Balboa 74mm OD "50mm" Spa Heater Gasket O-Ring Seal - Pair - Gecko / AEware / HydroQuip

Balboa 74mm "50mm" Spa Heater Gasket O-Ring — Pair

A pair of ribbed heater union gaskets for every Balboa 2" flo-thru heater tube, plus most Gecko/AEware, HydroQuip, Aquatemp, RMF and Spa Builders packs that use the same standard 2" heater union. If the old gasket measures 74mm across the outside, this is the correct seal. Colour varies between black, white and clear — the dimensions do not.

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

Will this fit my spa? The 2-Inch Union Rule

Fitment is decided by the heater tube, not the control pack. Ignore the badge on the box and check the union:

1. Switch the spa off at the isolator.
2. Find the heater tube — the stainless cylinder the water flows through, with a union nut at each end.
3. Undo one union nut and slide it back. The old gasket sits inside the union face.
4. Measure its outside diameter. 74mm means this product. 58mm means you need the 58mm "40mm" pair.

Worked example: a 2011 SpaPro X2 running a Balboa VS501Z pack with a 55496-01 heater tube. The union nut measures 63mm across the internal thread crest, the old gasket has compression-set to 73.6mm OD after eight summers, and the raised rib has flattened from 6mm to roughly 4.5mm. That spa takes this 74mm pair. The same owner's WWDG W3 swim spa next door runs the identical 2" union and takes the same gasket, which is the whole point of the rule.

74mm or 58mm? Choose in ten seconds

If your old gasket measures	Your heater is	You need
74mm outside diameter, 6mm thick	2" flo-thru, typically 15" long	This 74mm "50mm" pair
58mm outside diameter, 4mm thick	1.5", typically 10" long	The 58mm "40mm" pair

If you cannot get the old one out to measure, measure across the face of the union nut where the gasket seats instead. Do not guess from the pack model alone — Balboa fitted both heater sizes across overlapping years.

The exception: GS100, GS200 and VS100

These compact Balboa packs ship with a 1.5" x 10" heater rather than the 2" flo-thru, so they take the smaller 58mm OD "40mm" gasket. Every other Balboa control system listed below drives a 2" heater and takes this gasket.

Balboa control system compatibility

Grouped by family. All of the following use a 2" flo-thru heater tube.

• BP Series: BP100, BP200, BP500, BP501, BP600, BP601, BP1500, BP1600, BP2000, BP2100, BP2500, BP2600, BP6013, BP7, BP8, Revolution and Colossus — including the suffix variants BP501G1/G2/G3, BP501MS, BP501UX, BP601G1, BP601NJ, BP6013G1/G2/G3, BP2000G1, BP2100G0/G1, BP20P4BC, BP21P4BC, BP26P3BC and BP200UX.
• VS Series (Value Series): VS300, VS500, VS500Z, VS501Z, VS501SZ, VS501Z3K, VS503SZ, VS504SZ, VS510SZ, VS510DZ, VS511Z, VS511SZ, VS513Z, VS515Z, VS515SZ, VS520Z, VS520SZ, VS520DZ, VS523DZ, VS525Z. VS100 excluded — see above.
• GS Series (Global Series): GS500Z, GS501, GS501Z, GS501SZ, GS510SZ, GS510DZ, GS511DZ, GS520DZ, GS523DZ, GS504DZ. GS100 and GS200 excluded — see above.
• Signature / SCS era (very common in Australia): Sig10, Sig10L, SCS 500, SCS 501, SCS Spas packs, and the Sig100 (DU-SIG100-DCAH) and Sig200 Balboa Instruments systems. These are found on older Signature, Cyclone, Designer, Leisurite and other Spa Industries spas, and share the 3.0kW M7, 3.0kW non-M7 and 2.0kW (58145) heater assemblies — all 2" flo-thru. Sig10 is no longer supported and is normally retrofitted with a GS501z, which uses the same gasket.
• GL Series (Global Elite): GL2000, GL2000M3, GL8000.
• EL Series: EL1000, EL2000, EL2001, EL5000, EL8000.
• Legacy and metal-box Balboa: Value System, LE Series, Value/LE 1000, Lite Leader (CAT100, CAT200, CAT300), Lite Duplex, Duplex Digital, Super Duplex, Deluxe Digital, Generic Series, and pre-1997 packs with externally mounted heaters.
• Heater sensor generations: M2, M3 (Mach 3) and M7 (Mach 7) heater tubes all use the same 2" union and the same gasket. Board designations such as GL2000M3 refer to the sensor generation, not a different union. M7 replaced the pressure switch with dual digital sensors; it did not change the plumbing.

Other brands using the same 2" heater union

• Gecko / AEware: in.ye, in.yj, in.xe, in.xm, in.xm2, in.xl, in.k, SSPA, MSPA, Microspa and most Gecko AEware 2" heater tubes.
• SpaPro and WWDG control systems: all models, including SpaPro X1, SpaPro X2 and SpaPro X3, and WWDG W1, W3 and Wx systems. The SpaPro and WWDG packs use the standard 2" union, so this is the correct gasket for both.
• Others: HydroQuip, Onga, Pentair, HydroAir, Aquatemp, RMF, Spa Builders, United Spa Controls, and Spa-Quip (SpaQuip) flo-thru conversions, plus the many OEM re-badged packs built on these platforms.

