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"Does this crop up often ?"Not a particular weakness but may not have annealed them them when last done by po, heat to dull red and quench in cold water for copper as you're probably familiar with. Pinching them back tight again you may also have to tweak them with just a little speed to get them to bite, you'll feel if they just spin easily when you get near pinching them if they are going to tighten to compress the washer competently.