STAINLESS STEEL DOUBLE END STUDS, END 1.25XD DIN 939

Double End Studs, end 1.25xd DIN 939 — Double End Studs to DIN 939 share the same double-ended format as DIN 938 — threaded at each end with a plain shank between — but use a longer engagement length at the tapped-hole end, approximately 1.25 times the thread diameter rather than 1xd, specifically because it's intended for tapping into materials that are softer or lower-strength than steel, most commonly grey cast iron. Cast iron has lower thread shear strength than steel, so a 1xd engagement that would be perfectly adequate in a steel component can be more prone to stripping in cast iron, and the extra quarter-diameter of thread engagement in DIN 939 is what compensates for that, giving a joint strength broadly comparable to the stud's own rating rather than being limited by the parent material's thread. That makes the choice between DIN 938 and DIN 939 primarily a question about the tapped component's material rather than the stud's own grade or size: check what the housing, casing, or bracket the stud is going into is actually made from, and specify DIN 939 wherever that's cast iron or another comparably soft material, even though DIN 939 studs can generally also be fitted into steel where the longer engagement provides at least as much thread contact as the shorter standard. Typical applications mirror DIN 938 — flange joints, pump and valve bodies, machine housings — but specifically where the tapped component is a casting rather than a machined steel part. Grade selection for the stud material itself follows the same reasoning as elsewhere in the range: A2 stainless for general indoor mechanical assembly, A4 for washdown, marine, or process-chemical environments, particularly relevant here since studs in gasketed or sealed cast-iron housings (pumps, valve bodies, manifolds) are often in continuous contact with process fluid or external moisture. Diameter, thread pitch, and overall length should be selected to match the tapped hole depth in the casting and the nut and washer arrangement needed at the free end.

FAQs
Why does DIN 939 use a longer engagement length than DIN 938?
The longer engagement length is intended for tapped holes in materials such as grey cast iron, which generally require greater thread engagement than steel to achieve a comparably strong joint, given the lower thread shear strength of cast iron.

Can DIN 939 studs be used in steel components instead of DIN 938?
They can generally be fitted into steel as well, since the longer engagement provides at least as much thread contact, but DIN 938 is the standard specifically calibrated for steel and is more commonly specified there.

How can I tell which engagement length a stud has if it isn't marked?
Measure the shorter of the two threaded ends against the nominal diameter — roughly one diameter's length indicates DIN 938, roughly 1.25 times the diameter indicates DIN 939 — though checking the supplier's part reference is more reliable than measuring on a fitted stud.