Not a direct fit. Joining John Guest 5/16" push-fit to John Guest 3/8" push-fit needs a 5/16 to 3/8 push-fit reducer (John Guest). Sealing method, the real-world trap, and the exact part to buy.
Updated 05 September 2026.
Connect John Guest 5/16" push-fit to John Guest 3/8" push-fit
How to join 5/16" (8 mm) OD and 3/8" (9.5 mm) OD, and what stops it leaking.
Not a direct fit. Joining John Guest 5/16" push-fit to John Guest 3/8" push-fit needs a 5/16 to 3/8 push-fit reducer (John Guest).
The two ends
- End A
- John Guest 5/16" push-fit (5/16" (8 mm) OD)
- End B
- John Guest 3/8" push-fit (3/8" (9.5 mm) OD)
- A seals on
- single O-ring plus stainless collet grab-ring
- B seals on
- single O-ring plus stainless collet grab-ring
- Part needed
- 5/16 to 3/8 push-fit reducer (John Guest)
The part to buy
A John Guest 5/16 body takes only 5/16 tube and a 3/8 body takes only 3/8 tube, so stepping a 5/16 gas jumper up to a 3/8 beer run needs a 5/16 to 3/8 push-fit reducer, a collet on each size.
Do not shim a 5/16 tube into a 3/8 collet. The grab-ring and O-ring are sized to the exact tube wall, so an undersized tube pulls out under pressure. Use the reducer and cut each tube square.
Related connections
Questions and answers
Connect John Guest 5/16" push-fit to John Guest 3/8" push-fit?
Not a direct fit. Joining John Guest 5/16" push-fit to John Guest 3/8" push-fit needs a 5/16 to 3/8 push-fit reducer (John Guest).