The plastic preform (1) comprises a heat sealable neck finish (2) having at least a first upper heat sealable portion (20) which is terminated by a top opened end (200), a second intermediate threaded portion (21) for screwing a closure cap, and a third lower portion (22) that comprises a neck support ring (221a) of width (L1). The first upper portion (20) comprises an annular gripping recess (201) of predetermined height (H2) and depth (d). The inner face of the third lower portion (22) forms an inward transition step (221 b), and the outer diameter (D3) of the preform measured just below the neck support ring (221a) is less than the outer diameter (D2) of the neck finish measured just above the said inward transition step (221b).