SEED Guides Bolted Joints and Threaded Fasteners < >

7. Selection of Thread Systems

There is a large number of thread systems in existence and it is beyond the scope of this Guide to discuss them in detail. Table 7.1 gives a brief comparison of ISO Metric Coarse with British Association (BA) British Standard Fine (BSF), British Standard Whitworth (BSW), Unified Coarse (UNC) and Unified Fine (UNF).

The student designer is advised to consider ISO Metric Coarse series to be the current first choice for all new designs and to regard the other systems listed above as obsolete or obsolescent.

The user is advised to avoid threaded fasteners specified to inch dimensions except for British Standard Pipe thread which is widely used in the hydraulic industry and which can be used for hydraulic connections and plugs.

If the threaded fastener cannot be selected from the ISO Metric Coarse series then the ISO Metric Fine and Constant Pitch series may provide a solution but these series must be regarded as very expensive alternatives in most cases. It should be noted that standard ranges of fasteners are normally not produced to these alternative series.

 

7.1 Screw thread forms

7.1.1 Basic form. The basic ISO form of thread is shown by the heavy outline in Figure 7.1.

7.2 Selection of metric screw threads (B.S. 3643)

The form of the I.S.O. metric screw thread is shown in Figure 7.1. The basic form is exactly the same as the Unified (American Inch) thread form. (The pitch to diameter ratio of the I.S.O. coarse screw thread is equivalent to the B.S.F. series and NOT to the Whitworth inch series.)

In practice all standard fasteners (bolts, screws, set screws etc.) will be of the metric COARSE series with graded pitches. ONLY THE NOMINAL SIZES SHOWN IN TABLE 7.2 SHOULD BE USED. For a table showing all sizes and pitches see BS 18 section 4 table 2.

The I.S.O. FINE series provides a very fine pitched screw and is used only for aircraft and instrument making. It is impractical to tap I.S.O. FINE threads in soft metals. For large diameter threads (72-150mm) the 2mm. constant pitch series should be used. For even larger diameters (150-250mm.) the 3mm. constant pitch series should be used. Standard tooling is universally available to manufacture the above thread series only.

7.2.1 Classes of Fit
British Standard. BS.3643 provides for three classes of fit - Fine, Medium and Coarse, for precision, general and hot/dirty/corrosive conditions respectively. For medium fits the tolerance grades for the effective dia for both external and internal threads are 5, 6 or 7, while tolerance positions are ŒH¹ for internal threads and 'g¹ for external threads. (See SEED Design Procedural Guide GEN 1 for explanation of 'Limits and Fits').

7.2.2 Designation of metric threads
To dimension a thread on a drawing use the following designation (See also BS308, Pt 2, Section 6.9).

Table 7.2 Metric Threads: BS 3643.
Dimensions in mm DRG

NOMINAL
SIZE
PITCH
CORE
DIAMETER
MAJOR
DIAMETER
STRES
S AREA
MM2
TAPPING
DRILL
'B"
CLEARANCE
E DRILL
'A'
TAPPING
DEPTH
'C'
DRILL
DEPTH
'D'

M3
M4
M5
M6
M8
M10

M12
M14
M16
M18
M20
M22

M24
M27
M30
M33
M36
M39
M42

0.50
0.70
0.80
1.00
1.25
1.50

1.75
2.00
2.00
2.50
2.50
2.50

3.00
3.00
3.50
3.50
4.00
4.00
4.00

2.40
3.40
4.00
4.80
6.50
8.16

9.85
11.55
14.55
15.63
16.93
18.93

20.32
23.32
25.70
28.71
31.09
34.09
36.48

3.0
4.0
5.0
6.0
8.0
10.0

12.0
14.0
16.0
18.0
20.0
22.0

24.0
27.0
30.0
33.0
36.0
39.0
42.0

5.0
8.8
14.2
20.1
36.6
58.0

84.3
115.0
157.0
192.0
245.0
203.0

353.0
259.0
561.0
694.0
817.0
769.0
1120.0

2.5
3.3
4.2
5.0
6.8
8.5

10.2
12.2
14.0
15.8
17.5
19.8

21.0
24.3
26.5
30.0
32.0
35.5
37.5

3.4
4.5
5.5
6.6
9.0
11.0

14.0
16.0
18.0
22.0
26.0
24.0

26.0
29.0
33.0
35.0
39.0
41.0
45.0

4
5
6
8
10
12

15
17
20
22
24
27

29
33
36
40
44
47
51

6
9
10
12
16
19

23
26
28
33
35
37

42
45
51
54
60
63
69

 

