A preform for making a plastic container, comprising: a sidewall having an inner surface and an outer surface, said sidewall defining: a finish portion having at least one external thread defined thereon, said finish portion having a first sidewall thickness (T 1); a main body portion; an end cap, said end cap having a second sidewall thickness (T 2), said second sidewall thickness (T 2) being not less than said first sidewall thickness (T 1), the inner surface of the end cap being concavely spherically curved with substantially constant radius (R I); and a transition portion between said end cap and said main body portion, wherein the portion of the inner surface of the sidewall that defines the inner surface of the transition portion is shaped as a continuation of the inner surface of the end cap having substantially the same spherical shape and radius (R I); and the thickness of the sidewall that defines the transition portion increases from the distal end of the transition portion that joins the end cap to the proximal end of the transition portion that joins the main body portion; characterized in that the portion of said outer surface of said sidewall that defines said transition portion is concave as viewed in longitudinal cross-section. a sidewall having an inner surface and an outer surface, said sidewall defining: a finish portion having at least one external thread defined thereon, said finish portion having a first sidewall thickness (T 1); a main body portion; an end cap, said end cap having a second sidewall thickness (T 2), said second sidewall thickness (T 2) being not less than said first sidewall thickness (T 1), the inner surface of the end cap being concavely spherically curved with substantially constant radius (R I); and a transition portion between said end cap and said main body portion, wherein the portion of the inner surface of the sidewall that defines the inner surface of the transition portion is shaped as a continuation of the inner surface of the end cap having substantially the same spherical shape and radius (R I); and the thickness of the sidewall that defines the transition portion increases from the distal end of the transition portion that joins the end cap to the proximal end of the transition portion that joins the main body portion; characterized in that the portion of said outer surface of said sidewall that defines said transition portion is concave as viewed in longitudinal cross-section. a finish portion having at least one external thread defined thereon, said finish portion having a first sidewall thickness (T 1); a main body portion; an end cap, said end cap having a second sidewall thickness (T 2), said second sidewall thickness (T 2) being not less than said first sidewall thickness (T 1), the inner surface of the end cap being concavely spherically curved with substantially constant radius (R I); and a transition portion between said end cap and said main body portion, wherein the portion of the inner surface of the sidewall that defines the inner surface of the transition portion is shaped as a continuation of the inner surface of the end cap having substantially the same spherical shape and radius (R I); and the thickness of the sidewall that defines the transition portion increases from the distal end of the transition portion that joins the end cap to the proximal end of the transition portion that joins the main body portion; characterized in that the portion of said outer surface of said sidewall that defines said transition portion is concave as viewed in longitudinal cross-section.