OH NO!!!!! I just tore down my motor for a complete rebuild and have found the almost all of the serial number on the block was missing. It wasn't filed or ground off, just a few numbers of it were barely visable. The machine shop said this was common because the serial numbers were stamped on the block before any machining work is done. He also confirmed the motor was original and had never been rebuilt, but had high miles. One other thing he told me was the motor was not an '89 mustang, but rather an '88 thunderbird motor. He got this info from the block casting number. Has anyone had this problem with a missing serial number??? Please help me guys....