Abstract: In an internal combustion engine having a timing system which employs a crenellated or toothed timing belt and the lubrication is carried out under a substantial oil pressure, a device for maintaining the tension of said belt as close as possible to the initial calibration value, said device comprising the combination of two resilient members acting the said means being responsive to the pressure of the lubricating oil with the intermediary of a piston moved by the pressure of said oil. Thus, the belt tension is rendered insensitive to the size variations the engine undergoes as its temperature is changed, e.g. when the engine passes from standstill to running and vice versa.