Not a direct fit. Joining John Guest 5/16" push-fit to Duotight 8mm push-in needs a 8mm / 5/16 straight joiner. Sealing method, the real-world trap, and the exact part to buy.
Updated 05 September 2026.
Connect John Guest 5/16" push-fit to Duotight 8mm push-in
How to join 5/16" (8 mm) OD and 8 mm (5/16") OD, and what stops it leaking.
Not a direct fit. Joining John Guest 5/16" push-fit to Duotight 8mm push-in needs a 8mm / 5/16 straight joiner.
The two ends
- End A
- John Guest 5/16" push-fit (5/16" (8 mm) OD)
- End B
- Duotight 8mm push-in (8 mm (5/16") OD)
- A seals on
- single O-ring plus stainless collet grab-ring
- B seals on
- double EPDM O-ring on the tube outer wall
- Part needed
- 8mm / 5/16 straight joiner
The part to buy
Duotight 8mm and John Guest 5/16 both nominally take the same 8mm OD line, so the O-rings seal across both and brewers mix them at low pressure. Where you cross from one brand to the other, an 8mm straight joiner keeps the run clean.
5/16 is 7.94mm and 8mm is 8.00mm, so the join is fine on a short gas jumper but can weep on a long warm beer run. Standardise a run on one system rather than mixing brands joint after joint.
Related connections
Questions and answers
Connect John Guest 5/16" push-fit to Duotight 8mm push-in?
Not a direct fit. Joining John Guest 5/16" push-fit to Duotight 8mm push-in needs a 8mm / 5/16 straight joiner.