I have not got it happening on a imapct, but I had a similiar experiance with a wildcat tube. My mk2 wildcat was ammong the first batches wich had a old style reg (not externable adjustable), so I desided to install a huma reg, for the easier way of adjsuting it. After a couple of times taken the reg in an out trying different tunes, the threads started to go not as smooth halway in. What I did was to use force, trying to screw it all the way in, wich made the problem worse. After I got it about 70-80% in, it got completely stuck. Tried to attach it on a wise with some tools, but never got it off. I ended up bying a complete new tube, with AMP reg instead. Still have that old tube in the closet, with the endcap stuck on it. The lesson for me is to to be carefull with aluminium threads, they are not like steel, where you can use brute force, if they runs tight. Keep the threads clean before assembly. And do not count on using cilicone grease, as it does not help at all.