I use ball joint in Prime waist part like I used on Unicorn Gundam before. This part is only consist of 13 pages of patterns, a quick burst before modeling OP last part which is part 5 or legs. Keep the spirit and thanks to you guys who follow this project this far, the credit is for Michael Bay and his team who design this awesome character in the first place, leader of the Autobots, Optimus Prime (Transformers 2007).