Astralpool O-Ring 78104 - Pump Diffuser, 65mm Union Tail & Chlorinator Cell (BX/CX/TX/E/Viron XT)

Astralpool O-Ring 78104 - Pump Diffuser, 65mm Union Tail & Chlorinator Cell

This is the genuine Astralpool o-ring, part number 78104. One o-ring does three jobs across the Astral / Hurlcon range: it is the pump diffuser o-ring, the 65mm union tail o-ring, and the chlorinator / salt cell body o-ring. Despite being sold as a “65mm o-ring” it is not 65mm in diameter — it measures approximately 95mm outside diameter with a 5.2mm cross section. The 65mm refers to the union it seals, not the ring.

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

What This O-Ring Does — Three Roles, One Part

If any listing or parts diagram points you at 78104 for any of the following, this is the correct part:

• Pump diffuser o-ring — seals the diffuser to the volute on BX, CX, TX, E-Series, P280, P300, P320, P600 and Viron XT (P320 / P520) pumps. It seals both the small diffuser 40071A and the large diffuser 40071B.
• 65mm union tail o-ring — seals the barrel union tails on BX, XP, P300 and P600 pumps.
• Chlorinator / salt cell o-ring — the cell body / cell nut o-ring for VX, Viron, eQuilibrium and Filtrite chlorinators, Rolachem mixing cells, and older probe bodies.

The 95mm Rule (rev. ORING-1.0)

The single most common mistake is ordering by the “65mm” name and expecting a 65mm ring. Confirm by measurement, not by name:

1. Lay the old o-ring flat and measure across the outside edge to outside edge. The 78104 is approximately 95mm outside diameter.
2. Measure the thickness of the rubber (cross section). The 78104 is approximately 5.2mm.
3. If it measures ~95mm OD, this is your part whether it came off a diffuser, a union tail, or a chlorinator cell. If it is a smaller union o-ring, you likely need the 50mm union tail o-ring 78303 instead.

Worked Example

A Viron XT P320 stripped for a mechanical seal replacement had a flattened, glazed o-ring sitting between the diffuser and the volute. Measured flat it was 94.8mm across the outside and 5.1mm thick — within tolerance of the 78104 figures — so the same o-ring that seals the owner’s VX chlorinator cell was the correct diffuser seal. It was replaced at the same time as the diffuser and mechanical seal, since a hardened diffuser o-ring lets water bypass the impeller and drops flow even with a good impeller fitted.

Confirmed Compatibility

Astral / Hurlcon list 78104 across:

• Pump diffusers — BX, CX, TX, E-Series, P280, P300, P320, P600, Viron XT P320 & P520
• Union tails — BX, XP, P300, P600 (65mm barrel union)
• Chlorinator / salt cells — VX6, VX7, VX9, VX11, VX13, E25, E35, V18, V25, V35, V45, eQ18, eQ25, eQ35, eQ45, and Viron / eQuilibrium / Filtrite cell bodies
• Rolachem mixing cells and older probe bodies

Related Parts

• 40071A — small diffuser, CTX 180-400 / BX / E / Viron XT P320 (buy here)
• 40071B — large diffuser, CTX 500/610 / Viron XT P520 / BX large (buy here)
• 78303 — 50mm union tail o-ring, CTX / CX / TX / E-Series (buy here)
• 75508 / PS3868 — mechanical seal, BX / TX / CX / E / CTX (buy here)

When to Replace

Replace the o-ring whenever it is flattened, glazed, cracked, or has taken a permanent set, and any time the diffuser, union or cell it seals is opened up. An o-ring is cheap; a leak past a hardened one is not. On a diffuser it should be replaced with the diffuser and mechanical seal; on a chlorinator it should be replaced whenever the cell is opened for cleaning or cell replacement.

Frequently Asked Questions

Is this really 65mm?
No. It is named for the 65mm union it seals, but the o-ring itself is approximately 95mm outside diameter with a 5.2mm cross section. Measure the old ring flat to confirm.

Is the diffuser o-ring the same as the chlorinator cell o-ring?
Yes. Astral / Hurlcon use the same 78104 o-ring for the pump diffuser, the 65mm union tail, and the chlorinator / salt cell body. One part covers all three.

Does this fit the Viron XT diffuser?
Yes. 78104 is the diffuser o-ring for the Viron XT P320 and P520, and appears in the Viron XT MkII parts diagram. It seals both the small 40071A and large 40071B diffusers.

Which pumps use this as the diffuser o-ring?
BX, CX, TX, E-Series, P280, P300, P320, P600 and Viron XT P320 / P520.

My union o-ring is smaller than 95mm — what do I need?
You likely have the 50mm union, which takes o-ring 78303. Measure the old ring flat to confirm before ordering.

Is this a genuine Astralpool part?
Yes. We are an authorised Astralpool and Fluidra dealer and warranty agent, and this is the genuine 78104 o-ring.

Manufacturer Catalogues, Diagrams & Part Numbers

Need exploded views, spare part numbers, product numbers or diagrams? See the manufacturer catalogue: Astralpool Catalogue 2026

Before ordering: confirm this part is correct for your equipment. Use the catalogue above, seek professional assistance, or contact us with photos of your existing part, model details, serial numbers and any labels. As with all products, this must be diagnosed and fitted by a qualified and experienced person.

Common search terms: Astralpool 78104, 78104 diffuser o-ring, Hurlcon diffuser o-ring, Viron XT diffuser o-ring, 65mm tail union o-ring, VX chlorinator cell o-ring, Astral cell o-ring 95mm, BX P600 union o-ring, WWDG SpaPro.