Blow Your Own Letters
Thumbing through my back copies of OBA in spare time is a rewarding pastime, even if only to relish at the magnificent photos or follow the life experiences of Edgar Jessop. Blokes of my vintage can remember when young fellows like Terry Dennehy were burning up the bitumen at Bathurst on his first Honda 250 twin while the rest of us contemplated dropped valves, broken rings or ogled at back copies of Playboy centerfolds in the pits. So, to my reason for writing. OBA issue 50 Letters, has one entitled ‘The Bronze Age’, in which Bruce Anderson Qld. asked for some feedback.
I am now many years retired, but as a consulting metallurgist on metal castings, particularly defect analysis I had a close connection with the motorcycle fraternity for quite some time. Firstly, even back in the thirties, the casting technology for producing sound castings in cast iron, bronze, and aluminium was quite well established providing correct procedures were followed. The Specific Gravity from heaviest to lightest is Bronze 8.80, Cast iron 7.87, Aluminium 2.70. I actually have a 1934 copy of an original film showing the Bugatti car company foundry casting Aluminium heads for the type 59 racing model. Nobody can say they didn’t know what they were doing. The original liner Queen Mary launched in 1936 had three bronze propellers weighing 18 tonne each. They made four to have one as a spare but it was never needed. Good quality.
Secondly, all castings in all metals have a high propensity to develop porosity and cavities in the cast matrix if quality control procedures are not followed. Porosity is caused by gas in solution, which means the gasses are totally soluble in molten metal and only form as bubbles during solidification, which can’t all get out as the metal solidifies. Cavities are formed when the metal solidifies by the phenomenon called directional solidification contraction. Both are known as common unwanted defects; experienced by all and understood by few. Aluminium melts at 700°c; much lower than cast iron and bronze at over 1000°c, which made it easy for backyard operators to start a foundry, which many did. It’s easy; collect a heap of old Aluminium pots and pans, melt them in a steel container with coke or oxy and start casting pistons. Wrong, unless seizing, cracking and failing doesn’t worry you. Aluminium pots and pans are virtually pure Aluminium which is difficult to cast successfully. Molten Aluminium absorbs a lot of gases including water vapour which adds hydrogen into the liquid metal. You cannot make porosity-free Aluminium castings unless the hydrogen is purged out, degassed and deoxidized; not easy. Aluminium needs to be alloyed with Silicon in specificallycontrolled quantities from about 5% to 13% to make castings depending on the grade. Pistons are a special case needing controlled chemical analysis and heat treatment. If you want to make pistons, buy a proper furnace and melt specification ingot or better still contact a registered foundry. I have only covered the basics here but hope it helps.
Nev Murray NSW