Only the B18C block has a girdle. What it does is connect all of the main bearing caps together. The B16 and B17 blocks make do without it because they have a short enough stroke to not cause too much stress. The B18C however, is intended to rev just as high as the B16 and B17 and thus needs the reinforcement.

If you need an explanation of what the main caps are, etc I can probably copy something out of my Helms manual for you.