The first thing that trips up plumbers moving from copper to stainless press is the size list. The Standard Press 316 stainless range comes in 15, 22, 28, 35, 42, 54, 66.7, 76.1, 88.9, 108 and 168.3mm. There's no 20mm or 25mm, and the large sizes have decimals. Here's why, and how to work with it.
Stainless press sizes are tube outside diameters
Each size is the outside diameter of the metric stainless tube the fitting is designed for: 304 or 316 tube to AS 5200.053. A 22mm stainless fitting takes 22mm OD tube. It is not a 20mm copper fitting with a different label, and the two systems are not interchangeable.
- Don't press stainless fittings onto copper tube, or copper fittings onto stainless tube. Outside diameters, wall thicknesses and press profiles all differ (stainless is M-profile, copper is V). See M-profile jaws for stainless press.
- Buy tube to the right standard. PSA's stainless press listings are the fittings, valves and supports; source the tube to AS 5200.053 in 304 or 316 to suit the job.
- 304 or 316 tube? The fittings are 316L either way. Where you're using 316 for chloride resistance (coastal, pool plant, wash-down), use 316 tube too, so the whole line has the same corrosion resistance.
Which BSP thread goes with which size
The range's own threaded adaptors pair each tube size with its usual thread. This is the quickest way to translate a stainless size into the connections you'll meet on valves, meters and pumps:
| Stainless OD | Typical BSP pairing in the range |
|---|---|
| 15mm | 1/2" |
| 22mm | 3/4" |
| 28mm | 1" |
| 35mm | 1-1/4" |
| 42mm | 1-1/2" |
| 54mm | 2" |
| 66.7mm | 2-1/2" |
| 76.1mm | 2-1/2" or 3" |
| 88.9mm | 3" |
| 108mm | 4" |
Many adaptors also come in a step up or step down from the usual pairing (for example 22mm x 1/2" or 28mm x 3/4"), which can save a separate reducing bush. See the male BSP adaptor and female BSP adaptor for the full lists, and which threaded adaptor to use.
Not every fitting comes in every size
The core fittings (couplings, equal tees, 45° and 90° bends) run the full 15mm to 168.3mm. Others stop earlier:
- Slip couplings, end caps and the 3-piece press ball valve run to 108mm.
- Barrel unions and BSP unions run to 54mm.
- Male x male 90° bends run to 54mm.
Check the specific listing before you design around a fitting at 66.7mm and up.
Reducing down: reducer or reducing tee?
Reducer (spigot x press)
The stainless press reducer has a spigot (tube end) on the large side and a press socket on the small side. The spigot goes straight into another fitting's socket, so you reduce at a tee, bend or coupling without an extra length of tube. It comes in 30 combinations, from 22 x 15mm through to 168.3 x 108mm, including big steps like 54 x 15mm.
Reducing tee
The reducing tee drops the branch size in one fitting, from 22 x 15 x 22mm up to 168.3 x 108 x 168.3mm. Where you're branching off a main to a smaller line, a reducing tee is one fitting and two fewer press joints than an equal tee plus a reducer.
Spigot-ended bends
The same spigot idea applies to the male x female and male x male bends: they plug directly into another fitting, which tightens up offsets in plant rooms and saves short tube offcuts.
Frequently Asked Questions
Why is there no 20mm stainless press fitting?
Stainless press sizes are tube outside diameters for metric stainless tube to AS 5200.053. The size that fills the role of 20mm copper is 22mm stainless, which pairs with a 3/4" BSP thread.
Can I use stainless press fittings on copper tube?
No. The tube sizes and press profiles differ. Join copper and stainless through a threaded connection, not by pressing one system's fitting onto the other's tube.
What tube do I use with 316 stainless press fittings?
304 or 316 metric stainless tube to AS 5200.053. Use 316 tube where you chose 316 for corrosion resistance.
What's the largest stainless press size?
168.3mm for couplings, tees, bends and flange adaptors. Some fittings, such as slip couplings, end caps and the press ball valve, stop at 108mm.