Magnesium explained
OBA literature and letters in many issues refer to the use of Magnesium used for component parts in engine design, almost entirely, but not exclusively in AJS 7R and Matchless G50 for timing case covers and crankcases, but almost no other bike manufacturers advertise using pure Magnesium. Why is this so and why do we not see pure magnesium used today?
Pure Magnesium has been in use since the early 1800s, is the lightest of metals but with limited use which can be used for manufacturing engineered component parts by casting, forging, or direct machining from billet. In the early days a very small addition of Aluminium improved the strength and hardness which formed an alloy officially designated Magnalium, and registered in Germany as the metal Elektron.
Relative Specific Gravity(RSG) values from lightest to heaviest; Magnesium (Elektron) RSG 1.8, Aluminium RSG 2.8, Titanium RSG 4.54, Steel RSG 7.9. However Magnesium (Elektron) metal casting components are expensive to make, requiring unique foundry technology expertise to make and are dangerous to work with, i.e. during machining. It is liable to spontaneous violent explosion under appropriate ignition conditions and it burns with an intense brilliant white hot flame 3000 C and is very difficult to extinguish. This can also happen in the event of a serious crash. Magnesium castings can suffer from stress corrosion and have a high affinity to Oxygen and corrode readily if not painted with actual inert gold paint for protection.
In 1908 Elektron (90% Magnesium) was used in Germany to cast an entire aircraft crankcase specifically for lightness. From around 1920 designers recognised the value of lightness in motor engines and extensive use was made using Elektron magnesium in racing bikes and cars irrespective of the known dangers. A proposal was considered in Le Mans, France in 1955 to ban pure Magnesium use in racing car and engine design because of the danger. Further Research and development work was initiated to overcome the hazardous features. The new Magnesium grades created contained Aluminium which gave better mechanical properties of strength and hardness and was marketed under various fancy names. The late great Phil Irving, possessive of some metallurgical savvy, used a piece of Ceralumin (Magnesium Aluminium alloy – exact composition unknown) billet which was a piece of ‘Jetsam’ left over from the abortive Velocette rotary valve debacle. He machined a piston from the solid billet for the Velo Roarer when a standard Aluminium cast piston failed miserably. The new piston was a roaring success although the Velocette Roarer and Model O were never fully developed. Today extensive material science development work has used the benefits of new MagnesiumAluminium alloys which has created two of the most important and significant new metals used today worldwide, particularly in automotive applications. Elektron is still made in England by a specialist metal manufacturer. MagnesiumAluminium alloy has excellent improved mechanical properties to standard Aluminium, does not spontaneously catch fire, and is now widely used for complete engine blocks and many more engine and suspension components by companies such as Porsche, MV Agusta, BMW, FORD, GM, and VOLVO, and for motorcycle bottom halves, barrels and heads. Although designated Magnesium and covered by international ISO Standards and ASTM Mg alloy specifications they are actually special Aluminium alloys with only about 1%- 9% Magnesium but also Nickel, Copper and other additives involving post heat treatment. Various ‘brand’ names have been adopted; Duralium, Ceralumin, Y Alloy, among others. Manufacturers often adopt ‘brand’ names and patents for their alloys predominately to protect them divulging the actual chemical analysis. The improved performance characteristics of the modern Magnesium
Aluminium alloys allow lighter sections, strength, hardness and extensive design freedom. SG Cast
Iron modified with magnesium additive is a worldwide revolution in crankshafts, camshafts and conrods. (Another story another time).