Metals in Motorbikes

Even at the end of the '60s, when the AMC factory at Plumstead finally closed, metal components formed the highest proportion of parts needed in the manufacture of high-powered motorcycles.

Plastics had not yet become commonplace in the automotive field, with the exceptions of their use in electrical items (for their insulation properties) and in the latter years, in the form of fibre-glass, for petrol tanks, racing bike seats, number plate fairings and other lower load-bearing components.

On this page we hope to include information on the wide range of metals that went into the manufacture of AMC's range of bikes and, to do so, we will be grouping them according to their engineering characteristics.

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Iron and Steel

CAST IRON has always been used for cylinder barrels on air-cooled motorcycle engines, mainly due to its free flowing ability when molten which allows it to be cast into the closely-spaced, thin-section fins.

Its ease of machining and the fact that it is about the only metal that can be run against itself without seizure or rapid wear, even under conditions of great heat and scanty lubrication, also make it the material of choice for this, and other, applications.

Cast iron's frictional properties also made it the universal material for brake drums which, when aluminium wheel hubs came into use, was still used as a liner for the brake shoes to work against.

MALLEABLE CAST IRON was used quite extensively at AMC for the robust lugs that formed the connection points for the steel frame tubes.

Starting out as 'white' cast iron, an extremely hard and brittle material, it is then subjected to a lengthy annealing process by which most of the combined carbon is either removed or converted into small graphite flakes.

Although having a considerably lower tensile strength than mild steel, malleable cast iron is more tolerant of local stress concentrations or surface defects. It can also be cast in thicknesses down to 3mm, in hollow or ribbed sections, and machines easily, either wet or dry.

SPHEROIDAL GRAPHITE IRON (aka SG Iron, Ductile Iron, Nodular Iron) has much more impact and fatigue resistance than normal grey iron, due to the shape of the graphite particles. Whereas in grey cast iron the graphite exists in flake form, which can cause stress points in the metal matrix, SG Iron has rounded nodules which inhibit the creation of cracks.

This material was used at AMC for the crankshafts of their enlarged (600 and 650cc) vertical twins, as a replacement to the previous Meehanite iron castings.

AUSTENITIC CAST IRON is produced by taking a normal grey or ductile iron melt, controlling the carbon and silicon at lower levels and adding various alloys to produce a stable austenitic structure at ambient temperature. Nickel is the main alloying element, hence the alternative name of Ni Resist. Austenitic cast iron was used for the exhaust valve seat inserts fitted to aluminium alloy cylinder heads.

STEEL. It is unarguably true that without steel there wouldn't be motorcycles (or any other form of motorised transportation) as this material, in its many grades and forms, is only option for the majority of parts used in their manufacture.

The reason for this lies in steel's unique ability to form alloys with numerous elements that result in tailor-made properties of strength, hardness, heat resistance, etc., combinations of which are vital for achieving a long working life for a high-speed/power machine.

Itself an alloy of iron and carbon, steel is used in its most basic form for many lightly-stressed motorcycle parts; in sheet form for petrol/oil tanks, toolboxes, headlamp housings, mudguards, etc., whilst in its higher specifications, it is capable of withstanding the heat and stress imposed on engine components such as exhaust valves, camshafts, gears and sprockets.

(Click the exploded parts list image to see the number of steel parts - coloured red - in an AMC engine/gearbox unit)

Carbon is the most important element in the majority of steels, affecting hardness and strength by heat treatment and causing the ductility and weldability to decrease with increasing carbon content.

The following list identifies the chief changes in properties that the other main alloying elements provide:

Nickel - increases hardenability, tensile and impact values of steels.

Manganese - contributes to strength and hardness with variable carbon content.

Molybdenum - improves strength and hardness in nickel chrome alloy steels.

Lead, Phosphorous, Selenium and Sulphur - improves machineability in various combinations.

Vanadium - helps improve fatigue stress and wear resistance when used with other alloying elements.

Chromium - increases corrosion and oxidation resistance.

Prior to World War 2, the various grades of steels were only known by the trade names given by their manufacturers, who were predominately based in Sheffield.

Kayser Ellison favoured names of a numerical format, such as KE-805 (a direct oil-hardening nickel chrome steel widely used for gear making) and KE-965 (a special heat-resisting valve steel claimed to maintain a high tensile strength at temperatures up to 900°C, without becoming brittle).

W.T. Flather, another Sheffield firm, founded in 1817, who later specialised in steels for the cycle industry, choose to use stylish names to identify their products, such as 'Keystone', 'Silkut', 'Pinbar', 'Krombus', 'Nykos' and 'Ubas' Steels, the meanings of which are now lost in history. The derivation of UBAS, a low-carbon, case hardening steel, has been variously attributed to Universal Bright Alloy (or Best Axle) Steel, Un-Breakable Axle Steel and even Ultimate Bessemer Acid Steel.

However, with the onset of war and the need to avoid the possible confusion of different trade names for similar grades, a British Standard document was prepared to limit steel manufacturers and their customers to a restricted list of grades, identified by Emergency Numbers (EN).

Fifty-eight specifications (EN1 to EN58) were deemed sufficient to cover all the important variations, which included carbon steels, alloy steels, case hardening steels, spring steels and stainless steels, in ascending numerical order.

The 1941 publication of BS 970, as it was known, made no allowance for any new grades of steel that might be formulated in the future, to be inserted into the sequence of numbers, without upsetting the logic of the system.

A solution to this problem was found by simply adding a suffix letter to the EN number that had the closest match of composition/properties, resulting in steels such as EN1a, EN24t and EN58j.