Spas Direct, Sapphire, Alpine, Oasis, Vortex, Sunrans and the majority of Chinese-import spas sold in Australia run one of the platforms above.

Why the name never matches the measurement

Pool and spa plumbing uses nominal sizing: the size is a name, not a dimension. That is why a "50mm" gasket measures 74mm and a "40mm" gasket measures 58mm. As a reference across spa and pool systems:

• 50mm / 2" pipe = approx. 60-61mm outside diameter
• 40mm / 1.5" pipe = approx. 48mm outside diameter
• 25mm / 1" pipe = approx. 33-34mm outside diameter

The heater union gasket seals on the union face, which sits outside the pipe bore, so it is always larger again than the pipe OD. Measure the old gasket, never the pipe.

Measurements

• Outside diameter: 74mm (approx.)
• Inside diameter: 53mm
• Total thickness: 6mm
• Raised rib, square-edge side, centre-to-centre: 63mm
• Supplied as a matched pair — one for each end of the heater tube

Symptoms of a failed heater gasket

A heater union rarely fails as an obvious leak. Watch for a damp patch or mineral staining directly under the heater tube, white crusting on the union nut, corrosion creeping onto the heater terminals or element studs, or an RCD that trips intermittently once the heater has been running. Any of those means the gasket has taken a compression set and is weeping under thermal cycling.

When to replace

Replace both gaskets as a pair every few years, and always after a chemical imbalance event or any time the heater tube is removed. Ribbed gaskets lose rib height with heat cycling even when they look intact, so replacing one and reusing the other is a false economy. Do not over-tighten the union nut — these seal on the rib, so hand-tight plus about a quarter turn is enough. Over-tightening splits the union collar and turns a $11 job into a heater tube.

Replaces / may be stamped as

711-4031, 21619, B-21619, B21619, RGASHEA50M, PL-10170.

Fits these Balboa heater tube part numbers

All standard 15" x 2" Balboa M7 and non-M7 heaters, including: 26-58083-K, 26-58118-K, 50096, 50139, 53475, 53475-01, 53883, 53883-01, 53883-02, 53883-03, 53883-04, 53885, 53885-01, 53885-02, 53885-03, 53885-04, 53996, 54282, 54282-01, 54282-02, 54282-03, 54282-04, 54297, 54297-01, 54297-02, 54297-03, 54305, 54305-01, 54305-02, 54346, 54346-01, 54406, 54476, 54476-01, 54479, 54479-01, 54479-02, 54501, 54501-01, 54503, 54503-01, 54699, 54705, 54850, 54850-01, 55085, 55085-01, 55087, 55087-01, 55496, 55496-01, 55565, 55595, 55595-01, 55614, 55626, 55866, 55866-01, 55890, 56002, 56002-01, 56002-02, 56002-03, 56002-04, 56004, 56004-01, 56012, 56012-01, 56012-02, 56220, 56282, 56290, 56290-01, 56911, 58053, 58061, 58061-TUV, 58063, 58083, 58107, 58118, 58118KIT1, 58122, 58141, 58143, 58145, 58215, 58300 Series, 58301, 58306, 58433, 58450, 58454.

Also fits the Balboa Auxiliary Slave Heater and SCS 501 assemblies. Related parts: M7 3kW heater element, non-M7 element and tube assembly, and the SCS 501 / VS 500 / Sig10 spare parts index.

Questions we get asked

Does it matter if the gasket is black, white or clear?
No. Balboa and Gecko have used all three. Colour reflects the moulding batch, not the specification.

Why does a "50mm" gasket measure 74mm?
50mm is the nominal pipe name, not a measurement. A 50mm pipe is about 60-61mm OD, and the union face the gasket seals against is larger again. Measure the old gasket, not the pipe.

Which side faces the heater tube?
The raised rib faces the heater tube flange; the flat side sits against the union tailpiece. Fitted backwards it will usually still seal, then weep once it heat-cycles.

Do I need two?
Yes. A heater tube has a union at each end, and both should be replaced together. That is why this is sold as a pair.

My pack is Sig10 or an SCS 501. Will this fit?
Yes. The Sig10, Sig10L, SCS 500, SCS 501, GS501 and VS500 all use the 2" flo-thru heater and take this gasket, as does the GS501z that replaces them.

My spa is a rebadged brand, not a Balboa. Will it fit?
Almost certainly, if the heater has 2" unions. Most Australian spa brands buy a Balboa, Gecko or Gecko-pattern pack and put their own badge on the box. Measure the old gasket at 74mm and order with confidence.

Can I reuse the old gasket if it looks fine?
Not recommended. A gasket heat-cycled for years has lost rib height even when it looks intact, and a slow union weep will quietly corrode the heater terminals and the pack sitting under it.

Do I need to drain the spa to change these?
Usually only below the heater line. Isolate power first, close any shut-off valves, and have a towel ready — there will be water in the heater tube itself.

Common search terms

o-ring, oring, Balboa heater gasket, 50mm spa heater seal, 2 inch heater union seal, Gecko heater gasket, AEware heater gasket, spa heater o-ring, 74mm heater gasket, Balboa 21619, 711-4031, spa heater union washer, Sig10 heater gasket, SCS 501 heater seal, SpaPro heater gasket, WWDG heater gasket.