Stainless Steel Fastener Glossary (UK)

Stainless steel fastener glossary (UK)

This glossary explains the terms UK buyers meet when choosing stainless steel screws, bolts and nuts, such as set screw, grub screw, A2-70, galling and nyloc nut. Each entry gives a short definition first, then the Simfix range for that item. Simfix® is the UK stainless steel fixings brand of simfix.com, a trading name of Apex Stainless Fasteners Ltd (Company No. 02631068).

Set screw

In the UK a set screw is a bolt that is threaded along its whole length, with no smooth shank. In the US the same fastener is called a cap screw, and the US term "set screw" means something else. Simfix lists hexagon head set screws to DIN 933 and ISO 4017 in A2 (304) from M2 to M39 and in A4 (316) from M2.5 to M42. Browse hexagon set screws.

Grub screw

A grub screw is a headless screw that holds one part against another by pressure. It is not the same as a UK set screw, which is a fully threaded bolt with a head. Simfix lists grub screws as socket set screws to DIN 913 (flat point), DIN 914 (cone point), DIN 915 (dog point) and DIN 916 (cup point). They are stocked in A2 from M1.6 to M20 and in A4 from M2 to M20. Browse socket screws.

Fully threaded and part threaded

A fully threaded bolt has thread along its whole length, and a part threaded bolt has a plain shank under the head. Simfix lists fully threaded hexagon head set screws to DIN 933 and ISO 4017. It lists part threaded hexagon head bolts to DIN 931 and ISO 4014, in A2 from M5 to M45 and in A4 from M5 to M48. Browse bolts.

A2 (304) and A4 (316)

A2 and A4 are two of the austenitic stainless steel groups in BS EN ISO 3506. A2 has corrosion resistance equivalent to grade 1.4301 (304). A4 contains molybdenum and has corrosion resistance equivalent to grade 1.4401 (316). Almost every Simfix product is A2 (304) or A4 (316). Read A2 vs A4 stainless steel to choose between them.

A2-70 and A4-80

A2-70 and A4-80 are designations from BS EN ISO 3506 made up of a letter, a digit and a two-digit property class. The letter is the steel group, here A for austenitic. The digit denotes the alloy group within austenitic steels (A1 to A5), so A2 and A4 are different alloy groups. The final two digits are the property class, which describes the mechanical properties of the fastener. Simfix lists A2-70, A4-70 and A4-80 products.

Galling

Galling is a form of wear caused by adhesion between sliding surfaces, and it can make a thread seize. Stainless steel is particularly susceptible to galling, and it can often occur in screws and bolts. Grease and oil reduce galling. Read how to stop stainless steel bolts seizing.

Nyloc nut

A nyloc nut is a lock nut with a nylon insert that resists the nut loosening. It is a prevailing torque lock nut, which means the insert grips the bolt thread and the nut keeps some turning resistance. Simfix stocks nyloc nuts to DIN 985 in A2 from M2 to M36 and in A4 from M2.5 to M36. It also stocks DIN 982 nyloc nuts in M4 to M24 in both grades. Browse nuts.

Dome nut (cap nut, acorn nut)

A dome nut is a nut with a domed end on one side. It is also called a cap nut, domed cap nut or acorn nut. Simfix stocks DIN 1587 dome nuts in A2 from M3 to M30 and in A4 from M3 to M36. Browse nuts.

Coach screw, coach bolt and carriage bolt

A coach screw is a large wood screw with a hexagon head, and a coach bolt (carriage bolt) is a bolt with a shallow domed head and a square section under the head. The coach screw screws into timber with no nut. The coach bolt passes through a hole and takes a nut. In the UK a lag screw is called a coach screw. Simfix stocks DIN 571 hexagon head coach screws in A2 and A4 from M5 to M16. Browse coach screws.

Cup square bolt

A cup square bolt is a carriage bolt with a domed head and a square neck that stops the bolt turning in the hole. Simfix lists it as a cup square bolt to DIN 603, stocked in A2 and A4 from M5 to M16. Browse bolts.

Across flats (spanner size)

Across flats is the distance between two opposite flat sides of a hexagon head or nut, and it is the spanner size. An M8 bolt takes a 13 mm spanner, and an M10 bolt to ISO 4017 takes a 16 mm spanner. See the full metric bolt size, pitch and spanner chart.

Thread pitch (coarse and fine)

Thread pitch is the spacing of the thread in millimetres, and each metric diameter has a standard coarse pitch. An M8 bolt has a coarse pitch of 1.25 mm and an M10 bolt has 1.5 mm. A fine pitch is a smaller spacing for the same diameter, and only some products have one. See the metric bolt size chart and the tap drill size chart.

More guides: fastener head and drive types, DIN, ISO and BS standards explained and the fastener size guide.