still trying to figure that out myself havent gotten to that stage yet, sorry
I myself have not really completed many combiners, so I'm not the person to ask. However, I do know that people use a variety of different joint mechanisms in their projects, depending on the needs of the particular figure they're making.
Often these joints come from other toys. 3 of the most common connector methods use joints from some of the following toys...
Neo Shifters (a Mega Bloks brand imitation of Bionicles). These are fairly cheap figures, and you can get a lot of joints in one set. They aren't ratcheted, but they pivot and rotate, and hold very securely once plugged in.
Lego Knights These are getting hard to find lately... they're figures you can assemble out of specialized Lego parts. The joints are pretty strong, pivot and rotate, and have a sort of ratcheted plug so it will hold the weight of larger figures' limbs well.
Lastly, some folks use cannibalized Energon Connectors. These are from the combiner robots that were released during the Transformers Energon line. Each small robot had a socket-and-joint system in their chest, and the larger central robot had plugs in his shoulders and feet. 2 of these figures are getting a recent re-release as ROTF Superion and ROTF Bruticus (even though they aren't really movie-related toys). Not the cheapest way to get joints, but they're very strong and versatile. Those take more work to kitbash into a new design though...
Hope that helps
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