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Mild Steel

Steel for light duty bolts and screws and most nuts. Minimum ultimate tensile strength was 28tsi (430MPa) as set out in British Standards contempory to the AC 2 Litre (1940s). Steel hex. bolts and screws with no lettering/number markings are likely to be of this material (apart from stainless steel). Grade letters are still attributed to these mild steels (in the UK): A, B and C, for bolts and screws, and K for nuts. In the USA, grading is by numbers, and Grade 2 specifies minimum strength of 33tsi (510MPa). For metric fasteners, grade numbers show both the strength and also what the yield strength is as a proportion of ultmiate strength. "Yield strength" is the stress at which the material will begin to deform permanently. Metric grade 4.6 denotes a material of strength approximately 400MPa (26tsi), with a yield strength of 0.6 times its ultimate strength.

High Tensile Steel

Alloy steels that have been heat-treated to increase their strength. Fasteners have markings on them to indicate the strength grade, and are also marked to identify the manufacturer. In the UK, the grading system went from A to F, with markings required on bolts/screws for grades from D upwards. By the early 1950s, this system had changed. Grades formerly from D upwards, became grades R, S, T, etc. and this grading applied to Unified fasteners stocked in the UK as well as BSW, BSF, etc.

So, old British grade D = grade R = 45tsi (700MPa). Grade S = 50tsi (780MPa). Old grade E = grade T = 55tsi (850MPa).

In the USA, high tensile hex. bolts/screws were marked with radial lines: 3 equally spaced lines (pointing to corners) for grade 5, and 6 lines (pointing to the 6 corners) for grade 8. Plus many other variations of marking for other grades. Grade 5 ultmate strength = approx 110,000psi (49tsi/760MPa). Grade 8 ultimate strength = 150,000psi (67tsi/1000MPa). This description is far from comprehensive, but this website is geared towards an old British car, hence the British bias.

For metric, grade 8.8 upwards are marked on the bolts/screws, and grade 8 (approx. 800MPa) upwards marked on nuts. Metric grade 8.8 means approx. 800MPa ultimate strength, and a yield strength of 0.8 of that ultimate strength.

Stainless Steel

There are many types of stainless steel and these used to be listed under their various manufacturer's trade-names. Nowadays, these are identified as A1, A2, etc. (in Europe) which relates to their corrosion resisting properties, rather than strength. In the USA, the equivalent grades are 303 (for A1), 304 (for A2) and 316 (for A4). A4 has a higher resistance to attack by acids, salt water etc. and so is used for marine applications and in the chemical industry.

For information on strength, a grade number is also used, such as 50, 70, and 80. For example, grade A2-70: Multiply that 70 by 10, and you get the strength, 700MPa (45tsi). Grade A4-80 is also used extensively for fasteners. These types of stainless steel are non-magnetic, despite being largely made up of iron, due to the austenitic crystal structure of the material.

Yield strength is an important criteria, and it must be remembered that figures for stainless steel are quite low, relative to their ultimate strength. In fact, stainless steel's yield does not occur at a definite figure for stress, unlike most steels. This makes most commercially available stainless steel bolts unsuitable as replacements for high tensile steel.

There is a risk of galling (cold welding) of stainless threads, causing the fasteners to seize together. To reduce that risk, a different grade might be used for nuts and bolts: A1 (303) for the nuts. Or else a thread lubricant formulated for stainless steel (expensive!) may be used.


Generally lower in strength to mild steel (depending on component manufacturing processes), and so suitable for light duty joints. Widely used for electrical connections and interior trim for cars.


There are various types of bronze, and they generally have a very high resistence to corrosion. Their strength are comparable with mild steel.

Aluminium Alloy

These were used for weight saving on aircraft fasteners. BSF and BA aircraft bolts were made to different lengths, and thread length, from commercially available bolts. Bolt heads were marked with their part number prefixed by letter "L".


Alloys of titanium have a very high strength to mass ratio, making them ideal for aircraft and top level motor sport. This is a whole subject on its own and rather outside the scope of the AC motor car restoration! Tensile strength can typically be 60tsi (920MPa), but its relative density is only 4.4 (steel is 7.9).


For light weight, light duty, and inexpensive fasteners. Tensile strength can be as high as about 6tsi (100MPa).


Self Colour

Material left bare without any coatings or treatment for either protection or appearance.

Chemically Blackened / Bluing

An oxidising process that leaves a blue-black appearance, and may provide some slight protection against rust, in the case of steel.

Chromium Plated

For appearance, chromium gives the best results, although it offers limited protection against corrosion. For steel, three layers of metal plating are required: Copper, nickel and then chromium. Commonly used for steel and brass (the latter with just the nickel and chromium layers).

Cadmium Plated

This material is often associated with high grade bolts. It offers corrosion resistance and also reduces friction in threads, but is expensive.

Zinc-Plated (Electro-plated)

Not to be confused with "galvanising" (see below), electro-plated zinc provides rust resistance for steel. The plating material deteriorates (is sacrificed) in the process of saving the steel underneath by electrolytic corrosion. Appearance is not as attractive as other types of plating.


Dipping components in a hot zinc solution is generally known as galvanising (although any zinc coating provides galvanic protection). It gives a thicker and more protective coating than electro-plating. Unfortunately, the coating is too thick and uneven for precision screw threads. It is sometimes used for low grade structural bolts such as coach bolts.

Nickel Plating

An attractive form of plating with some degree of rust protection when used on steel. Commonly used for brass too.

<< Page 5 ****** Page 7 >>

Page 1 - Screw Threads

Page 2 - Bolts, Screws and Studs

Page 3 - Nuts

Page 4 - Nuts (Self-Locking)

Page 5 - Washers

Page 7 - Tightening

Page 8 - Wood Screws and Nails

Page 9 - Fasteners for the AC 2 Litre Saloon



Website started 29th December 2006