Waterco Trimline MkIII Cartridge Filter Half Union - 40/50mm - 122257BLK
Waterco Trimline MkIII Cartridge Filter Half Union - 40/50mm - 122257BLK is temporarily out of stock and will ship as soon as it's available.
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Don't Guess - Ask Us!
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Waterco Trimline MkIII Cartridge Filter Half Union - 40/50mm - 1222...
Waterco Trimline MkIII Cartridge Filter Half Union - 40/50mm - 122257BLK is temporarily out of stock and will ship as soon as it's available.
Waterco Trimline MkIII Half Union - 40/50mm
Genuine Waterco half union, part number 122257BLK, in black. This is the barrel union that connects the filter inlet or outlet to your pipework, supplied as the half union assembly. It takes either 40mm or 50mm pipe, so one part covers both plumbing sizes.
This is the post-2001 MkIII union. Replace it when the union nut has cracked, when the thread is stripped, or when the union weeps and a new o-ring has not fixed it.
Specifications
| Part Number | 122257BLK |
| Manufacturer Description | 40/50mm Half Union MK3 (post-2001) |
| Pipe Size | 40mm or 50mm |
| Colour | Black |
| Brand | Waterco |
| Quantity Per Filter | 2 — one inlet, one outlet |
Fits
Waterco Trimline MkIII cartridge filters manufactured from 2001 onward — C25, C50, C75, CC50, CC75, CC100.
The C100 and CC150 use a 40mm half union, part number 122243B. Pre-2001 MkII filters use a different union again, with union o-ring 620341. If you are not certain which you have, send us a photo of the existing union before ordering.
Fitting
A filter has two unions, inlet and outlet. If one has failed the other is usually the same age, so it is worth replacing both while the system is drained down rather than doing the job twice. Hand-tighten the union nut only — these seal on the o-ring face, not on thread tension, and overtightening cracks the nut.
Related Trimline Parts
- 122243B — 40mm half union, C100 and CC150
- 620341 — Union o-ring, 40/50mm MkII
- 620342 — Half union o-ring, BS332 Viton
- 62026 — MkIII lid o-ring, 214mm
Manufacturer Manuals, Diagrams & Part Numbers
Need exploded views, spare part numbers, diagrams, or more product information? See Waterco's Trimline Cartridge Filter manual.
Important Before Ordering: Please ensure you order correctly. Confirm this product is suitable before ordering. Use the manual above, seek professional assistance, or contact us with photos of your existing part, model details, serial numbers, and any labels.
As with all products, these must be diagnosed and fitted by a qualified and experienced person.
To the layperson, plumbing sizes and their names can be confusing due to the nominal sizes and naming conventions used.
This is not something we made up. Plumbers and manufacturers have used nominal (meaning ‘by name only’) sizing for decades.
Simply put, in plumbing, named sizes are often not the actual measurements of the pipe, fitting, or part. A PVC pipe called “50mm” or “2"” does not measure 50 mm anywhere. Instead, it follows nominal sizing, an internationally-standardised naming system.
Almost all fittings we use in the pool & spa industry use “pressure PVC” pipe & fittings. Do not confuse with “DWV” or other pipe & fittings and their names and sizes.
Use the Outside Diameter (OD) to check your pipe against fittings. Below is a quick reference for common spa & pool pipe sizes:
| Imperial Name | Metric Name | Pipe Diameter (OD) |
|---|---|---|
| 1/2" | 15mm | ~21.2–21.5 mm |
| 3/4" | 20mm | ~26.6–26.9 mm |
| 1" | 25 mm | ~33.4–33.7 mm |
| 1.5" | 40 mm | ~48.1–48.4 mm |
| 2" | 50 mm | ~60.2–60.8 mm |
| 2.5" | 65 mm | ~75.2–75.5 mm |
To reiterate, these are nominal sizes and are designed to fit matching fittings — not to measure what their name suggests. These are based on PVC Pressure Pipe AS/NZ 1477 Dimensions1.
- º Internal diameters vary slightly by manufacturer and class.
- 1 Not all pool/spa fittings match these dimensions exactly. Some do not follow Australian or international standards. Chinese spas may use proprietary fittings.
Understanding Plumbing Sizes & Nominal Measurements
Plumbing sizes can be confusing. PVC and many other pipe systems use nominal sizes—the size is a name, not the exact measured dimension. This convention is used across most pool and spa systems and other plumbing areas so that pipes and fittings remain compatible.
Why pipe sizes don’t match the measurements
The Nominal Pipe Size (NPS) system standardises pipe dimensions so that pipes, fittings, valves, and accessories interoperate.
Plumbers work with these nominal names every day, but DIYers can be tripped up if they try to match by a tape-measure alone.
How to Measure Pipe Correctly
-
Pipe size is identified by:
- The pipe’s Outside Diameter (OD).
- The fitting’s Inside Diameter (ID) (socket opening).
- Pipe Class / Schedule (e.g., Class 9, 12, 18 or Sch 40) relates to wall thickness and therefore the Internal Diameter (ID) and pressure rating.
- The outside diameter of a fitting is not useful (wall thickness varies by manufacturer).
Hose & Tube Dimensions
Common flexible hose sizes used in pool/spa systems (approximate IDs).
| Imperial Name | Metric Name | Inside Diameter (ID) |
|---|---|---|
| 3/8" | 9–10 mm | ~9 mm |
| 3/4" | 19 mm | ~19 mm |
BSP – British Standard Pipe Thread Sizes
BSP thread names are also nominal. The thread OD will not equal the nominal name.
| BSP Thread Size Naming | Outside Diameter (mm / inch) | TPI |
|---|---|---|
| 1/16" BSP | 7.7 mm / 0.304" | 28 |
| 1/8" BSP | 9.7 mm / 0.383" | 28 |
| 1/4" BSP | 13.16 mm / 0.518" | 19 |
| 3/8" BSP | 16.66 mm / 0.656" | 19 |
| 1/2" BSP / 15 mm | 20.99 mm / 0.825" | 14 |
| 5/8" BSP | 22.99 mm / 0.902" | 14 |
| 3/4" BSP / 20 mm | 26.44 mm / 1.041" | 14 |
| 7/8" BSP | 30.20 mm / 1.189" | 14 |
| 1" BSP / 25 mm | 33.25 mm / 1.309" | 11 |
| 1-1/4" BSP | 41.91 mm / 1.650" | 11 |
| 1-1/2" BSP / 40 mm | 47.80 mm / 1.882" | 11 |
| 2" BSP / 50 mm | 59.61 mm / 2.347" | 11 |
| 2-1/4" BSP | 65.71 mm / 2.587" | 11 |
| 2-1/2" BSP | 75.18 mm / 2.960" | 11 |
| 3" BSP | 87.88 mm / 3.460" | 11 |
| 4" BSP | 113.03 mm / 4.450" | 11 |
| 5" BSP | 138.43 mm / 5.450" | 11 |
| 6" BSP | 163.83 mm / 6.450" | 11 |
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122257BLK
