Astralpool / Zodiac Air Bleed Plug D-Thread & O-Ring
Astralpool / Zodiac Air Bleed Plug D-Thread & O-Ring is temporarily out of stock and will ship as soon as it's available.
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Astralpool / Zodiac Air Bleed Plug D-Thread & O-Ring
Astralpool / Zodiac Air Bleed Plug D-Thread & O-Ring is temporarily out of stock and will ship as soon as it's available.
Astralpool / Zodiac Cartridge Filter Air Bleed Plug & O-Ring — D-Thread — 78705 / 78103
Genuine Astralpool air bleed plug (air release / bleed screw) complete with its O-ring, part numbers 78705 / 7870510 (plug) and 78103 (O-ring). It threads into the top of the filter to release trapped air on start-up. The twin-start "D" thread is designed so the plug will not pop out under pressure. Fits the entire Astralpool / Hurlcon cartridge filter range and most other filters using the same common thread.
- Physically Stocked, unlike most "drop-shipping" websites.
- Shipped from VIC, NSW, WA, QLD
The 1/4-inch BSP Thread Check
This is a common-thread part, so it fits by thread, not by filter model:
- Look at the threaded air-bleed port on top of your filter.
- If it is the common 1/4" BSP (~13mm) thread, this plug and O-ring fit — whatever brand the filter is.
The Zodiac ZXC is the Zodiac-badged Astralpool ZX, so ZXC filters use this plug too.
Fits Across The Astralpool / Hurlcon Range
- Astralpool / Hurlcon ZX & Zodiac ZXC / ZCF cartridge filters (ZX50 to ZX310)
- Astralpool / Hurlcon QX, CL, CF, UF, QL cartridge filters
- Astral IX / IX50 in-line leaf canisters
- WWDG and many other brands using the same 1/4" BSP air-bleed thread
Because it is a common-thread part, it also suits many filters outside the Astralpool range — confirm your port is the 1/4" BSP (~13mm) thread and it will fit.
Fitting Tip
Use a small amount of silicone lubricant on the thread to stop the plastic binding. Hand-tighten only.
Also Need The Lid Or Tank Seal?
ZX / ZXC lid assembly — SP7820502 · ZX / ZXC tank O-ring — 78110 · Full ZX / ZXC range & spare parts
Part Numbers & Search References
Plug: 78705 / 7870510 · O-ring: 78103 · Old numbers: 78205, 78207 · air bleed plug · air release valve · bleed screw · D-thread air bleed · ZX filter air bleed · Zodiac ZXC air release · 1/4 BSP cartridge filter bleed plug.
Frequently Asked Questions
Will this fit my filter?
If the air-bleed port on your filter is the common 1/4" BSP (~13mm) thread, yes. It fits the whole Astralpool / Hurlcon range and most other brands using that thread.
What is the D-thread?
A twin-start thread that stops the plug being blown out under pressure, while still letting you open it by hand to bleed air.
Does it come with the O-ring?
Yes. The plug is supplied with its sealing O-ring (part 78103).
My filter is a Zodiac ZXC. Is this the right plug?
Yes. The Zodiac ZXC is the same filter as the Astralpool ZX, just Zodiac-badged, so this plug fits.
Manufacturer Catalogues, Diagrams & Part Numbers
Need exploded views, spare part numbers, product numbers, diagrams, or more product information? See the manufacturer catalogues: Astralpool Catalogue 2026 | Zodiac Catalogue 2026
As with all products, these must be diagnosed and fitted by a qualified and experienced person.
In plumbing, named sizes are not the actual measurements. A pipe called “50mm” or “2"” does not measure 50 mm across. Instead, it follows nominal sizing, an internationally-standardised naming system.
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 |
These are nominal sizes and are designed to fit matching fittings — not to measure exactly 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 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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