VTEC B series engines have a girdle on the bottom of the block to help brace the block so that the crank can rotate up to those higher RPM. When you build a xx/VTEC, you need to add one since the non-VTEC blocks don't come with one, and would not be stable if you ran them up to that high an RPM without a girdle. Stock VTEC B series block girdles don't fit non-VTEC blocks. Z10 makes block girdles for non-VTEC B18 and B20 blocks, but you can't just slap em on and expect them to work properly. They need to be installed with dowell pins to keep them in position.

Here is a detailed install complete with pictures:
http://honda-tech.com/zerothread?id=263598