Inch Fasteners

Inch Fasteners

Inch fasteners were standardized by four American organizations, namely:
ASME: American Society of Mechanical Engineer
ASTM: American Society for Testing and Materials
SAE: Society of Automotive Engineers
IFI: Industrial Fasteners Institute
The definition of a bolt, and we include here also washer or nut, in imperial units (inches) consists of the following parameters:
[fastener type-standard] [thread diameter]-(pitch)x(length) [material] [coating]
If we compare them with the European fasteners defined by DIN and ISO standards, we find the following differences:
DIN or ISO standards have a standard for each type of bolt, i.e., just by naming the standard, you refer to the type of bolt you are looking for. For example, the DIN 933 standard refers to fully threaded hex head bolts. This is not the case with the American standards, since the same standard can define several types of bolts, so to define it, it is necessary to name the type of screw itself, and it is usually accompanied by the standard so that it does not lend itself to confusion.
In DIN and ISO standards, the thread pitch is usually omitted if it is coarse and is indicated after the metric if it is fine. For inch size fasteners, the size of the thread diameter must always be indicated along with the number of threads per inch, which will determine whether it is coarse or fine pitch.
All measurements shall be given in inches or fractions thereof.
Un ejemplo de la definición completa de un tornillo podría ser:
Hex Cap Screw-ASME B18.2.1 3/8-16x2 SAE Gr. 5.2 zinc-plated
(Hex cap screw according to ASME B18.2.1, size 3/8”, pitch 16 threads per inch, length 2”, material SAE grade 5.2, coating zinc-plated)


Each European standard has its counterpart in inches, but they are not compatible. DIN and ISO fasteners are harmonized but will never be interchangeable with American ones.
The table below shows the equivalent standards.
DINAmerican Standard
DIN 931Hex Cap Screw-ASME B18.2.1
DIN 933Hex Cap Screw-ASME B18.2.1
DIN 912Hexagon Socket Head Cap Screw-ASME B18.3
DIN 7984Low Head Hexagon Socket Cap Screw-ASME B18.3
DIN 125Plane washer type B regular-ASME B18.22.1
DIN 433Plane washer type B narrow-ASME B18.22.1
DIN 9021Plane washer type B wide-ASME B18.22.1
DIN 127Regular helical spring lock washer-ASME B18.22.1
DIN 934Hex Nut-ASME/ANSI B18.2.2
DIN 936Hex Jam Nut-ASME/ANSI B18.2.2


The current American standard refers to inch threads for Unified fasteners, designated by the letters UN. Generally, each thread is associated with a coarse pitch (UNC) and a fine pitch (UNF). The pitch is defined by the number of threads per inch.
Size (")Size (")Size (mm)Pitch (UNC)Pitch (UNF)Thread (*)Metric DIN
nº 6
nº 80.1644.16563236AllM4
nº 100.194.8262432AllM5
(*) it expresses the length up to which the hex head screws are fully threaded.
In European standards there is a standard for fully threaded hex head screws. It does not exist as such in inch screws. The only fully threaded screws are those with 1/4" thread diameter or smaller, from this size on, depending on the length of the screw, it will be fully threaded or not, so it must be taken into account for the design of parts joints.


In Europe there are 4 grades of carbon steels and 2 grades of stainless steels for fasteners.
Calidad 6.8Calidad 6
Calidad 8.8Calidad 8
Calidad 10.9Calidad 10
Calidad 12.9Calidad 12
For carbon steels, the former is not used for high strength fasteners and the latter is difficult to find, as it can suffer hydrogen embrittlement when the coating is applied, so the most used are the intermediate ones. In stainless steels, the A2 grade has the particularity of rusting in marine environments due to high salinity.
Equivalent materials for European and American fasteners are compared.
GradeYield [MPa]Tensile [MPa]BoltsNuts
cal. 6.8600A307 Gr. AA563 Gr. A
A307 Gr. B
SAE Gr. 1SAE Gr. 2
SAE Gr. 2
cal. 8.8640800SAE Gr. 5A563 Gr. B
A449 Type 1
A325 Type 1
SAE Gr. 5.2SAE Gr.5
A449 Type 2
A325 Type 3
cal. 10.99001000SAE Gr. 8A563 Gr. D
A354 Gr. BD
A490 Type 1SAE Gr.8
SAE Gr. 8.2A563 Gr. DH
A490 Type 3
cal. 12.910801200A574
A2210500AISI 304
A4450700AISI 316


Various types of coatings are usually used for fasteners. In this case it is shared for European and American. The fastener coating will depend on the application and the environment in which it is installed.
For comparison of coatings, saline mist chamber tests are usually performed according to ASTM B-117, ISO 9227 and DIN 50051, which are equivalent.
Common coatings such as electrolytic zinc plating withstand less than 200 hours to the appearance of red rust, which is usually low for demanding applications. In these cases, surface treatments from the automotive industry such as Geomet B are used which resists about 1000 hours.
Another treatment that holds out against this number of hours is Dacromet but it is in disuse because of its hexavalent chromium content which is carcinogenic and is currently being replaced by other treatments with trivalent chromium such as Geomet.
Hot-dip galvanizing can be a good option for durability. If compared in salt spray chamber tests it is usually around 900 hours until the appearance of red rust. But the results can be misleading, as the zinc in the galvanizing alloys with oxygen from the environment repairing any small cracks. Salt spray chamber tests are performed without oxygen so, in a real application, hot dip galvanizing will tend to last longer than the tests say it will. In contrast, hot-dip galvanizing is a coating of several tens of microns so it can clog small metric threads. The standard ISO 10684 regulates hot-dip galvanizing for fasteners
Tests can be a good option to compare different surface treatments but the results of these tests are difficult to correlate with real application environments, so experience becomes the best ally to decide the fastener coating for our application.