Screws
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Frequently asked questions
Are DIN 912 and ISO 4762 socket head cap screws the same?
Essentially yes: ISO 4762 replaced DIN 912 and head dimensions are practically identical. They are typically supplied in class 12.9 black steel or A2-70 stainless. State the property class when ordering, since the same head style can differ greatly in load capacity.
What is the difference between self-drilling and self-tapping screws?
Self-tapping screws need a pilot hole and form their own thread in thin sheet, plastic or wood. Self-drilling screws have a drill point that drills and taps in one pass, widely used on metal roofing and light steel framing. Choose a drill point length greater than the total material thickness to be penetrated.
Which drive type resists cam-out best?
Hex socket and Torx (hexalobular) drives transmit torque far better than Phillips or slotted drives. Phillips is designed to cam out under excess torque, so it strips easily when overtightened. On lines using powered screwdrivers, Torx reduces bit wear.
What should I watch when fitting stainless screws into aluminum?
Stainless in contact with aluminum in damp conditions causes galvanic corrosion, attacking the aluminum. Use anti-seize or an insulating layer, and consider thread inserts so the aluminum thread survives repeated assembly. Stainless into stainless is also prone to galling when driven fast without lubrication.
What pilot hole size suits self-tapping screws in sheet metal?
Pilot hole diameter depends on screw size and sheet thickness and is normally taken from the screw maker's table. Too small a hole makes driving hard and risks breaking the screw, while too large a hole gives shallow thread forming that strips easily. In thin sheet the pilot hole is usually close to the screw's minor diameter.
What countersink angle do flat head screws need?
Metric countersunk screws to ISO 10642 and ISO 7046 have a 90 degree head, so the countersink must also be 90 degrees. Inch screws commonly use 82 degrees, and mixing them gives uneven head contact that loosens easily. Countersink depth should leave the head flush or slightly below the surface.
What should be watched when tightening class 12.9 socket head screws?
Class 12.9 screws are very strong but less ductile, so they are sensitive to overtorquing. The mating thread must be strong and deep enough, or it will strip before the screw reaches its preload. Use a torque wrench and good quality hex keys to avoid rounding out the socket.
What is the EPDM washer on roofing screws for?
A steel washer bonded to an EPDM rubber layer seals around the screw shank, keeping rain from entering the drilled hole in roof sheeting. Tighten just until the EPDM bulges slightly at the washer edge, since overtightening crushes the rubber and breaks the seal. EPDM resists UV and weather better than ordinary rubber.
