I had a similar problem assembling the Tekno SCT410 diffs. Really, its not something just associated with the Tekno to be honest. Basically when I put them together, really have to make sure that the gears all seat together correctly, as if you try spinning the cups at all even by mistake, thing can shift enough that when you snug down the screws, it will bind up. I had that happen like 3 times before I figured it out, and, it did that once when I put the steel spur on the EB48.