PET bottle thread sizes: PCO-1881, PCO-1810 and the 38 mm neck, and how to print a cap that fits

17 September 2026

basicsgeometry

Nearly every soda and water bottle sold today has a PCO-1881 neck: 24.94 mm across the thread, 2.7 mm pitch, a single start, about 11 mm of thread height. Older and some imported bottles carry PCO-1810, the same diameter with a 3.18 mm pitch and a taller 14.5 mm thread. Juice and milk bottles use the wide 38 mm neck: 37.2 mm across, 4 mm pitch, three starts. Those three finishes cover the recycling bin, and they are the presets in the PET cap generator.

Why the neck is standardized at all

Bottles are blown in one factory and capped in another, sometimes on another continent, so the neck finish was standardized decades ago by the closure industry. PCO stands for plastic closure only, and the number is a specification, not a size. 1810 is the older, taller finish. 1881 came around 2009 and shortened the neck to save about a gram of PET per bottle, which at billions of bottles a year was worth retooling every capping line on the planet. For a 3D printer that history is a gift. A cap that fits one Coke bottle fits all of them.

PCO-1881, PCO-1810 and 38 mm thread dimensions, side by side

• PCO-1881: 24.94 mm thread diameter, 2.7 mm pitch, 1 start, 11 mm of engagement. Current soda, water and most sparkling bottles.
• PCO-1810: 24.94 mm, 3.18 mm pitch, 1 start, 14.5 mm of engagement. Older bottles, some imports, many 2 liter bottles from smaller fillers.
• 38 mm: 37.2 mm, 4 mm pitch, 3 starts, 10 mm of engagement. Juice, milk, kefir, some sports drinks.

Telling 1881 from 1810 by eye: the 1810 neck has a longer threaded section above the support ring, about 3 mm more, and its threads are spaced a touch wider. If a spare cap from a new bottle spins on and never seats, you have a 1810 neck.

The 38 mm finish is the interesting one for printing. Three starts means three separate helices, so the cap seats in a third of a turn. The tooth is 1.6 mm deep, generous for a 0.4 mm nozzle, and the pitch of 4 mm gives twenty 0.2 mm layers per turn. It prints better than the soda thread, not worse.

Clearance: the number everybody copies wrong

A printed cap on a printed jar wants about 0.35 mm of radial clearance, because both halves carry the same errors and they partly cancel. A printed cap on a molded neck is a different job. The neck is perfect, your print is not, and all the slop has to live on your side.

My first caps used the jar number, 0.35 mm. They started fine and jammed on the second turn, every one of them, on every brand I had in the kitchen. At 0.5 mm they screw down and grip, and 0.5 is now the default. The generator lets you go from 0.3 to 1.0 mm, and the note under the download button says what I learned the slow way: necks vary by brand by a tenth or two, so if a cap runs tight on your bottle, regenerate with 0.1 mm more instead of sanding.

Printing a cap that lasts

PETG, not PLA. The cap has to flex a little onto the neck's sealing lip, and PLA cracks at the first hard twist; PETG springs. Print it top down, as the generator exports it, with the threads facing up so every turn of the helix is a stack of rings with no overhang. 0.16 mm layers make the 2.7 mm pitch noticeably smoother than 0.2. The wall is 2 mm, the top 2.4 mm, and there is an optional knurl 0.6 mm deep that makes a wet cap easier to open.

For a cap that has to hold liquid on its side, add the seal: a washer printed in TPU that sits under the top and presses on the neck lip. Without it a printed cap is dry-tight, which is enough for a parts scoop or a bird feeder and not enough for a bottle in a bag.

Three things people make with it

A spare cap, obviously; that is why the generator exists. A cap with an eyelet, so a bottle hangs from a carabiner as a feeder or a garage scoop. And the coupler: two sockets back to back that screw two bottles into one container. Both sockets are right-hand threads, which surprised me until I turned one over in my hand: flipping a helix does not change its handedness.

The library has the nine I print most, with the dimensions already set: the PCO-1881 spare cap, the PCO-1810 version, the 38 mm juice cap, the bottle coupler and the sealed cap with a TPU washer. All of them are under bottle caps.

What I have not done is the tamper band. A real cap tears off and leaves its ring on the neck, and my caps simply stop at the end of the thread and leave the ring where it is. A skirt that clips down over the ring, so the printed cap looks like it came with the bottle, is the next thing I want to try.