Standard Taper Joints Explained: 14/23, 19/26, 24/29 and How to Match Them

Short answer

The marking 24/29 means a ground cone with a large-end diameter of 24 mm and a ground length of 29 mm, tapering 1:10. Two joints of the same size are interchangeable. Two joints of different sizes are not, and need a reducing adapter.

What the two numbers describe

Standard taper (ST, also written NS or "ground joint") defines three things at once: the taper angle of 1:10, the diameter at the wide end in millimetres, and the length of the ground surface in millimetres.

• 14/23 - small joints used on 25-250 ml flasks, thermometers and small adapters.
• 19/26 - the middle size for 250-1000 ml vessels and general apparatus.
• 24/29 - large general-purpose joints, common on 500-2000 ml flasks and separatory funnels.
• 29/32, 34/35, 45/40 - large joints on reactors, columns and high-flow apparatus.

Because the taper is fixed, the same size always seals; the length differs only so that joints of different series do not half-engage by accident.

Sizes in practice

How to match a joint to your existing glassware

1. Read the marking engraved on the joint itself, not the catalogue page of the vessel it came with.
2. Measure the wide end with a caliper if the marking is worn: 24 mm across the top of the cone is a 24 series, not a 19.
3. Check the ground length too - a 24/29 cone and a 24/40 cone share a diameter but sit at different depths, so the pair may seal loosely.
4. If the sizes differ, order a reducing adapter rather than forcing a joint: a mismatched joint leaks, seizes and eventually chips.

Grease, sleeves or clips

There are three ways to hold and seal a joint, and they are not interchangeable in use:

• PTFE sleeves and rings - no grease contamination, easy to clean, the usual choice for analytical and biological work.
• Grease - traditional, gives a good vacuum seal, but introduces a contamination risk and must be removed before the next use.
• Keck clips - mechanical retention only. Clips stop the joint from separating under pressure; they do not seal it by themselves.

For vacuum work, use a sleeve or a thin grease film plus a clip. For reflux at atmospheric pressure, a clip and a clean dry joint are usually enough.

Frequently asked questions

Are joints from different manufacturers compatible?

If both follow the standard taper definition and the same size marking, yes. The 1:10 taper is standardized, so 24/29 from one supplier seats in 24/29 from another. Fit still varies slightly at the ground surface, so check the first assembly.

Why did my joint seize?

Usually because it was assembled dry under vacuum, heated while clamped, or stored assembled. Separate joints after use, clean and dry them, and store them apart.

Can I use a 19/26 stopper in a 24/29 socket?

No. Different diameter, different taper engagement. Use the matching stopper or a reducing adapter.

Where to look in the catalogue

Matching parts are listed with their joint size on each page: adapter fittings for reducing and connecting between sizes, standard-joint glassware for cones, sockets and stoppers, and three-neck flasks or condensers where the joint size decides what fits.

Tell us which joint your existing apparatus uses through the contact form and we will confirm the matching item before you order.