Spa Heater Union Split Nut Repair Fitting - 58mm ID - Balboa, Gecko, Aquatherm

Name: Spa Heater Union Split Nut Repair Fitting - 58mm ID - Balboa, Gecko, Aquatherm
Brand: Balboa
SKU: mp0121108020
Price: 16.00 AUD
Availability: InStock

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Heater Union Split Nut Repair Fitting - Balboa, Gecko, Aeware 2" heaters.

Physically Stocked, unlike most "drop-shipping" websites.
Shipped from VIC, NSW, WA, QLD

Heater union to suit most 2" Balboa, Gecko, Spa Power 2000 and other Thermcore heaters. Comes as two pieces. ~58mm internal diameter. Also replaces unions that used these heater split retainer collars, as new union has thicker shoulder in place of the retainer.

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 Dimensions^1.

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.

Quick example: A 2" / 50 mm fitting is named for the pipe it connects to, but the pipe’s actual outside diameter (OD) is approx. 60.3 mm (typically ~60.2–60.5 mm in AS/NZ pressure pipe).

Another example: a 2” / 50mm o-ring may refer to a barrel union, etc that suits 50mm pipe. Eg: a “50mm Balboa Gasket” refers to a heater gasket that measure 74mm across, but suits a Balboa barrel union made for 50mm pipe (and the 50mm pipe does not measure 50mm either!)

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).

Important: For pool/spa work, most plumbing uses Pressure Pipe (AS/NZ standards) — not DWV. Always check OD and the pipe class against your equipment specifications.

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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