What does it refer to in the following sentence?

If the gap is too big or small, you adjust it by tightening or loosening the nut at the base of the finger.

What does it refer to in the following sentence?

Oil will only work if you let it soak in for some time.

What does they refer to in the following sentence?

There are more than 6 billion mobile phones in use in the world. In many countries they are more popular than landlines.

What does they refer to in the following sentence?

The combustion gases turn the turbine and the compressor round as they escape from the combustion chamber.

What does it refer to in the following sentence?

The pitch depends on how many times the sound producer vibrates. The faster it vibrates, the higher the pitch.

What does it refer to in the following sentence?

The water is stored in the radiator and the engine jacket. It is pumped along the bottom hose from the radiator to the engine jacket.

What does it refer to in the following sentence?

The radiator cap keeps the water under pressure. This helps stop it boiling.

What does it refer to in the following sentence?

The fuel and air mixture enters the cylinder. The piston rises and it compresses the mixture.

What does it refer to in the following sentence?

On an aircraft the crankshaft is connected to the propeller. In a car it transmits power to the wheels.

What does it refer to in the following sentence?

The compressor sucks air into the engine and compresses it.

What does it refer to in the following sentence?

As the piston rises again, it pushes the exhaust gases out.

What does they refer to in the following sentence?

As the combustion gases expand, they push the piston down, and the crankshaft turns.