whats the diff...

between the RE super flex large, and the re superfles builder ball with 5/8 or 3/4 hardware?

i am getting ready to start mt radius arm set up, and am not sure which joint to buy

Beuller??

The HD superflex joints are bigger and stronger than the large joints. I bought two of the HD 5/8" joints for my build. The HD joints also have twice the missalignment (34 degrees), than the large joints (13 degrees). Also, the width of the body is different, the HD joints are 1.5" while the large joints are 1.875". Keep this in mide when you attach them to your arms. I welded 1.25" all thread to them and taped out the tube.