Between the poor joint design and rubber power cables, the arms of the original kit were left with just about zero articulation. I wanted a lot more motion than that, and I've never seen a Hi-Zack model that properly replicated both the look and movement of the arms (most versions just leave off the power cables!). I originally came up with a joint system that had a ball joint at the top of the elbow (shown at left), but it didn't look particularly good, and the movement gained didn't justify the aesthetic sacrifice.
I ended up using a joint system like the one on the Master Grade Zaku's. First, I built a new bicep part by constructing a mold from plastic sheet, then filling it with Bondo putty. I sawed off the rounded bottom of the original bicep, and made a half circle elbow connecting joint, using a T shaped polycap to plug it into the bicep. A brass rod is attached to the bottom of this part, which plugs into a large polycap in the forearm (held in place with more Bondo). The bicep plugs into the shoulder part with another polycap and plastic peg.
I decided I didn't like the soft look of the squared off parts of the forearms, so cut them off and replaced them with parts built from styrene sheet. This also added a sectioned armor look, which is nice. This required adding a support between the two halves, also from plastic sheet. I took this opportunity to goop some Bondo in the elbow to hold the polycap that attaches to the upper arm. These ended up a little wider than the original arms, so I widened the gray part and filled the cut out (that was supposed to allow the elbow to bend, but didn't do any good) with plastic sheet as well. The power cables in the forearms were replaced with some elastic strings that had the right crosshatch texture, which are plugged in to short slices of brass tubing (the power cables coming from the torso plug in to the other end of these tubes, inside the elbow).
I wanted a more detailed joint for the shoulder, and hoped for some more forward/backward movement as well. So I chopped up the kit supplied block shoulder joint and built a detailed version using brass tubing, plastic rod, a cut up T shaped polycap, and a homemade minus mold. The middle section of the kit part was discarded, and I used the ends in my final piece. It's difficult to describe the construction, so I'll just let the pictures speak for themselves. By itself the joint moves all over the place, but the power cables added later ended up severely limiting the movement.
Keep reading to check out the mods to the legs on the model.