The internal diameter of the bottle neck opening, commonly identified as the I dimension on a neck-finish drawing.
Also called: bottle bore diameter; inside neck diameter; I dimension; bottle mouth internal diameter.
Which bottle dimension is the actual opening diameter?
On a neck-finish drawing using these dimension conventions, I identifies the internal neck diameter and T includes the outside thread. They answer different fit questions. Check the dimension letter and its drawing definition because opening or neck width can otherwise be ambiguous.
The minimum internal diameter matters for filling-tube clearance. Obtain the relevant internal profile and tolerance rather than assuming that a nominal screw-cap size supplies the opening available to the filling equipment. Identify any fitment that is part of the proposed package.
For an ideal round tube centered in an ideal round opening, the diameter difference is twice the clearance on each side. This geometric check can clarify a drawing discussion; it does not establish an acceptable operating clearance or demonstrate that a moving package remains centered.
Does a larger opening establish a faster bottle-filling rate?
The opening dimension establishes neither the counter’s event handling nor the complete filling sequence. It is one interface to check. Keep tube fit, the proposed product discharge and the required bottle rate as separately evidenced questions rather than treating a bore dimension as a throughput rating.