Model yachtsmen have a pretty simple life as regards threaded fixings compared to the model engineering world where a bewildering array of 'standards' is used and the model engineer is generally expected to be able to cut his own threads.
There are four screw systems the model yachtsman may come across and really old boat's threads may be non standard, made by the yacht builder on a one off basis.
The four common threads are: BA UNF ME and Metric (ISO)
Determining the thread type can be a problem. The age and origin of the model will give a guide as to the thread standard. Measure the diameter and count the threads per inch (tpi) to identify the probable thread specification. Final identification is usually by trial and error if no specialised measuring equipment is available. Note that items like bottle screws have a right hand screw thread on one end and a left hand on the other. Otherwise left hand threads are fairly rare.
General Hints.
Restoring and cutting threads.
Dies are used for cutting male threads, eg bolts. Damaged threads often clean up quite well by running a die along the thread. Otherwise it is best to replace the threaded item if possible. In a worst case it is possible to recut the thread to the next smallest size but the mating part will have to be replaced.Note that if a nut appears to start on a thread then jams after a couple of turns it may well be that the pitches are different, for example a coarse thread bolt and fine thread nut.
Taps are for cutting female threads, eg nuts. When you have a non standard, damaged or simply gunged up threaded fitting the easiest solution is often to clean it up by running a tap through it. If this fails the hole will have to be retapped to the next larger size. To do this, firstly drill the hole to the correct tapping size. You may not have an exact drill size for the core diameter required so use the nearest one available but the correct sizes are available from Chonos and similar suppliers. There is a little latitude but an undersized hole risks tap breakage and an oversized hole risks weak loose threads.
- Taps come in sets of three; taper, second and plug for precision work. For simple modeling the second (no.2 ) tap is sufficient unless you need a thread in a blind hole where the thread must reach the bottom. This requires a plug (no 3 ) tap.. Carbon taps are usually cheap & manufactured to wider tolerances than HSS taps and are quite adequate for our purposes.
- Always use very sharp taps & dies & lubricate well.
- It is virtually impossible to tap a hole 100% vertically by eye. Whenever possible, use a jig to ensure that the tap is vertical. Use of a tapping jig will virtually eliminate broken taps.
Loose threads can usually be locked using 'Loctite'.
Suppliers
Suppliers to the model engineering hobby stock a huge range of fixings and basic ferrous and non ferrous metals in appropriate sizes and and are well used to supplying small quantities.
For tools, including drill bits, taps and dies:- Chronos
For sheet, rod, bar and tube in brass, copper and steel, plus a range of fixings:- Bruce Engineering
For a huge range of fixings at excellent prices and small quantities - no internet presence:-
Item Mail Order Tel: 01427 848880 and ask for catalogue.
British Association (BA)
This thread has been around for a long time and is by far the most common on models. It was first formulated in 1884 and was standardised in 1903. It is slowly being replaced by ISO metric but nut, bolts, taps and dies are readily available from model engineering suppliers.
There is a tendency for electrical applications to use even number BA threads and mechanical applications to use odd BA sizes but this is by no means universal. Sizes are simply quoted as, for example, '8BA'
Screw Numbers |
Diameter of Screw
Over Thread |
Threads per inch |
Tapping Drill Size |
16 BA |
0.0310 inch |
133.3 tpi |
0.6 mm
|
15 BA |
0.035 inch |
120.5 tpi |
0.7 mm
|
14 BA |
0.039 inch |
109.9 tpi |
0.8 mm
|
13 BA |
0.047 inch |
102 tpi |
0.98 mm
|
12 BA |
0.051 inch |
90.9 tpi |
1.05 mm
|
11 BA |
0.059 inch |
81.9 tpi |
1.2 mm
|
10 BA |
0.067 inch |
72.6 tpi |
1.4 mm
|
9 BA |
0.075 inch |
65.1 tpi |
1.55 mm
|
8 BA |
0.086 inch |
59.1 tpi |
1.8 mm
|
7 BA |
0.0982 inch |
52.9 tpi |
2.05 mm
|
6 BA |
0.11 inch |
47.9 tpi |
2.3 mm
|
5 BA |
0.126 inch |
43 tpi |
2.65 mm
|
4 BA |
0.1420 inch |
38.5 tpi |
3 mm
|
3 BA |
0.1610 inch |
34.8 tpi |
3.4 mm
|
2 BA |
0.1850 inch |
31.4 tpi |
4 mm
|
1 BA |
0.2090 inch |
28.2 tpi |
4.5 mm
|
0 BA |
0.2360 inch |
25.4 tpi |
5.1 mm
|
UNF
United National Fine. This is found on American market equipment, including a lot supplied by the Far East. The sizes up to 1/4inch are quoted as, for example, 0-80 (size-number tpi) 1/4in and above are quoted by size.
