Pistons provide airpressure together with the pistonhead.
There are a lot of different types and sizes:

Aluminium/Full Metal Pistons: These Pistons are very strong and very durable.
A downside however is the weight, which causes more stress on the gearbox.
The metal teeth can also damage the gears in case of a gearbox defect.

Half Teeth Pistons: These Pistons are specific for special gears, which are usually Torque Up gears.

Plastic Pistons: Light and strong. These Pistons are used the most, and often have the last teeth made of metal. In the event of a defect, the plastic teeth will break, instead of expensive gears.

Please note! Make sure that the Piston and Sector Gear engage propperly, to prevent wear and damage!
This is called Angle of Engagement.