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I have put small strips of plumbing tape on the contact surfaces of the male ends of the bits. Make sure nothing ends in the bore and then in the valves. Also make sure to take the bits and neck apart after use to avoid them getting stuck together.

Klaus Smedegaard Bjerre

if you're brave and CAREFUL....try pushing the receiver apart a bit, where the screw is to tighten around the tenon of the neck. If the neck is simply too small, it won't do any good, and the tape suggestion is the best. If the receiver has been pushed out of shape a bit, expanding it may do the trick. If you aren't confident with doing that, either take it to a repair shop or just use the tape.