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Do Duotight and John Guest push-fit fittings interchange?

Duotight and John Guest are both push-in systems, but they are sized to different outer diameters and do not reliably swap.

Both grip the tube, differently

Push-in fittings hold tubing by clamping a collet around the outer diameter and sealing on an O-ring. Duotight and John Guest both work this way, which is why people assume they mix. The catch is the exact OD each is machined for.

Duotight fittings are built around EVABarrier double-wall tubing in metric outer diameters. John Guest push-fit is built around its own imperial and metric sizes. A push-in system only seals reliably when the tube OD matches the collet and O-ring it was designed for.

Duotight and EVABarrier sizes

The common Duotight sizes are 8mm, 9.5mm and 6.5mm, each matched to a specific EVABarrier tube. An 8mm Duotight takes 5mm ID by 8mm OD EVABarrier. A 9.5mm Duotight takes 6.35mm ID by 9.5mm OD EVABarrier. The 6.5mm size suits the thinner EVABarrier used for smaller runs.

Because the fitting is chosen by OD, you order tubing and fittings together. Put 8mm tube into a 9.5mm fitting and it will leak or pull out.

Where John Guest does not line up

A John Guest 5/16 inch push-fit is close to 8mm but not the same, so it does not reliably interchange with Duotight 8mm. Sometimes a tube will push in and appear to hold, then weep or release under gas pressure, which is exactly the failure you do not want on a pressurised line.

If you are standardising a build, pick one system and stay with it, or use a proper adapter between the two rather than trusting a near-fit.

Practical rule

Keep your tubing and your fittings in one ecosystem. If your keg gear is EVABarrier and Duotight, buy Duotight throughout. Match every fitting to the tube OD printed on the reel, and reserve John Guest for the JG tubing it was designed around.

Related

Sources: KegLand Duotight and EVABarrier data · John Guest