AND

SCREW-THREADS

Note that definitions of some terms used in this subject, have varied over the years, vary across different continents, and many folks are adament that they are using the "correct" words. The arguments are pointless, just as long as it is made clear what items/points are under debate.

Also please note that the following is information to assist classic car restorers. If you intend to make design changes, or are looking for data on modern engineering design, please consult official sources for more detailed up-to-date data.

Basic Threadforms

Most threadforms follow the basic shape illustrated, with flanks set at a pre-determined angle, and the roots and crests either flat or rounded to given dimensions. The following notation is used to give detailed dimensions to each type of threadform:

F = angle between flanks; H = triangular height; p = pitch; Sc = shortening at crest; Sr = shortening at root; h = thread depth; Rc = radius at crest; Rr = radius at root.

This was the first standardised screw thread system. The Whitworth threadform is illustrated below, showing a 55 deg. angle between flanks and a rounded root and crest. In practise, the crests of the internal threads (nuts) may be at least partially flat rather than the full rounding off. British Standard Whitworth (BSW) sets out the number of threads per inch for each nominal diameter of screw thread. Nominal diameter is the outside diameter of the thread. The Whitworth threadform is also used for other British thread systems and also special threads.

F = 55 deg.; H = 0.960491p; Sc = Sr = H/6; h = 0.640327p; Rc = Rr = 0.137329p.

Although largely obsolete, BSW is still the standard thread for microphone stand and camera tripod fittings.

This used the Whitworth threadform, but with a finer pitch, as laid out in the table:

BSP also uses the Whitworth threadform. Because this is usually a tapered thread (for plumbing), nominal diameter is the bore of the pipe that the thread is associated with. The taper is 16 degrees but the flank angles remain equal to the axis (as for non-tapered Whitworth threadforms). BSP has become the international standard thread for pipes.

This thread uses the Whitworth threadform, and relates to conduit couplers and fittings.

This thread uses the Whitworth threadform.

This thread uses the Whitworth threadform, 26TPI regardless of diameter.

This thread uses the Whitworth threadform.

This thread uses the Whitworth threadform.

The BSC threadform is shown here, with 60 deg. between flanks and fairly large radius roots and crests. It is not to be confused with British Standard Conduit threads which use the Whitworth threadform. Note that although the 3/16" and 1/4" diameters have the same threads per inch (TPI) as the BSF thread, different threadforms should NEVER be mixed. It is rather like trying to push a square peg into a round hole! There is also a small series of BSC threads for spokes.

F = 60 deg.; H = 0.8660p; Sc = Sr = p/6; h = 0.5327p; Rc = Rr = p/6.

A very distinctive threadform with 47.5 deg. between flanks and large radius root and crest. Instead of nominal diameters, size numbers are given, from 0 to 25 (25 being the smallest). These are small diameters used mostly for electrical equipment and intruments. Dimensions are usually given in millimetres, with thread pitch indicated rather than TPI.

F = 47.5 deg.; H = 1.13634p; Sc = Sr = 0.268p; h = 0.6p; Rc = Rr = 0.18p.

If you are in the USA, you may drop the letter "A"! The American National threadform is shown here. This is another one with 60 deg. between flanks. The crests and roots of these threads (internal and external) are flat. However, with worn tooling, the roots may be rounded, and this threadform then becomes asymmetric.

Nominal diameters are quoted in inches from 1/4" upwards. Smaller diameters are given size numbers from 0 to 12 (12 being the largest) and are often quoted by a combination of size number and TPI. For example, 10-24 refers to a number 10 thread with 24TPI, which makes it an ANC thread.

F = 60 deg.; H = 0.8660p;

For external thread: Sc = H/8 (flat); Sr = H/6 (flat); h = 0.61342p.

For internal thread: Sc = H/4 (flat); Sr = H/8 (flat); h = 0.54125p.

The coarse pitch version of the American National thread.

The fine pitch version of the American National thread.

An even finer version.

There were a number of pipe thread standards using the ATP threadform, which was based upon the AN threadform, but adapted to tapered threads (both flank angles equal to axis). Thread series using the ATP threadform were National Taper Pipe (NPT), Dryseal Pipe and American Petroleum Institute (API).

Based upon the above taper pipe threadform.

