The story of leaded petrol involves Thomas Midgley, Jr. He was a chemist, back in the days when "Better Living through Chemistry" had nothing to do with drugs. Originally, his ideas were thought to be brilliant. Only later did they turn out to be disasters.

In 1921, he found that tetraethyl lead was a superb chemical to add to petrol to stop "knocking" in car engines. Today, children in cities all over the world are getting lower IQs, because their brains have been poisoned by lead.

He then invented chlorofluorocarbons - and we all know how these wonderful chemicals have punched a "hole" in the ozone layer. In fact, the "hole" in 1992 is the widest and deepest ever recorded.

In 1940, Thomas Midgley, Jr. was paralysed by polio. He built an assembly of pulleys and ropes so he could move himself between his bed and his wheelchair. In 1944, he became entangled in the ropes, and strangled to death in his own invention.

But in the 1920s, tetraethyl lead was a wonderful invention. When your spark plug fires, it ignites a flame front which travels through the combustion chamber, burning the petrol:air mixture. Tetraethyl lead made the flame front travel more slowly, and also less turbulently. Lead virtually eliminated knock. Overnight, compression ratios jumped from 4:1 to 7:1, and the modern high-output engine was born.

But leaded petrol had even more benefits. As it burns, tetraethyl lead turns into a tan-coloured layer of lead oxide, which covers the valves and the combustion chamber. The valves hit hard against the valve seats several thousand times each minute. The lead oxide acts as a cushioning agent, and protects the valve and the valve seats. The lead oxide is also a lubricating agent. This reduces wear in the valve guides, as the valves slide inside them.

If you start running your old leaded engine on unleaded petrol, the lead oxide quickly wears away. Damage begins, but only if your engine was made with "soft" metallurgy, and only in high-temperature areas - exhaust valves, exhaust valve guides and exhaust valve seats.

At high temperatures, iron oxides form on a "soft" cast-iron valve seat. These oxides flake off and actually embed themselves (as hard nodules) in the "soft" face of the exhaust valve. These hard nodules work like tiny grinding wheels as the valve rotates. They grind away at the valve seat, making it recede into the head. At first, the valve-adjusting mechanism takes up the slack. Eventually, the adjustment is all used up, and the valve no longer closes completely. Then the edges of the valve quickly burn away - so you get what the mechanics call a "burnt valve".

It's quite easy to modify your leaded cylinder head to run on unleaded petrol and never get damaged (if it has "soft" metallurgy). There are just three changes.

The first step begins with the valve guides. You can install silicon-aluminium-bronze valve guides, but usually only in Double OverHead Cam (DOHC) engines. Silicon-aluminium-bronze is fairly soft, and wears fairly rapidly. This is not important in DOHC engines, where the valve just goes up and down, and doesn't rub on the side of the valve guide. In an engine with tappets, the end of the tappet moves in an arc, and so the valve rubs a little on the wall of the valve guide. In this case, use steel valve guides.

The next step is to install valves that are faced or tipped with hard Stellite. The Lancer engine finally died after 240,000 km, so my mechanic installed Stellite-tipped valves, which had also been nitrite-hardened. Valves with "hard" metallurgy cost about $30-$40. You need to replace only the exhaust valves - you can reuse/recondition the inlet valves. The last step is installing the "hard" valve seats. There are several companies (Federal-Mogul, ACL, etc) that supply after-market valve seats with superior metallurgy. You can ask for Stellite, or hard high-chromium or 60%-alloy valve seat inserts.

Valve seat inserts are usually supplied slightly oversize, to give the correct "interference fit". The installation is critical. The head is heated, while the inserts are cooled in dry ice. It's important to use a heat-transfer compound, to fill in the microscopic roughness in the walls between the head and the valve seat insert. In fact, silicone or oil assembly lubricants can have the opposite effect, and actually behave like a heat barrier.

It was easy to convert the Pug 504. Peugeot made a later engine, which was lead-free. We looked up the catalogue, and retro-fitted the later valves, valve guides and valve seats.

In Oz, only 36% of petrol sold is unleaded. But many people are running their old cars (with the "hard" metallurgy) on leaded petrol, even though they could change to unleaded. If these people did change, unleaded petrol sales would jump to about 60% of total petrol sold. Other older cars can run perfectly well on only one leaded tankful for every four unleaded tankfuls.