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You shall better grind off up and down of the ball to prevent Splited off the parts, it require considerble force to conect each other it's mean is hard to conect the parts.
it once parts conected each other they shall able to rotate about 10~15 degrees in design and All parts not same size as it grow 3% per parts from head to Set 21 and shrink 2% per part untill reach the tail tips.
i hope 9th cross section image shall help you understand it's structure.
As many builders said this dragon is quite hard to build :
Thank you.