Metric Socket Countersunk Head Screws
Described as Flat head in the USA are heat-treated to give a material strength grade of 12.9 for the USA market and 10.9 for the UK and European market, the exceptions in the UK and Europe are Unbrako and Holo-krome who are both 12.9. Due to the head geometry of the 12.9 Countersunk Head, the service load in the application is only equivalent to strength grade 10.9. The included head angle for Metric Socket Countersunk Head Screws to DIN 7991 up to and including M20 is 90° and over M20 60°. BS 4168, however, specifies 90° for M24 and above. ANSI/ASME B18.3.5M metric standard states 90°, so care needs to be exercised. Inch products have a different head angle. Unified are 82° and British Imperial 90°. Socket countersunk Head screws are used in applications where a flush head is required.