GENERAL. This standard for Metric Flat 90 degree countersunk Head Type 1A Cross Recess Tapping Screw of thread forming Types AB and B and thread cutting Types BF and BT conforms in all essentials with the American National Standard covering metric tapping screws. It should be noted that, for most screw sizes, the head diameters specified herein differ from those documented on previously issued detail drawings. Therefore, it is recommended that any countersink provisions in existing designs, based on earlier metric flat head screw dimensions, be reviewed and adjusted accordingly. In general, screws made in conformance with this standard are considered to be functionally interchangeable with parts produced to the latest ISO standard for recessed countersunk head tapping screws. Also, for the most part, the Type 1A cross recess configuration will serve to identify these screws as being metric.