BOLT HEAD SIZES

You may have little confusion while choosing the head sizes or shank diameter of the bolts. Because when you measure the actual sizes of the bolts they may vary. For instance, I took one M5 bolt from one company and measured the shank diameter which was 4.85 mm diameter and then I took one bolt from another company and measured the shank diameter of the bolt which was 4.9 mm. Why does the size vary? Let us find answer and refer bolt sizes from the table.

BOLT HEAD SIZES

    Sizes of bolts head and body are very crucial in industrial applications. Because Fasteners are used widely to fasten the materials alternate to welding. Welding is a permanent joint and so we cannot dismantle the materials for servicing, seperating for easy movement, etc. 

    Design Engineers also have to design the materials with appropriate fasteners with correct sizes. So if the size varies from one fastener to another fastener how can we choose the correct size? To throw out these confusions ISO (International Standard Organisation) has been standardized  the sizes of the bolts' heads and their bodies giving the minimum and the maximum allowable values. So according to our application we have to choose the maximum value or the minimum value.

    Coming to our example, We need to know the shank/ body diameter of M5 bolt. From the table (Given  Below) we know that the minimum value is 4.82 mm and the maximum value is 5 mm. So we can choose any value between 4.82 and 5. But note that it is not a common practice to choose 5 mm ( the maximum value). Practically all bolts' shank/ body diameter will be less than the maximum value. This is applicable only for the diameter of the body. Other values can be used as per the given table.

    Follow the table to know the correct sizes of the bolts. For simplicity we have given the sizes for the most commonly used metric bolts. They are Allen bolts, Hexogonal bolts and CSK Bolts (Counter Sink Bolts).

The table of the Sizes of heads and the bodies of the Metric Bolts:

Metric Thread

Shank (mm)

Allen Bolt (mm)

Hex Bolt (mm)

CSK (mm)

ISO

Dia

OD

Height of the Head

Width across flats

Width across corners

Height of the Head

OD

Height of the Head

M1.6

1.46

1.6

2.86

3

1.46

1.7

3.08

3.2

3.48

3.7

.975

1.225

-

-

M2

1.86

2

3.62

3.8

1.86

2

3.88

4

4.38

4.6

1.275

1.525

-

-

M2.5

2.36

2.5

4.32

4.5

2.36

2.5

4.88

5

5.51

5.8

1.575

1.825

-

-

M3

2.86

3

5.32

5.5

2.86

3

5.38

5.5

6.08

6.4

1.875

2.125

5.7

6

1.7

M4

3.82

4

6.78

7

3.82

4

6.85

7

7.74

8.1

2.675

2.925

7.64

8

2.3

M5

4.82

5

8.28

8.5

4.82

5

7.85

8

8.87

9.2

3.35

3.65

9.64

10

2.8

M6

5.82

6

9.78

10

5.70

6

9.78

10

11.05

11.5

3.85

4.15

11.57

12

3.3

M8

7.78

8

12.73

13

7.64

8

12.73

13

14.38

15

5.35

5.65

15.57

16

 

4.4

M10

9.78

10

15.73

16

9.64

10

16.73

17

18.9

19.6

6.82

7.18

19.48

20

5.5

M12

11.73

12

17.73

18

11.57

12

18.67

19

21.1

21.9

7.82

8.18

23.48

24

6.5

M14

13.73

14

20.67

21

13.57

14

21.67

22

24.49

25.4

8.82

9.18

26.48

27

7

M16

15.73

16

23.67

24

15.57

16

23.67

24

26.75

27.7

9.82

10.18

29.48

30

7.5

M18

17.73

18

26.67

27

17.57

18

26.67

27

30.14

31.2

11.785

12.215

32.38

33

8

M20

19.67

20

29.67

30

19.48

20

29.67

30

33.53

34.6

12.785

13.215

35.38

36

8.5

M22

21.67

22

32.61

33

21.48

22

31.61

32

35.72

36.9

13.785

14.215

35.38

36

13.1

M24

23.67

24

35.61

36

23.48

24

35.38

36

39.98

41.6

14.785

15.215

38.38

39

14

M27

26.67

27

39.61

40

26.48

27

40.38

41

45.63

47.3

16.785

17.215

-

-

M30

29.67

30

44.61

45

29.48

30

45.38

46

51.28

53.1

18.74

19.6

-

-

 Follow the link below  for Spanner/Wrench or Allen key sizes of the bolt and to distinguish the differences among bolts with figure:

Spanner/wrench or allen key sizes for metric thread bolts

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