By the late 1940s, the Unified thread was introduced to try and rationalise British, Canadian and American fasteners. In practice, this was virtually a duplicate of the American National thread series. The threadform appears to differ slightly in that the roots are rounded. The crest of the external thread (bolts and screws) may be either round or flat, the latter making the bolt/screw a fraction smaller in diameter than its nominal figure. The crest of the internal thread (nuts) is flat.

Like the American National thread series, sizes below 1/4" dia. are given size numbers 0 to 12, and are listed in the format 10-24, 10-32 etc. (size number and TPI).

F = 60 deg.; H = 0.8660p;

For external thread: Sc = H/8 (if rounded); Sr = H/6 (rounded); h = 0.613434p.

For internal thread: Sc = H/4 (flat); Sr = H/8 (rounded); h = 0.541266p.

Note that the size 10 thread diameter is not 3/16" as is often quoted, but is slightly larger.

The coarse thread of the Unified series. The TPI figures are the same as for ANC. Apart from 1/2" dia., TPI figures are the same as for BSW. However, UNC and BSW fasteners should NEVER be mixed!!!

The fine thread of the Unified series. The TPI figures are mostly the same as for the ANF series, with the exception of 1" dia. fasteners.

The extra fine thread of the Unified series.

There is also a constant pitch series available with a fixed TPI figure for each diameter in its range. For example, 8-UN has a TPI of 8 for each diameter.

This is for very small diameters, and dimensions are given in millimetres.

Germany, France and Switzerland all had separate systems, athough the threadforms were closely related. The threadforms had 60 deg. between flanks and the basic form was for flat crests and roots, possibly with rounded roots and crests in practice. In fact it was very similar to the American and Unified threadforms. Each metric system offered a choice of thread pitches (distance between adjacent threads). The French also had a Metric Automobile thread. Most of the differences between the systems relate to diameters below 6mm and also very large diameters that are not relevant to motor cars.

France's version of the metric thread was an asymmetric threadform with a flat crest and either flat or rounded root.

F = 60 deg.; H = 0.8660p; Sc = H/8 (flat); Sr = H/16 (flat or rounded); h = 0.704p; Rr = 0.054p.

The French Automobile threadform set out dimensions for both nut and bolt thread with rounded roots. Fasteners were offered in a choice of pitches for most diameters.

F = 60 deg.; H = 0.8660p;

For external thread: Sc = H/8 (flat); Sr = H/8 (rounded); h = 0.650p; Rr = H/8.

For internal thread: Sc = 3/16 x H (flat); Sr = H/12 (rounded); h = 0.632p; Rr = H/12.

This threadform has a kind of symmetry between nut and bolt threads, one being the the inverted version of the other. Thread roots are rounded

F = 60 deg.; H = 0.8660p;

For external thread: Sc = H/8 (flat); Sr = H/8 (rounded); h = 0.650p; Rr = H/8.

For internal thread: Sc = H/8 (rounded); Sr = H/8 (flat); h = 0.650p; Rc = H/8.

This is very similar to the German threadform, but with a smaller radius for the root.

F = 60 deg.; H = 0.8660p;

For external thread: Sc = H/8 (flat); Sr = H/15 (rounded); h = 0.6995p; Rr = 0.058p.

For internal thread: Sc = H/8 (flat); Sr = H/15 (rounded); h = 0.6995p; Rr = 0.058p.

Inevitably, a standardised system emerged, the S.I. Metric thread, although this did not cover sizes below 6mm. S.I. Metric is closely based upon the French Metric thread series. The basic threadform is, again, with flat roots and crests and is asymmetrical. In Britain, a slight variation of this system was introduced during World War 2. This permitted emergency use of metric threads when required, but to British manufacturing practices. The difference was to have a larger root radius for the screw/bolt threads, and an increased inner diameter for the nut thread for compatibility.

Sparking plug threads were standardised at 14mm diameter and 1.25mm pitch.

F = 60 deg.; H = 0.8660p;

For external thread: Sc = H/8 (flat); Sr = 7/32 x H (flat); h = 0.5683p.

For internal thread: Sc = 7/32 x H (flat); Sr = H/8 (flat); h = 0.5683p.

Roots of threads may be rounded when using a worn tool to form the thread.

This system has become the internationally accepted series of threads and was 'launched' in the UK in the 1960s. Its threadform is just a fraction altered from the SI version, and the basic form has flat roots and crests (in practice, roots are usually rounded, giving greater thread depth). In fact, it was only the coarse series that was introduced in the UK, although the fine series has appeared to some extent. Unlike the S.I. system, the ISO includes diameters below 6mm.

