What is the diameter of 3 inches

The "inch" tube

The inch and the pipe

Or why a 1 "pipe does not have an outer diameter of 25.4 mm.

At the beginning of the 20th century, the dimensions related to the inner diameter. Since the inside diameter is the value that is important for pipes, a 1 "pipe had a clear width of 25.4 mm. With the steel quality of that time, the outside diameter was ~ 33 mm. The fittings and tools were also manufactured for this purpose. Later, when the steel quality was improved, the pipe walls could also be thinner. Since the fittings and threading tools were manufactured according to the outer diameter, the inner diameter became larger. In heating and sanitary engineering, the customs values ​​no longer correspond to today's dimensions. "Inch" pipe dimensions are now defined metrically. The DIN EN ISO 228-1 standard also specifies this. If you now find a pipe with an outside diameter of ~ 33 mm, then it is and remains a "1 inch pipe." This standard also defines the thread identification The inside diameter of the pipe is not described in the standard, and it can also be here do not convert anything from inches to mm.

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As you can see, it is not easy to keep track of the thread "jungle". Do-it-yourselfers better find out more before buying their spare parts or ordering them online. There is a wide variety of thread and screw dimensions in the automotive sector. So if you want to work independently, for example on the gearbox or drive, you should either be very knowledgeable or buy from a reputable provider who will certainly be happy to provide the necessary information.

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Outside diameter according to DIN EN ISO 228-1 and DIN 2999
Thread identificationOutside diameter min.
mm
Outside diameter max.
mm
1/16"7,517,72
1/8"9,519,73
1/4"12,9113,12
3/8"16,4116,66
1/2"20,6720,96
5/8"22,6322,91
3/4"26,1626,44
7/8"29,9230,20
1"32,8933,25
1 1/8"37,5437,90
1 1/4"41,5541,91
1 1/2"47,4447,80
1 3/4"53,3953,75
2"59,2559,61
2 1/4"65,2765,71
2 1/2"74,7575,18
2 3/4"81,1081,53
3"87,4587,88
3 1/2"99,99100,33
4"112,60113,03
4 1/2"125,30125,73
5"138,00138,43
5 1/2"150,70151,13
6"163,40163,83

The nominal diameter is specified in EN ISO 6708 by the designation DN (Diameter Nominal) followed by an approximate specification of the inner diameter in millimeters, as a dimensionless number.

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Outside diameter according to DIN EN ISO 6708 and EN 10255
Thread identificationDNOutside diameter mm
1/4"813,5
3/8"1017,2
1/2"1521,3
3/4"2026,9
1"2533,7
1 1/4"3242,4
1 1/2"4048,3
2"5060,3
2 1/2"6576,1
3"8088,9
4"100114,3
5"125139,7
6"150168,3
8"200219,1
10"250273,0
 

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