Mason jars: regular mouth and wide mouth

You have a preserving jar in your hand and you want to print a lid for it: a shaker, one with a handle, an insert to separate solids from liquids. Before you draw anything, you have to answer a question that looks trivial and isn't: which mouth does the jar have? Mason-type jars — the continuous-thread preserving jars so common across North America — come in two standard mouths that are not interchangeable, and a lid modelled for one won't thread onto the other, not even close. Pick the wrong mouth and no amount of flank clearance will save you: it simply won't go on.

Two mouths, two diameters

The Mason system standardised two mouth diameters over a century ago — two, and only two — and the whole two-piece-lid system depends on them: the flat sealing disc paired with the threaded ring that clamps it down. The regular mouth has a thread outside diameter of roughly 70 mm. The wide mouth has a thread outside diameter of roughly 86 mm. That is the entire difference that matters to you: 16 mm of diameter that decide which lid you model.

Both use the same kind of thread: a continuous helix with a generous pitch, meant to let a ring drop one or two turns and clamp the disc against the rim. It's not a fine precision thread; it's a lid thread, the kind you spin on with your fingers and no tool. That, as you'll see, works in your favour on FDM.

Why a jar thread is forgiving on FDM

A printed Mason lid threads on well, for two concrete reasons: the pitch is wide and it doesn't have to seal anything. With the axis vertical, the helix climbs comfortably layer by layer — there are plenty of layers per turn to form crest, flank and root — and, because the pitch is large, the helix angle is shallow and the underside flank of each turn overhangs very little. Compared with the thread on a carbonated-drink bottle cap, which is small, short-pitched and has to seal against pressure as well, a jar thread is forgiving: it does not need to seal, and there is room to spare for clearance.

Watch out for one misconception: the large diameter does not dilute contour error. A bead that runs 0.15 mm wide is 0.15 mm of interference at 70 mm, exactly as at 6 mm; the circumference doesn't spread it out. What the 70 or 86 mm do give you is a long thread, with many turns in contact, and that's exactly why it pays to be a little more generous with clearance: any point that binds adds up along the whole engaged length.

Otherwise the rules of any printed thread apply. The external thread comes out oversize — the bead pushes the contour outward — and the internal thread comes out undersize — the bead eats into the gap — so the flanks would clash if you modelled both parts to exact size. Start more generously than on a small metric thread: 0.3–0.4 mm of total flank clearance, and close it up by iteration — on a large diameter, 0.1 mm all but guarantees the thread will bite and refuse to go down. Split it as you would on any printed thread: leave the part that mimics the jar at its nominal size and open up the one that threads over it, so you have a single dimension to adjust if it binds. All of that reasoning, with its failure modes, is in Modelling threads.

Orientation is not negotiable: print the lid with the thread axis vertical and the thread pointing up. That way each turn of the helix forms on top of the previous one and the underside flank comes out acceptably clean. Lay it on its side and half the thread hangs in the air, droops, and the fit you measured is gone. And because the wide mouth has the longer thread, poor orientation ruins more millimetres of engaged thread: because it is larger, it punishes careless orientation more. Why the layer plane governs all of this is explained in How FDM shapes your design.

The seal comes from the rim, not the thread

Here's the part most people get wrong. On a Mason jar the thread doesn't seal: it only clamps. Tightness comes from the flat disc of the metal lid pressing against the mouth rim — the top lip of the glass, flat and smooth — with its sealing gasket in between. The threaded ring is only the mechanism that lowers the disc and keeps it pressed against that lip.

This completely changes how you model a functional lid. If you print a one-piece lid — a shaker, one with a handle, one with a pouring spout — the seal can't come from the plastic thread, which will never be watertight anyway. It has to come from a gasket against the rim, seated in an annular pocket on the inner face of your lid: a silicone or EPDM gasket sized to compress against that glass lip when you tighten. The thread only supplies the clamping force; the rim and the gasket supply the tightness. Design with that in mind and a printed lid will close a jar of dry goods — or even liquids, with the right gasket — without relying on the plastic thread to do a job it isn't fit for.

That principle — thread that clamps, face that seals — is the same one that governs any reusable jar-and-lid pair; it is covered in detail in Reusable jars and lids: designing the thread-and-cap pair.

From adapter to egg tray: what to print for each mouth

With the two mouths identified and the seal sorted, the range of useful parts is wide. Custom lids to replace the metal one that rusts or goes missing: plain, with a seated gasket, or with a shaker spout for spices, seeds or liquids. Lids with a handle to turn a jar into a jug. Inserts that rest on the rim without threading — an egg tray, a strainer, a solid-and-liquid separator — held between the disc and the glass lip. Storage holders that use the known mouth diameter to slot jars into a drawer or onto a wall.

And one part that's especially rewarding: the mouth adapter. Because the two diameters are fixed and known, you can make a wide-mouth lid serve on a regular-mouth jar with a double-thread ring. The regular-mouth jar has a ~70 mm external thread, so the adapter carries a regular internal thread (~70 mm) in its bore to screw onto the jar, and a wide external thread (~86 mm) on its outer surface for the wide-mouth lid to run over. Each thread is drawn at its own nominal diameter, with the flank clearance only on the internal faces, and both printed in the same vertical orientation. If you want the opposite — a regular lid on a wide jar — you flip the pair: wide internal thread inside, regular external thread outside.

Whatever the part, the order of work is always the same: identify the jar's real mouth first with the calliper, model the thread at its nominal diameter with a rounded profile, leave your flank clearance only on the internal part, orient the axis vertically, and handle the seal with a gasket against the rim. If this is the first lid thread you've modelled, start with Modelling threads and come back here with the two mouth dimensions in mind.