Table 7.3 Hexagon and Cap Head Screws: (Dimensions in mm)

PRECISION HEX HEAD BOLTS (BS EN24016) and SCREWS (BS EN 24017)
SOCKET HEAD CAP SCREWS
(BS 4168 part 1)
NOMINAL
SIZE
HEAD
DEPTH
nominal
WIDTH ACROSS FLATS
max
WIDTH ACROSS CORNERS
min
HEAD DEPTH
max
HEAD DIAMETER
max without knurling
ALLEN KEY
SIZE A/F
nominal

M3
M4
M5
M6
M8
M10

M12
*M14
M16
*M18
M20
*M22
M24
M27
M30
M33

2.0
2.8
3.5
4.0
5.3
6.4

7.5
8.8
10.0
11.5
12.5
14.0
15.0
17.0
18.7
21.0

5.5
7.0
8.0
10.0
13.0
16.0

18.0
21.0
24.0
27.0
30.0
34.0
36.0
41.0
46.0
50.0

6.08
7.74
8.87
11.05
14.38
18.90

21.10
24.48
26.75
30.14
33.53
35.72
39.98
45.63
51.28
55.80

3.0
4.0
5.0
6.0
8.0
10.0

12.0
14.0
16.0
18.0
20.0
22.0
24.0

5.5
7.0
8.5
10.0
13.0
16.0

18.0
21.0
24.0
27.0
30.0
33.0
36.0

2.5
3.0
4.0
5.0
6.0
8.0

10.0
12.0
14.0
14.0
17.0
17.0
19.0


*:- NON-PREFERRED SIZE

Table 7.4 Socket set screws metric series to BS 4168 (Parts 2-5)
Dimensions in mm

NOM
SIZE
D

HEX
SOCKET
SIZE 'J'

 

 


nom

HEX
KEY
DEPTH
'K'

 


min

CUP POINT
DIA.
'H'

 

 


max    min
CONE POINT
90 deg.š FOR
THESE
LENGTHS AND
OVER: 120š
FOR SHORTER
LENGTHS
'Y'

FLAT
POINT
DIAMETER
'V'

 



max    min

DIAMETER
'P'

 

 


max    min

DOG POINT LENGTH
'Q'

short series long seris

 



max     min       max     min
M3
M4
M5
M6
M8
M10
M12
M16
M20
M24
1.50
2.00
2.50
3.00
4.00
5.00
6.00
8.00
10.00
12.00
1.20
1.50
2.00
2.00
3.00
4.00
4.80
6.40
8.00
10.00
1.40     1.15
2.00    1.75
2.50    2.25
3.00    2.75
5.00    4.70
6.00    5.70
8.00    7.64
10.00  9.64
14.00  13.57
16.00  15.57
4
5
6
8
10
16
16
20
25
30
2.00    1.75
2.50    2.25
3.50    3.20
4.00    3.70
5.50    5.20
7.00    6.64
8.50    8.14
12.00  11.57
15.00  14.57
18.00  17.57
2.00
2.50
3.50
4.00
5.50
7.00
8.50
12.00
15.00
18.00
1.75
2.25
3.20
3.70
5.20
6.64
8.14
11.57
14.57
17.57
1.00
1.25
1.50
1.75
2.25
2.75
3.25
4.30
5.30
6.30
0.75
1.00
1.25
1.50
2.00
2.50
3.00
4.00
5.00
6.00
1.75
2.25
2.75
3.25
4.30
5.30
6.30
8.36
10.36
12.43
1.50
2.00
2.50
3.00
4.00
5.00
6.00
8.00
10.00
12.00

 

Table 7.5 Dimensions of slotted and recess countersunk head screws
Dimensions in mm

GENERAL
 SLOTTED HAD To BS EN ISO 2009 RECESSED HEAD To BS EN ISO 7046-1
NOMINAL
SIZE &
THREAD
DIA. 		HEAD
DIA.
Max.
'D' 		HEAD
HGT
Max.
'k'

THREAD
LENGTH

'B'

 WIDTH OF
SLOT
Nom.
'n'

DEPTH OF
SLOT
Max.
't'

