The part under the neck ring (9) of a preform for blowing a PET bottle of predetermined capacity not larger than 75 cl and with a max. material weight of 15 g, is inclined by an angle α less accentuated than normal, max. 17°, which allows to make a preform with a thickness t2 of the wall thinner than 1.9 mm and a thinner inner diameter of the preform body in order to allow a lower stretch ratio starting from a larger preform to obtain the final bottle of predetermined capacity.