Timing Belt Tensioner: Description and Operation

Automatic Timing Belt Tensioner:
Basically the tensioner consists of a spring and a damper.

When the belt is too slack, a piston is pressed out by a spring and oil flows, via a non-return valve, to the other side of the piston until oil pressure is equal on both sides of it.

If the belt becomes too tight oil is pressed back past the piston through a leak-off channel until pressure is once again equal on both sides of the piston. This keeps the belt evenly and correctly tensioned regardless of its temperature.