RECESS
AND DRIVER POINT NUMBER
M1.6 3.00 1.00 15.0 0.40 0.50 0
M2 3.80 1.20 16.0 0.50 0.60 0
M2.5 4.70 1.50 18.0 0.60 0.75 1
M3 5.50 1.65 19.0 0.80 0.85 1
M4 8.40 2.70 22.0 1.20 1.30 2
M5 9.30 2.70 25.0 1.20 1.40 2
M6 11.3 3.30 28.0 1.60 1.60 3
M8 15.8 4.65 34.0 2.00 2.30 4
M10 18.3 5.00 40.0 2.50 2.60 4

* Threaded up to head only

Table 7.6 Lengths of metric bolts LENGTH (mm)

LENGTH
mm)
DIAMETER
M3
M4
M5
M6
M8
M10
M12
M16
M20
M24
5 C C
6 C C
8 CV CV CV CV
10 CV CV CV CV CV
12 CV CV CV CV CV CV
16 CV CV CV CVH CVH C
20 CV CV CV CVH CVH CVH CVH
25 C CV CV CVH CVH CVH CVH H
30 C CV CV CVH CVH CVH CVH CH
35 C C C CVH CVH CVH CVH CVH V
40 C C CVH CVH CVH CVH CVH CV
45 C C CV CH CVH CVH CH CV
50 C CV CH CVH CVH CVH CVH CH
55 CV CH CH CH CH CH H
60 C CH CH CH CH CVH CH
65 CV CH CH CH CH CH H
70 CV CH CH CH CH CVH CH
75       CV CH CH CH CH CH H
80 H H CH CH CH CVH CH
90 CH CH CH CVH CH
100 CH CH CH CVH CH
110 H CH CH H
120 CH CH CH CH
130 H H CH H
140 H H CH H
150 H H CH CH

KEY
C = CAP HEAD SCREWS
H = HEX. HEAD SCREWS
V = C/SUNK HEAD
SCREWS

Table 7.7 Typical Dimensions of Spanners and Sockets
Dimensions in mm
SPANNER
SOCKET
NOMINAL
SIZE

WIDTH
ACROSS
FLATS

THICKNESS
DIMENSION
'A'
DIMENSION
'B'
DIMENSION
'C'
DIMENSION
'D'

M6
M8
M10
M12

M14
M16
M18
M20

M22

10.0
13.0
16.0
18.0

21.0
24.0
27.0
30.0

34.0

5.0
5.5
6.0
6.0

6.5
6.5
7.0
7.0

 

7.5
10.0
12.5
16.0

17.0
17.0

20.0

26.0

14.0
20.0
22.0
27.0

30.0
32.0

37.0

4.50

20.0
28.0
32.0
28.0

41.0
46.0

55.0

66.0

15.0
19.0
25.0
26.0

30.0
32.0

43.0

50.0

NOTE: Ring spanners will generally be 1 - 2 mm larger in diameter than corresponding sockets.

Table 7.8 Metric Washers

SPRING WASHERS To BS 44664
BRIGHT PLAIN WASHERS To BS 4320
NOMINAL
SIZE
INSIDE
DIAMETER
'A' Max.
OUTSIDE
DIAMETER
'B' Max.
THICKNESS
'C' Nom.
INSIDE
DIAMETER 'A'
Min.
OUTSIDE
DIAMETER'B'
max.
THICKNESS
Normal range
'C' Nom.

M3
M4
M5
M6
M8

M10
M12
M14
M16
M18

M20
M22
M24
M27
M30
M33
M36
M39
M42

3.3
4.35
5.35
6.4
8.55

10.6
12.6
14.7
16.9
19.0

21.1
23.3
25.3
28.5
31.5
34.6
37.6
40.8
43.8

5.5
7.0
8.6
9.6
12.8

15.9
17.9
21.1
24.3
26.4

30.5
32.7
35.7
38.9
43.9
47.0
52.1
55.3
60.3

1.0
1.2
1.5
1.5
2.0

2.5
2.5
3.0
3.5
3.5

4.5
4.5
5.0
5.0
6.0
6.0
7.0
7.0
8.0

3.2
4.3
5.3
6.4
8.4

10.5
13.0
15.0
17.0
19.0

21.0
23.0
25.0
28.0
31.0
34.0
37.0
40.0
43.0

7
9
10
12-25
17

21
24
28
30
34

37
39
44
50
56
60
66
72
78

0.5
0.8
1.0
1.6
1.6

2.0
2.5
2.5
3.0
3.0

3.0
3.0
4.0
4.0
4.0
5.0
5.0
6.0
7.0

 

