I took a photo after every impressioning step (= look for marks, file, take photo), so I can see the progression. Pin 4 is the torpedo pin. It stops marking in step 20, and in step 21 there were no markings of any pins at all.
Animated GIF, click to open the image to see the animationIn an attempt to get markings I tried many techniques; various types of wiggling and the "pull-out" method. None of them yielded any pin marks for pin 4. The normal wiggling movement does appear to feel different when it's hung up on this torpedo pin, but I cannot describe the feeling.
After filing cut 4 another 0.2 mm deeper, pin 4 starts marking again, so I could finish impressioning.
Solutions I tried:
Visually detect any torpedo pins in the lock by peering down the keywayThis works reasonably well, but it gets hard after pin position 3. Not all torpedo pins are copper, I also have a lock with a stainless steel torpedo pin.
Detecting torpedo pins by touchNo luck so far.
Adding ink to the blank with a permanent markerDoes not yield any good pin marks.
Solutions to try:
* Altering the blank key for a more "sloppy" fit
* Filing the blank exactly on pin depth
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