In 1955, the British Standards Institution revised the BS 970 specification, to bring it into line with the then current needs, and increased the number of grades to around 200, allowing the appearance of EN61, EN207, EN355, amongst many others.

In 1970 (just after the closure of AMC's Woolwich factory) the EN method of steel classification was "officially" superseded by an entirely new six character classification method, which was designed to relate more directly with the steel's chemical composition and strength but, like many things in mechanical engineering, the old, well-understood EN grades are still in general use to this day.

Choosing a particular grade of steel for a motorcycle component was, in most cases, usually based on 'what had been used before'. A conservative method that had some merit when dealing with high-powered machinery whose riders would often extract the maximum performance in, sometimes, less than perfect conditions. More often, it was changes in heat treatment processes and finishes, rather than the base material specification that brought about improvements to a components working life.

The before-mentioned KE-805 direct hardening steel (later standardized as EN 24) was the material usually selected for highly-stressed parts, such as gears and crankpins, whilst UBAS low carbon steel (EN 16 equivalent) was used extensively for engine mainshafts, cams, rocker arms, oil pump plungers, to name but a few parts.

Cyanide hardening would have been the preferred heat treatment method for UBAS parts subject to extremes of wear, although carbo-nitriding took over most of the role when a large-scale plant was installed at AMC.

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Stellite

Stellite is a cobalt-chromium alloy that was developed in the early 1900s and is the trademarked name of the Kennametal Stellite Company.

Displaying outstanding hardness, toughness, wear and corrosion resistance, Stellite is ideally suited for cutting tools which can maintain a good cutting edge at high working temperatures but has another use in motorcycles as a hard-facing material.

Examples of this application are the cam interfaces of Norton Atlas tappets, where the grade 14 cast iron items have Stellite material added which is subsequently ground to a mirror finish, and the tips of Matchless G80 valves.

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Aluminium Alloy

At one time also used for cylinder heads, cast iron gave way to aluminium alloy quite early on in AMC's history for reasons of weight saving, heat transference and lack of hot spots in their high compression ratio engines.

Aluminium, though, was already the norm for use in all the main engine, gearbox and transmission housings as well as wheel hubs and many other smaller components.

As with steel, aluminium was used in a multitude of grades depending on the strength required, its resistance to corrosion and its form.

The majority of aluminium parts would need to be formed by casting and, for this process, the various choices would have been covered by BS 1490 - Specification for Aluminium and aluminium alloy ingots and castings for general engineering purposes.

LM4 was generally selected for the production of sand and permanent mould castings, where its moderate mechanical properties were acceptable, allowing thin-walled, pressure tight housings at the cheapest cost.

For more arduous duty, and higher resistance to corrosion, LM25 would be used due to its ability to be heat-treated to various levels - -TE (Precipitation treated), -TB7 (Solution treated and stabilized) or -TF (Fully heat treated).

More specialized aluminium alloys were used for highly stressed parts and those subject to extreme temperatures, examples being:
Y-alloy (a nickel-containing aluminium alloy),LM 13-WP for pistons, RR53-B for cylinder heads and L83 / 2L65 or L77 for connecting rods.

Duralumin (usually referred to a Dural) was an aluminium alloy containing 4% copper and smaller amounts of magnesium and manganese which, after quenching, slowly hardened when left at room temperature.

It was used for big-end roller cages, push rod tubes and the rear wheel sprockets of some competition bikes.

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Magnesium Alloy

Even lighter than aluminium (specific gravity of 1.8 compared to 2.8), magnesium was reserved for use on only the AMC racing machines, being utilised as castings for the crankcases, timing gear casing and conical wheel hubs of the AJS 7R and Matchless G50 bikes.

This exotic, and sometimes unsafe material to machine (due to its tendency to self-ignite if unsuitable tools were used) was usually known as Elektron, the trademark of its British company Magnesium Elektron Ltd.

Having a gold paint finish was not just a cosmetic choice for these parts (although it certainly added to the glamour of the machines) as the otherwise unprotected magnesium alloy would soon have corroded away.

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Copper (Brass and other alloys)

The use of copper, in its unalloyed form, was restricted in its use in motorcycles because of its ductility (softness) and corresponding lack of strength. However, together with its high thermal conductivity, this apparent drawback made it the perfect choice of material for gaskets and sealing washers that were needed in the hottest parts of the engines.

Copper was also used as a selective coating when carburising steel parts, such as solid rocker arms, to ensure that only the working ends and pivot bore would be subsequently hardened, leaving the remainder in its more ductile, shock absorbing state.

Brass (an alloy of copper and zinc) and its cousins bronze (copper and tin), aluminium bronze, phosphor bronze also make an appearance in many motorcycle components, where their electrical, heat and wear resistance properties are required (e.g. valve seat insets, plain bearings and electrical contacts).

Aluminium bronze is a type of bronze in which aluminium is the main alloying metal added to copper, in contrast to standard bronze (copper and tin) and it's use in motorcycle engines was limited to the inlet valve seat inserts in aluminium cylinder heads.

Oilite is a bronze material, formed by powder metallurgy, which has its porous structure vacuum impregnated with oil to improve its bearing ability.

Although in some cases bearings were machined from barstock to bespoke forms at AMC, it was more normal for standardized components to be incorporated into designs as very sharp cutting tools are necessary to avoid burnishing the important lubrication release surfaces.

Once fitted, Oilite bearings rarely showed any wear if used under the specified loadings.