One can “crush” the neck back down to the appropriate size, with a proper sized threaded rod/arbor so that it fits in the threads to prevent making it too small. Think a band clamp, but a tad stronger.. Knowing the thread size and a friendly machinist that can thread can make the plug, then just figure out how to collapse the neck back down to size… bonus info, if you know what size the threads are you can purchase a cheap Chinesium tap and then make the neck a tad smaller and recut the threads. By you SIG I am guessing that the motor is the 1930 model?
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