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Flare, push-in or barb: which line connection should I use?
A flare seals on its cone face, a push-in fitting grips the tube OD, and a barb relies on the tube ID and a clamp, so each needs a different tubing choice.
How each connection seals
A flare fitting is the SAE 45 degree flare defined in SAE J512. The seal happens where the coned flare face meets the fitting, not on the thread, which is why you never wrap PTFE tape on a flare. MFL is the male flare and FFL is the female flare, both 1/4 inch SAE.
A push-in fitting like Duotight grips the outside diameter of the tube with a collet and seals on an internal O-ring. A barb pushes inside the tube and holds by friction plus a clamp, sealing against the tube inner wall.
Match the fitting to the tubing
Barbs are sized to tube inner diameter, so a 1/4 inch barb suits typical liquid line and a 5/16 inch barb suits gas line. Get the barb size wrong and the tube either will not go on or blows off under pressure.
Duotight push-in fittings are sized to tube outer diameter and are matched to EVABarrier double-wall tubing. An 8mm Duotight takes 5mm ID by 8mm OD EVABarrier, and a 9.5mm Duotight takes 6.35mm ID by 9.5mm OD. A John Guest 5/16 inch push-fit is a different size and does not reliably interchange with 8mm Duotight.
When to choose which
Flare is the workhorse of reconfigurable draft setups because it disconnects and reconnects cleanly with a spanner and reseals every time. Push-in is fast, tool-free and tidy, which suits people who rebuild their gas manifold often. Barbs are cheap and permanent-feeling, best where a line rarely comes apart.
Many builds mix all three: a flare at the regulator, push-in along the manifold, and a barb into the disconnect.
The tape mistake
The single most common leak is PTFE tape wrapped onto a flare thread. Because the flare seals on the face, tape on the thread does nothing useful and can hold the cone off its seat. Tape belongs on tapered pipe threads such as NPT, not on flares and not on parallel BSP faces that seal on a washer.
Get the right fitting
Open the KegFitFinder toolMatch a connection to your tubingFind MFL flare to duotight fitting on eBayaffiliateRelated
Sources: SAE J512 flare standard ยท KegLand Duotight and EVABarrier data