If this is the piece you're talking about, it's a knock sensor. Detects predetonation which the ignition box or ECM will retard timing when it occurs, avoiding engine damage like cracked pistons and rings, damaged bearings, etc. If your ignition setup uses this, it should be installed. Commonly installed in block drain holes, do not replace with a drain plug unless you're running an old style ignition with no provision for a knock sensor.