Choosing the Right Torque Tester Fixture for PCO Neck Finishes (1881 / 2925)
By Amy Zhao, Factory Technical Lead, KHT Instrument
A torque tester fixture is the part of the instrument that physically holds the bottle or cap steady during a torque test, and choosing the right one starts with knowing your exact neck finish — because a fixture that works for one PCO neck size won't necessarily clamp another cleanly.
PCO (Plastic Closure Only) neck finishes are among the most common standards in bottled beverage and liquid packaging, but "PCO" isn't one single size. Two of the most widely used are PCO 1881 (28mm) and PCO 2925 (29mm) — close enough in size that it's easy to assume one fixture handles both, and different enough that assuming so can be a mistake.
Why Neck Finish Matters for Fixture Selection
A torque tester's clamping fixture needs to grip the bottle (or cap) firmly enough that all of the rotational force goes into the cap-thread interface being measured — not into the bottle slipping in the clamp. If the fixture doesn't match the bottle's neck geometry precisely, you introduce exactly the kind of measurement noise that undermines the whole point of torque testing: a reading that reflects how well the fixture gripped, rather than the actual torque performance of the cap and closure.
A Real Selection Scenario (One-Hand Experience)
We recently worked through this exact question with a closures manufacturer supplying two bottleneck finishes: 28mm (PCO 1881) and 29mm (PCO 2925). Rather than assuming a standard fixture would cover both, the right first step — and the one we took — was requesting photos of the actual bottle caps in question, so the fixture choice could be evaluated against the real geometry rather than the nominal spec sheet numbers alone.
This is worth naming as the correct approach, not a formality: nominal neck-finish specifications describe the standard, but slight variations in real-world caps (wall thickness, thread engagement depth, cap skirt design) can affect whether a single fixture clamps both sizes cleanly or whether a quick-change adapter is the safer choice.
Common Mistake: Assuming "Standard Fixture" Covers Every PCO Variant
The mistake to avoid is treating "PCO" as if it were one fixed geometry. PCO 1881 and PCO 2925 differ by a full millimetre in neck diameter — small on paper, but potentially significant for a clamp designed around one specific size. Committing to a fixture before confirming it handles both of your actual neck finishes risks discovering the mismatch only after the instrument is already in production use, when a re-fixture or adapter order becomes a line-stoppage problem rather than a pre-purchase decision.
What the Instrument Side Looks Like
On the KHT NLY-20S automatic cap torque tester, the bottle body clamping range is Φ5mm–Φ200mm and the cap clamping range is Φ8mm–Φ80mm — wide enough to physically accommodate both 28mm and 29mm neck finishes within the instrument's overall range. Whether that means a single standard fixture handles both without adjustment, or whether a quick-change fixture adapter is the more reliable path, is a mechanical fit question that needs evaluation against the actual cap and bottle samples — not something to assume from the clamping-range spec alone.
Fixture Decision Path
| Situation | Recommended approach |
|---|---|
| Single, consistent neck finish across your product line | Standard fixture sized to that neck finish is usually sufficient |
| Multiple neck finishes on the same production line (e.g. 28mm + 29mm) | Request an engineering evaluation with actual cap/bottle photos or samples before committing — don't assume one fixture covers both |
| Uncertain whether your neck finish is a standard PCO size | Measure the actual neck diameter and thread profile, or send samples, rather than relying on a generic "PCO" label |
Frequently Asked Questions
Q1. What's the difference between PCO 1881 and PCO 2925? They are both PCO (Plastic Closure Only) neck finish standards commonly used on beverage bottles; PCO 1881 corresponds to a 28mm neck, and PCO 2925 corresponds to a 29mm neck. The two are visually similar but not interchangeable.
Q2. Can one torque tester handle bottles with different neck sizes? The instrument's clamping range (e.g. Φ5–200mm bottle body, Φ8–80mm cap on the NLY-20S) can span multiple neck sizes, but whether the fixture itself clamps two different neck finishes without swapping parts depends on the specific fixture design — this needs to be confirmed against your actual samples.
Q3. Should I send bottle/cap samples before ordering a torque tester? If your production line uses more than one neck finish or an uncommon closure design, yes — sending photos or physical samples lets the fixture be evaluated against your real product rather than nominal spec numbers, and can identify upfront whether a quick-change adapter is needed.
Q4. What happens if the fixture doesn't match the neck finish properly? An improperly matched fixture can allow the bottle to slip or shift during the test, which introduces measurement error unrelated to the actual cap torque — the reading may show inconsistency that looks like a product quality issue but is actually a fixture-fit issue.
See full specifications and configurations on the NLY-20 Digital Cap Torque Tester product page.