A spring fastener characterized by a structure having a cavity in which the rib of a first part, such as a plastic panel for example, like a pillar, may be engaged, and is 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. The spring fastener comprises engagement springs having a peak, engagement regions and-side cuts in the vicinity of the peak, preferably within the engagement regions. The side-cuts, of preferably triangular shape, increase considerably 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). It is highly preferable that the fastener comprises barbs with inwardly bent front ends, and that the engagement regions further have hindrance portions, the combination of which highly improves the balance of insertion to removal ratio.