the guys in the Kahr shop pre-break in recoil springs by slipping them over a screwdriver shaft and fully compress the spring 4-5 times by pressing it against the edge of a bench. I got a new spring today and it was at least 3/4 inch longer then the original spring. I did the pre-break in trick and the spring was the same length as the original spring but feels stronger. |
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This process is referred to as "scragging." It introduces a residual stress field in the wire that opposes the direction of loading, which permits the spring to achieve a higher apparent torsional shear stress and a higher strain energy than would otherwise be possible had the spring been produced at the "correct" free length to begin with. Read page 70 of this document for a succinct explanation of scragging: Evaluation of Soviet Automatic Aircraft Guns 37mm NS and 37mm N If you examine U.S. firearm blueprints carefully, you will find that the overwhelming majority of springs *might* need to be scragged in order to be produced, if the wire tensile strength happens to fall at the lower end or even the middle of the permissible range. Pistol magazine springs almost always lose 10-20% of their initial free length after loading. That's totally normal, and desirable. |
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I bought a cheap CW9 a few years back that would fail to fire sometimes, the forums said replace the recoil spring. I thought: weird right? I ordered a new spring. Let me tell you what, in 30+ years of handguns I've NEVER had a recoil spring so hard to replace. I had put the slide in a vise to install it. I'll remember "scragging" if it'll help. The new recoil spring fixed the fail to fire. |