STAINLESS STEEL POZI FLANGED AB SELF TAPPING SCREWS BS 4174

Pozi Flanged AB Self Tapping Screws BS 4174 — The defining feature of BS 4174 Pozi Flanged AB self tapping screws is the integral flange moulded beneath the head — effectively a captive washer that spreads clamping force over a much wider footprint than a plain pan head, without adding a separate loose part to the assembly. That matters most in thin sheet steel, aluminium, GRP, and other soft or low-stiffness materials, where a narrow pan head can dig in, deform the surrounding surface, or pull straight through the material during vibration; the flange resists all three. Typical uses include roofing and cladding fixings, ductwork, cable tray, and general sheet-metal assembly across construction, agricultural building, and HVAC work, anywhere a wide bearing surface is needed but a separate washer would slow assembly. The AB point self-starts into thin gauge material without a pre-formed hole in most cases, though a pilot hole is still worth drilling for accurate, straight location, especially through pre-painted or coated sheet where a wandering point can mark the finish. BS 4174 is the British Standard covering this flanged head format specifically — its head dimensions, flange diameter, and tolerances are set independently of the German DIN 7981 range, so a part called up to BS 4174 shouldn't be assumed dimensionally identical to a DIN flanged equivalent. Grade choice follows the same logic as elsewhere in the range: A2 stainless handles the great majority of internal and general external fixing, while A4 earns its premium on coastal roofing and cladding, external cladding facing prevailing weather, or anywhere chloride exposure or washdown chemicals will attack a lower grade over years of service. Choose a screw diameter and length that gives full thread engagement in the base material with two or three threads showing through, and make sure the flange diameter comfortably covers the clearance hole in the fixed component.

FAQs
Is a separate washer still needed with these screws?
Generally not, since the flange performs the washer's function. Where the clearance hole is oversized relative to the flange, a separate washer may still be appropriate.

Can a BS 4174 flanged screw be substituted where a drawing specifies a DIN 7981 pan head?
Not directly. The two are separate standards with their own dimensional and tolerance requirements, and a flanged head changes the bearing area a design may be relying on, so substitution should only be made after checking the drawing's intent.

Does the flanged head need a wider clearance hole than a plain pan head?
The shank clearance hole is the same; it's the flange diameter that determines how much bearing area is available, so match the flange size to the load-spreading requirement of the joint.

Are these available in A4 as well as A2?
Yes, A4 (316-equivalent) is available for coastal, marine, or washdown environments where sustained chloride exposure would otherwise attack the finish.