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Understanding Engineering Thread Forms On Vehicles

UNDERSTANDING ENGINEERING THREAD FORMS ON VEHICLES

Experienced engineers understand the system of measuring threads that are used on vehicles, but how about the novice or person new to vehicle restoration; it must appear a minefield of technical jargon and sets of figures. This information page will hopefully take some of the mystery out of thread forms and how to identify what thread the item is in your hand or on the bench. A screw pitch gauge is simple, inexpensive and vital along with a digital caliper gauge, again relatively inexpensive. Both items are readily available from Engineers stores/merchants or even of the web. If you are struggling with obtaining either of these items we will supply them, they are as important as a set of spanners when starting off any restoration project. Firstly do not panic all is explainable in a logical simple way.

Metric

1) Identify where the machine was manufactured and when; if the vehicle is of European or Japanese manufacture and relatively modern the chances are that the majority of the thread forms used will be Metric (Metric has 4 main sub-divisions within the classification; Metric coarse, Metric fine, Metric extra fine and Metric super fine). Metric coarse is by far the most commonly used thread form, with fine secondly, extra fine next then superfine lastly. Firstly measure the diameter of the shank and note that down, this is the starting point. Metric thread forms are classified by their pitch (that is how many teeth there are per millimeter), for diameters up to M5 there are more than 1 tooth per millimeter, M6 is standard at 1 tooth per millimeter, above M6 there are less than 1 tooth per millimeter. The simplest way is to offer a rule to the thread and count how many teeth there are in say 10 millimeters (if you count 8 teeth in the 10mm then the pitch is 1.25mm i.e. 10mm divided by 8 = 1.25, 6 teeth and a little of the next tooth in the 10mm 10 divided by 6 = 1.67 so the pitch would be 1.5mm, 5 teeth plus a little of the next tooth in the 10mm 10 divided by 5.5=1.82 so the pitch would be 1.75mm) . The problems begin when there appears to be more teeth counted for the given diameter to be standard (this would indicate that it is one of the fine pitch sub-divisions) again count the teeth divided into 10mm, this will give you a pitch that goes with the diameter. Check on a thread chart below to identify what the classification is. The Left Hand list is Metric Fine; Right Hand Metric Coarse

JIS (Japanese Industrial Standard)

The same information as per Metric above is applicable

Metric Fine Metric Coarse
Diameter Pitch
3.000.35
3.500.35
4.000.50
4.500.50
5.000.50
6.000.75
8.001.00
10.001.00
10.001.25
12.001.25
14.001.25
14.001.50
16.001.50
18.001.50
20.001.50
22.001.50
24.001.50
25.001.50
27.001.50
27.002.00
28.001.50
30.001.50
32.001.50
33.002.00
40.001.50
50.001.50
63.001.50
Diameter Pitch
1.000.25
1.100.25
1.200.25
1.400.30
1.600.35
1.800.35
2.000.40
2.200.45
2.500.45
3.000.50
3.500.60
4.000.70
4.500.75
5.000.80
6.001.00
7.001.00
8.001.25
9.001.25
10.001.50
11.001.50
12.001.75
14.002.00
16.002.00
18.002.50
20.002.50
22.002.50
24.003.00

British Standard

2) If the machine is of British Manufacture then the chances are the majority of the threads will be to British Standard (again with 4 sub-divisions BA, BSCycle, BSF, BSW) BA (British association) is claimed to be the odd thread form as it is also measured in teeth per millimeter for some strange reason as BA thread forms preceded the UKs entry into Europe. However I and others would dispute this scenario as Fastener manufacturers describe BA as Teeth per inch, so again ascertain the diameter of the shank and count the number of teeth in an inch. The same applies to BSF (British standard Fine) and BSW (British Standard Whitworth). BSCYcle is relatively straightforward as there are only 2 different amounts of teeth per inch recognized; 26 teeth Per Inch which applies to all diameters up to 3/8, thereafter either 20 teeth per Inch or 26 teeth per Inch are used. Again offer a rule up to the thread and count the number of teeth per Inch. Again refer to the chart below.

BA (British Association)

BAOuter DiameterThreads per inch
00.2362 in / 6 mm25.38
10.2087 in / 5.3 mm28.25
20.1850 in / 4.7 mm31.35
30.1614 in / 4.1 mm34.84
40.1417 in / 3.6 mm38.46
50.1260 in / 3.2 mm43.10
60.1102 in / 2.8 mm47.85
70.0984 in / 2.5 mm52.91
80.0866 in / 2.2 mm59.17
90.0748 in / 1.9 mm64.94
100.0669 in / 1.7 mm72.46
110.0591 in / 1.5 mm81.97
120.0511 in / 1.3 mm90.91

BSW (British Standard Whitworth)

Whitworth sizeThreads per inch
1/8"40tpi
5/32"32tpi
3/16"24ti
7/32"24tpi
1/4"20tpi
5/16"18tpi
3/8"16tpi
7/16"14tpi
1/2"12tpi
9/16"12tpi
5/8"11tpi
11/16"11tpi
3/4"10tpi
13/16"10tpi
7/8 "9tpi
15/16"9tpi
1"8tpi

BSF (British Standard Fine)

fractional inchesthread per inch
3/1632tpi
7/3228tpi
1/426tpi
5/1622tpi
3/820tpi
7/1618tpi
1/216tpi
9/1616tpi
5/814tpi
11/1614tpi
3/4"10tpi
13/16"10tpi
7/811tpi
15/16"9tpi
1"10tpi

BSCycle (British Standard Cycle Thread)

0.250
(1/4) inch
26 Tpi
0.281
(9/32) inch
26 Tpi
0.3125
(5/16) inch
26 Tpi
0.375
(3/8) inch
26 Tpi
0.4375
(7/16) inch
20 Tpi
26 Tpi
0.5
(1/2) inch
20 Tpi
26 Tpi
0.5625
(9/16) inch
20 Tpi
26 Tpi
0.625
(5/8) inch
20 Tpi
26 Tpi

Unified Coarse and Fine

Again the same basic rules apply to ascertain where the vehicle was manufactured, if it is an American vehicle then the chances are the vast majority of the threads used will be to Unified Standard.

3) Ascertain the diameter of the shank then offer a rule up to the thread and simply measure the amount of teeth per inch and refer to the chart below. Unified only comes in two formats, so is somewhat simpler to find the thread, UNC (Unified National Coarse) and UNF (Unified National Fine).

Major diameter [in]Threads per inch
Coarse (UNC)Fine (UNF)Extra fine (UNEF)
#0 = 0.060080
#1 = 0.07306472
#2 = 0.08605664
#3 = 0.09904856
#4 = 0.11204048
#5 = 0.12504044
#6 = 0.13803240
#8 = 0.16403236
#10 = 0.19002432
#12 = 0.21602428
1/4"202832
5/16182432
3/8162432
7/16142028
1/2132028
9/16121824
5/8111824
3/4101620
7/891420
181420

THE ABOVE INFORMATION IS FOR GUIDANCE ONLY AND IN NO WAY IS INDENDED TO BE A DEFINATIVE GUIDE.

Not all vehicles contain just one Standard or type of thread form, some no matter where they were manufactured or by whatever maker use a combination of Fasteners and Thread forms.

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