A spring fastener engages the rib of a first part, such as a plastic panel for example, like a pillar for example, may be engaged, and they are also suitable to be engaged reversibly in a slot of second part, such as a metal sheet for example, like the frame of a car for example. The spring fastener comprises engagement springs having a peak, engagement regions and-side cuts in the vicinity of the peak, for example within the engagement regions. The side-cuts, of preferably triangular shape, increase the lever/angle pulling force (pulling force applied on the first part and away from the spring fastener, in order to separate the first part from a second part connected together with the spring fastener). According to one embodiment, the fastener comprises barbs with inwardly bent front ends, and the engagement regions further have hindrance portions.