Modern Motorcycles, the Wonders of Elastic Metal

The idea mixture of fuel to air in a motorcycle engine is around 13.2:1, so one part fuel to 13.2 parts air. In each cylinder when the fuel/air mixture goes bang at high revs the resulting pressure release around 90 to 95 Bar, or 1305 to 1377psi. This is why it's so important to do up cylinder head studs well.

Also in your modern motorcycle engine the spark plug is right in the tick of it all. The spark plug makes the fuel/air combination in the combustion chamber go bang. With todays 600cc sportsbike running at 13,500rpm the plug sparks 6,750 times a minute (half the number of revs as the engine only fires every other revolution), or about 113 times a second. Each spark is 3kV, which is enough to ignite the fuel vapor. Now here is the impressive park, at high revs the plug is constantly running at around 800 degrees centigrade due to the constant explosions, which is very hot. Tin melts at 231, lead at 327, aluminum at 660 and iron at 1538 centigrade.

Why doesn't this melt the aluminum cylinder or piston? Although the temperature reaches 800 the engine's cooling keeps temperatures down. If this cooling fails or the engine runs too hot, bits will burn and melt.

When things are spinning around very fast and forces are very high in an modern engine, the metal starts to develop slightly elastic properties. When the piston hits top dead center and the con rod pulls it back downwards the rod will actually stretch slightly due to the forces involved in changing the piston's direction.

How much will the metal stretch? According to engine tuners the minimum area you can leave between the piston and cylinder head is 0.6mm, any less and the piston will hit when the con rod goes 'elastic.' The worst time for engine elasticity is on over-run, which is why the advent of slipper clutches, which take the pressure off the engine on over-run, has helped reduce the number of engine failures.