Bottle Neck Finishes Explained: GPI, ROPP, Crown, Cork

What is a bottle neck finish?

The finish is the molded glass section above the shoulder: the sealing surface at the top (the land), the thread or bead, and often a transfer bead or ring below. In glassmaking it is formed first, in the neck ring, which is why the word "finish" is used even though it is made at the start of the process.

Every finish is defined by a drawing with key dimensions: T (thread outside diameter), E (root diameter under the thread), H (height from top to shoulder or bead), S (start of thread) and I (inside bore). Closures are designed around these dimensions, which is why two bottles that "look the same" may not take the same cap.

How do you read GPI/SPI codes like 28-400 and 33-400?

GPI (Glass Packaging Institute) and SPI (now the Plastics Industry Association) finishes use a two-part code:

• First number: nominal T diameter in millimeters, such as 18, 20, 22, 24, 28, 33, 38, 43, 48, 53, 58, 63, 70, 89.
• Second number: thread style, which defines thread height, pitch and number of turns.

So 28-400 means a 28 mm neck with a 400 thread, and 33-400 is a 33 mm neck with the same thread style. Some suppliers write these reversed (400-28, 400-33); the meaning is the same.

Aluminum-plastic GPI caps for spirits and beverage bottles commonly use 28-400 or 33-400 finishes. The plastic inner carries the thread, and the aluminum outer shell gives a metal look.

What is an ROPP finish?

ROPP stands for roll-on pilfer-proof. The closure is a plain aluminum shell with no pre-formed thread. On the capping machine, rollers press the aluminum against the glass thread, forming the thread in place and rolling a perforated band under the neck bead. When the consumer twists the cap, the band separates, which shows the bottle has been opened.

Common ROPP sizes include:

• 28 mm ROPP (often 28×18 or similar), widely used on spirits, water and beverages.
• 31.5×24 mm, a standard olive oil and vinegar finish, often combined with a plastic pourer or non-refillable fitment inside the cap.
• 30×60 mm (and similar long-skirt formats), used for wine screw caps.

ROPP requires a capping head set up for the closure size. If you hand-cap or use a small line, confirm that suitable equipment is available before choosing it.

What are crown, bar-top, swing-top and lug finishes?

26 mm crown is the international standard for beer and soft drink bottles. Pry-off and twist-off crown finishes are different; a twist-off crown needs a twist-off neck.

Bar-top stoppers are T-shaped: a shank (synthetic cork or natural cork) fitted to a head made of wood, acrylic, glass or zinc alloy. Shank diameters such as 19.5 mm and 21.5 mm are common. The shank must be slightly larger than the neck bore so it compresses and seals; the right size depends on the bottle's inside bore diameter and its tolerance, so request the bore drawing.

Lug finishes are common on food jars and are named by diameter, such as 63, 70 or 82 mm. They are often used with hot fill and vacuum sealing.

How do you match a cap to a bottle?

Follow this sequence before ordering in volume:

1. Get the bottle's finish drawing with T, E, H, S and I dimensions and tolerances.
2. Ask the closure supplier for the cap drawing and confirm it was designed for that exact finish.
3. Choose the liner for the product: for example PE foam, EPE, TPE or saran-tin liners, depending on alcohol content, oil, carbonation and oxygen sensitivity.
4. Order samples of both bottle and cap from the production sources, not substitutes.
5. Fill with your actual product and apply caps with the method you will use in production.
6. Measure application and removal torque, and run leak tests: inverted storage, warm storage, and if relevant, transport vibration.
7. Check cork extraction force and fit for bar-top stoppers at different temperatures.
8. Record the approved combination and lock it in the purchase order.

Why should you test torque and leakage?

A cap that fits is not necessarily a cap that seals. Application torque that is too low can let liquid seep out; too high can strip plastic threads, crack the glass finish, or make the bottle hard to open. Tolerances on both glass and closure add up, so a combination that works on ten samples can fail on a percentage of a production run if the parts are near their limits.

Alcohol can soften some liners and plastics, and oils can migrate past poor seals. Temperature changes during sea freight also stress seals. Testing with your product, under realistic conditions, catches these problems before they reach a container.

Can one supplier provide matched bottles and closures?

Buying bottle and closure from one source reduces the risk of mismatch and simplifies testing. Mars Glass supplies aluminum screw caps, GPI aluminum-plastic caps in 28-400 and 33-400 style, 28 mm ROPP pilfer-proof caps, 31.5×24 mm oil pourer caps, 26 mm crown finish bottles and synthetic cork T-stoppers in 19.5 and 21.5 mm with wood, acrylic, glass or zinc-alloy heads. Listed cap MOQs typically range from 4,000 to 100,000 pieces.

Key takeaways

• The neck finish defines which closure fits; always work from a drawing.
• In GPI codes like 28-400, 28 is the diameter in millimeters and 400 is the thread style.
• ROPP caps are threaded on the line; they need suitable capping equipment.
• 31.5×24 mm is a common olive oil finish; 26 mm is the standard crown size.
• Bar-top stopper shank size must match the bottle bore with some compression.
• Test torque, leakage and liner compatibility with your real product before mass production.

Frequently asked questions