Screw number |
TPI |
Diameter |
Tapping Drill Size |
number 0 |
80 |
0.06 |
1.25 mm
|
number 1 |
72 |
0.073 |
1.6 mm
|
number 2 |
64 |
0.086 |
1.9 mm |
number 3 |
56 |
0.099 |
2.15 mm
|
number 4 |
48 |
0.112 |
2.4 mm
|
number 5 |
44 |
0.125 |
2.7 mm
|
number 6 |
40 |
0.138 |
2.95mm
|
number 8 |
36 |
0.164 |
3.55 mm
|
number 10 |
32 |
0.19 |
4.1 mm
|
number 12 |
28 |
0.216 |
4.65 mm
|
ME
Model engineering threads. These will only be found on British models. There are two thread pitches, 40tpi and 32tpi and the thread form is based on the traditional whitworth standard. Much used in steam plant, unlikely to be found on other commercially produced yacht fittings. Taps and dies readily available, but very hard to find commercially produced nuts and bolts. Much favoured by scale modelers because the across flats nut size is not specified so undersized nuts which look much closer to scale can be used.
Diameter |
TPI |
Tap drill (metric) |
1/8 |
40 |
2.40 |
5/32 |
40 |
3.25 |
3/16 |
40 |
3.90 |
7/32 |
40 |
4.75 |
1/4 |
32 |
5.6 |
Metric
Metric threads come in two series, fine and coarse. Taps & dies are available as are nuts and bolts.
Metric Threads - Fine Thread Pitches
Size - Nominal Diameter
(mm) |
Pitch1)
(mm) |
Tap Drill
(mm) |
| M 1.0 |
0.20 |
0.80 |
| M 1.1 |
0.20 |
0.90 |
| M 1.2 |
0.20 |
1.00 |
| M 1.4 |
0.20 |
1.20 |
| M 1.6 |
0.20 |
1.40 |
| M 1.8 |
0.20 |
1.60 |
| M 2 |
0.25 |
1.75 |
| M 2.2 |
0.25 |
1.95 |
| M 2.5 |
0.35 |
2.10 |
| M 3 |
0.35 |
2.60 |
| M 3.5 |
0.35 |
3.10 |
| M 4 |
0.50 |
3.50 |
| M 4.5x |
0.50 |
4.00 |
| M 5 |
0.50 |
4.50 |
| M 5.5 |
0.50 |
5.00 |
| M 6 |
0.75 |
5.20 |
Metric Threads - Coarse Thread Pitches
Size - Nominal Diameter
(mm) |
Pitch
(mm) |
Clearance Drill
(mm) |
Tap Drill
(mm) |
| M 1.60 |
0.35 |
1.8 |
1.25 |
| M 2.00 |
0.40 |
2.4 |
1.60 |
| M 2.50 |
0.45 |
2.90 |
2.00 |
| M 3.00 |
0.50 |
3.40 |
2.50 |
| M 3.50 |
0.60 |
3.90 |
2.90 |
| M 4.00 |
0.70 |
4.50 |
3.30 |
| M 5.00 |
0.80 |
5.50 |
4.20 |
| M 6.00 |
1.00 |
6.60 |
5.00 |
|