F = 60 deg.; H = 0.8660p;

For external thread: Sc = H/8 (if flat); Sr = H/4 (flat); h = 0.54127p; Rr = 0.1443p (if rounded).

For internal thread: Sc = H/4 (flat); Sr = H/8 (if rounded); h = 0.54127p.

Roots are usually rounded in practice.

The above list is no where near comprehensive, but does include most types that will be encountered on cars, especially British cars up to the 1950s. For those who arrived here looking for thread data, rather than AC 2 Litre cars, the following should be of interest:

F = 52 deg.; H = 0.8906p; Sc = 0.2453p; Sr = 0.1395p; h = 0.50586p; Rr = 0.12055p; flat crest.

F = 52 deg.; H = 0.7813p; Sc = 0.1906p; Sr = 0.0977p; h = 0.49298p; Rr = 0.09298p; flat crest.

Asymmetric thread. Upper flank: F = 29 deg.; Rc = Rr = 0.015 inches. Lower flank: F = 35 deg.; Rc = Rr = 0.022 inches.

Sc = 0.0166 inches; Sr = 0.0331 inches; h = 0.642 inches (7TPI) or 0.0832 inches (6 TPI).

Square threadform.

p = 0.3333 inches; Rc = 0.07267 inches; Rr = 0.094 inches; h = 0.209 inches; thread thickness = 0.1453 inches.

Size E40: Pitch 0.25in; outside dia. of cap 1.5in

Size E27: Pitch 0.14in; outside dia. of cap 1.03in

Size E14: Pitch 0.11in; outside dia. of cap 0.54in

Size E10: Pitch 0.07in; outside dia. of cap 0.37in

Size E5: Pitch 0.04in; outside dia. of cap 0.21in

There is a wide tolerance on the above diameters.

F= 60 degrees.

F= 29 deg.; H = 1.93336p; Flat root and crest.

Crest width (nut) = 0.3707p; root width (nut) = 0.3707p - (0.256 x major dia. allowance)

Root width (screw) = 0.3707p - (0.259 x minor dia. allowance - effective dia. allowance)

Depth of engagement = p/2

F = 29 deg.; H = 1.93336p; Flat root and crest.

Crest width (screw) = 0.4224p

Root width (screw) = 0.4224p - 0.52c, where c = clearance of 0.010" for up to 10TPI or 0.005" for finer pitches.

F = 60 deg.; H = 0.8660p; Flat root and crest.

Crest width (screw) = 0.250p

Root width (screw) = 0.227p

F = 5 deg.

Crest width (screw) = 0.4563p

Root width (screw) = 0.4563p - 0.17c, where value of c varies depending upon application.

This had a TPI of 27 for all diameters, and used the unified threadform.

F= 33 deg.; leading flank angle = 3 deg.; trailing flank angle = 30 deg.; Sc (screw) = 0.52507p (flat); Sr (screw) = 0.33921p (rounded); Rr (screw) = 0.12427p; Sc (nut) = 0.45696p (flat); Sr (nut) = 0.52507p (flat); h (screw) = 0.86777p; h (nut) = 0.75p; width o crest (screw) = 0.26384p

F = 60 deg.; H = 0.8660p; Sc = Sr = 0.10825p; h = 0.6495p; flat root and crest.

External thread: F = 60 deg.; H = 0.866p; h = 0.34p; Rc = 0.263p; Rr = 0.2p; root is flat in middle.

Internal thread: F = 30 deg.; H = 1.866p; h = 0.34p; Rc = 0.263p; Rr = 0.2p; root is flat in middle.

Originally a sharp V form, later truncated.

F = 53 deg. 8 minutes; H = 1.0p; h = 0.75p; Sc = Sr = 0.125p; flat root and crest.

F = 30 deg.; H = 1.866p; h (screw) = 0.5p = clearance at root of nut; h (nut) = 0.5p - clearance at root of screw; flat roots and crests.

Page 2 >>

Page 2 - Bolts, Screws and Studs

Page 3 - Nuts

Page 4 - Nuts (Self-Locking)

Page 5 - Washers

Page 6 - Materials and Strength Grades

Page 7 - Tightening

Page 8 - Wood Screws and Nails

Page 9 - Fasteners for the AC 2 Litre Saloon