Patented is a hook fastener pre-form product for subsequent formation of a loop engaging hook fastener product. The pre-form product has a base sheet with a surface of thermoplastic resin and a plurality of stem formations formed integrally with the surface to protrude there from. The stem formations are arranged for engagement with a field of loops after deformation of terminal ends. Each of the stem formations includes a first portion extending upwardly from the surface and a second portion extending upwardly from the first portion to a distal end to define a height of the stem formation relative to the surface. Each first portion has only one second portion extending there from. An intersection of the second and first portions occurs at a distance from the surface equal to at least half the height of the stem formation. The intersection defines a discrete transition in a cross-sectional area of the stem formation that corresponds to a flat upper surface of the first portion. The second portions are more susceptible to deformation energy than the first portions and are sized to form heads that overhang sides of the first portions for releasable engagement with a loop product, when deformed.