Table 7.9 Metric Nuts
Dimensions in mm

NOM
SIZE
'C'
PITCH
WIDTH
ACROSS
'FLATS
'A' max.
WIDTH
ACROSS
CORNER
'B' max.
THICKNESS
OF BLACK
NUT 'D'
max. to BS
EN 24032
Style 1
Grade
A & B
THICKNESS OF NUT 'D' max. to BS EN24033
Grades A
& B
HEX. THIN NUT
'E' To BS EN
24035
THICKNESS OF SLOTTED NUT 'F'
To BS 7764
DEPTH OF LOWER FACE TO SLOT BOTTOM
'G'
SLOT WIDTH
'H'

M3
M4
M5
M6
M8

M10
M12
M14
M16
M18

M20
M22
M24
M27
M30
M33
M36
M39
M42

0.50
0.70
0.80
1.00
1.25

1.50
1.75
2.00
2.00
2.50

2.50
2.50
3.00
3.00
3.50
3.50
4.00
4.00
4.50

5.5
7.0
8.0
10.0
13.0

17.0
19.0
22.0
24.0
27.0

30.0
32.0
36.0
41.0
46.0
50.0
55.0
60.0
65.0

6.3
8.1
9.2
11.5
15.0

18.5
20.8
24.2
27.7
31.2

34.6
39.2
41.6
47.3
53.1
57.7
63.5
69.3
75.1

2.4
3.2
4.7
5.2
6.8

8.4
10.8
1.8
14.8
15.8

18.0
19.4
21.5
23.8
25.6
28.7
31.0
33.4
34.0

-
-
5.1
5.7
7.5

9.3
12.0
14.1
16.4
-

20.3
-
23.9
-
28.6
-
34.7
-
-

1.8
2.2
2.7
3.2
4.0

5.0
6.0
7.0
8.0
9.0

10.0
11.0
12.0
13.5
15.0
16.5
18.0
19.5
21.0

-
5
6
7.5
9.5

12
15
16
19
21

22
26
27
30
33
35
38
40
-

-
3.2
4.0
5.0
6.5

8
10
11
13
15

16
18
19
22
24
26
29
31
-

-
1.2
1.4
2.0
2.5

2.8
3.5
3.5
4.5
4.5

4.5
5.5
5.5
5.8
7.0
7.0
7.0
7.0
-

 

Table 7.10 Installation Information for ISO Metric Fasteners

A

Clearance Hole Diameter
(Medium Fit Series BS EN 20273)

F
 Counter-bore Diameter for Hex Head bolt to enable socket wrench access
B Clearance Hole Diameter
(Free Fit Series BS EN 20273)
G
 Counter-Bore Depth to avoid Head Protrusion to allow for standard steel washer
C Counter Bore Diameter for Screws to Socket Head Cap Screw
H
 Counter-Bore Diameter for Counter-Sunk Head avoid Head Protrusion
D Counter-Bore Depth for SHCS to avoid Head Protrusion
J
 Counter-Bore Depth
E Spot-Face Diameter for Hex. Head Bolt
K
 Minimum Plate thickness

(SHCS socket head cap screws)

THREAD
SIZE 		A
Medium

B
Coarse

C D E F G H J K
M3 3.4 3.6 6.0 3.5 8 12 3.5 7.4 2.0 2.5
M4 4.5 4.8 7.5 4.5 10 15 4.5 9.7 2.6 3.2
M5 5.5 5.8 9.0 5.5 11 16 5.5 11.7 3.1 4.0
M6 6.6 7.0 10.6 6.5 13.5 18 6.5 13.8 3.6 5.0
M8 9.0 10.0 14.0 .5 18 23 8.0 18.6 4.8 6.3
M10 11.0 12.0 17.0 11.0 22 26 10.0 23.0 6.0 8.0
M12 13.5 14.5 20.0 13.0 25 29 11.5 28.0 7.0 9.0
M14 15.5 16.5 23.0 15.0 29 33 12.5 31.0 7.5 9.5
M16 17.5 18.5 26.0 17.0 31 36 14.0 34.0 8.0 10.0
M20 22.0 24.0 32.0 21.0 38 46 17.0 40.0 9.0 12.5
M24 26.0 28.0 38.0 25.0 45 53 20.0      
M27 30.0 32.0 43.0 28.0 51 60 23.0      
M30 33.0 35.0 48.0 31.0 57 66 26.0      
M36 39.0 42.0 57.0 37.0 69 80 32.0      

All dimensions in mm