PET bottle threads: PCO-1881 and PCO-1810

You want to print a cap that screws onto a fizzy-drink bottle, or join two bottles neck to neck, or fit a pouring spout to a container you already own. It sounds trivial: measure the neck, sketch a female thread, done. It isn't. The neck of a PET bottle is not just any thread you can eyeball and copy — it's an industry-standard neck finish, dimensioned in fine detail and shaped to be blow-moulded and opened in a single twist — not to be printed in FDM. If you don't know which finish you're looking at, your cap will come out with the wrong pitch; it won't catch for more than half a turn; and you'll write it off without ever understanding why. This article tells you what those finishes are, what dimensions they actually have, and how to translate them into a printed thread that engages and — with luck — holds the pressure of a carbonated drink.

What a PCO finish is (and why there are two)

The threaded neck of a PET drinks bottle isn't designed bottle by bottle. It follows standardised neck finishes shared by bottle makers, bottlers and closure manufacturers, so that a cap from one supplier seals a bottle from another. For carbonated drinks — fizzy pop, sparkling water, PET beer — the two finishes you'll meet almost every time are PCO-1881 and PCO-1810.

They're close relatives, not the same finish. PCO-1810 is the older one: a tall neck, more plastic, with a thread of roughly three turns. The PCO-1881 is the current one, introduced to cut the neck down and save material — a few grams of PET per bottle, multiplied by billions of bottles a year. They share the thread diameter (around 27.4 mm), the sawtooth profile and the pitch, so a cap will start on either neck. What changes is the neck height and, with it, the thread length: the 1810 engages about three turns; the 1881, barely two.

Here's the catch: a cap starting on both necks doesn't mean it seals well on both. The engagement, the thread height and the sealing surface all differ, so a cap made for the two short turns of the 1881 ends up loose, or leaks, on an 1810, and an 1810 cap bottoms out before it seats on an 1881. With gas inside, that difference is exactly what makes a cap leak or blow off. Don't design for "a PCO neck" in the abstract: identify the specific finish on your bottle and model for that one.

The dimensions that actually matter

The thread runs to about 27.4 mm outside diameter and is a single start: one lead that gives a little over two turns on the 1881. Even so, a fizzy-drink bottle opens in one long turn, not the three or four of a metric thread — it manages that with a coarse pitch, a few turns of engagement, and the vent slots cut into the neck, which let the gas escape before the thread fully releases. The profile isn't a symmetric V: it's a buttress-style sawtooth, with one face almost perpendicular to the axis to take the push of the internal pressure and a ramped face for the cap to climb. That asymmetric profile belongs to the family of profiles that behave best in FDM, for the reasons Modelling threads sets out.

From dimension to FDM clearance

A cap is a female thread, and female threads are the perennial problem — made worse here by how fine this one is. In FDM the bead spreads into the gap, so the female comes out tight: model the cap's thread at the neck's nominal dimension and the flanks clash, jamming the cap on the first turn. You don't print the bottle — it's PET moulded to its exact size — so there's no male part here to fatten up: the whole correction goes into the cap, by opening it out.

Leave 0.15–0.25 mm of clearance on the cap's flank diameter, a touch more than on an ordinary metric thread, for two reasons. First, the thread is shallow — around 1.15 mm — and the helix rises very little per turn, so its lower flank steps layer by layer and comes out rough; that roughness eats into the gap. Second, you're going to screw and unscrew this thread by hand over and over, and you want it to run smoothly, not fit to the micron. Don't overdo it: with too much clearance the cap wobbles, loses contact with the flanks and works itself loose under the gas pressure. The aim is for it to run home with your fingers, with no tight spot, and to nip up the seal when it reaches the stop. The way you split these tenths of a millimetre, and the physics behind it, is the same as for any printed fit; Real printed clearances spells it out. Modelling the helix itself is covered by Modelling threads.

Orientation: axis vertical, mouth up

Like any printed thread, this one lives or dies by how you orient it on the bed. Print the cap with the thread axis vertical, square to the bed, closed end down and mouth up. That way the internal helix climbs layer by layer: each turn sits on top of the last, the crest and root come from clean stacked runs, and no thread hangs in mid-air. You also avoid supports inside the cavity, which on a fine thread are impossible to remove without wrecking the thread.

Lay it on its side and you break both rules you already know from Modelling threads at once: the helix stops climbing layer by layer and turns into a string of overhangs wrapped round the cylinder, and half the thread ends up unsupported on its underside, so it droops and the fit you measured vanishes. With a thread this coarse and this short, you've no margin to spare: it comes out furry and won't catch. Axis vertical, mouth up, and the rest sorts itself out.

The seal is the lip, not the thread

Here's what almost everyone overlooks. The thread doesn't seal: it holds. What stops the drink — and above all the gas — from escaping is the lip of the bottle's mouth pressing against the inner face of the cap. The thread only supplies the force that pushes that lip onto its seat; the seal is made where those two surfaces meet. That's why a commercial cap has a disc or a gasket of soft material inside: the PET lip bites into it and plugs the gap.

With a carbonated drink it gets genuinely tricky. It can sit at around 3–4 bar of internal pressure, and that pressure drives the gas towards any micro-leak. A sealing face printed in FDM is scored with layer lines: however flat you draw it, porosity remains between bead and bead, and the gas works through it, leaving your drink flat within a day. For carbonated contents, don't count on printed plastic to seal on its own.

What to print: caps, adapters and spouts

With the thread sorted and the seal understood, the possibilities open up. The most obvious is a cap — a spare, or one with an added function: a plug with a handle, a pouring spout, a narrow-neck pourer for decanting without spills. Bottle-to-bottle adapters are handy too: a part with a female PCO thread on both faces joins two fizzy-drink bottles neck to neck, and from there you can build reservoirs for vortex experiments, inverted drip irrigation or self-watering seed starters — or simply move liquid cleanly between containers.

Remember, too, that the thread diameter and profile are common to the 1881 and the 1810, so nearly everything you design starts on both families without touching the helix — though the pressure seal does depend on the specific finish, as you've already seen. Focus on what really decides whether your part works: the right pitch, measured on your bottle; the flank clearance in the cap; the vertical axis; and a gasket, with plenty of turns of engagement, if there's gas involved. If you're not yet clear on the thread profile you're going to model, start with Modelling threads; and if you want to understand why the layer plane governs all of this, go on to How FDM shapes your design.