What Is a Bonded Seal?
A bonded seal is two parts working as one: the metal ring takes the clamping load and the rubber does the sealing. The rubber stands slightly proud of the metal before tightening. As the fitting closes, the rubber is squeezed until the ring is clamped between the fitting shoulder and the port face, so the thickness of the ring sets how far the rubber can be compressed. Line pressure then pushes the rubber outward against the ring, which stops it extruding and presses it harder onto both faces.
No groove is needed, only two flat faces. That is why a bonded seal suits ports and fittings that were never cut for an O-ring.
BSP Bonded Seal and Dowty Washer Size Chart: G1/8 to G2
A bonded seal is named after the thread it fits, so a G1 seal goes on a G1 thread and its inside diameter is a little larger than the thread. The chart gives the standard washer series in millimetres, with the thread beside it so you can check a part with calipers.
| Thread | TPI | Thread OD | Seal ID | Seal OD | Thickness |
|---|---|---|---|---|---|
| G1/8 | 28 | 9.73 | 10.37 | 15.88 | 2.00 |
| G1/4 | 19 | 13.16 | 13.74 | 20.57 | 2.00 |
| G3/8 | 19 | 16.66 | 17.28 | 23.80 | 2.00 |
| G1/2 | 14 | 20.96 | 21.54 | 28.58 | 2.34 |
| G3/4 | 14 | 26.44 | 27.05 | 34.93 | 2.34 |
| G1 | 11 | 33.25 | 33.89 | 42.80 | 2.34 or 3.25 |
| G1 1/4 | 11 | 41.91 | 42.93 | 52.38 | 3.25 |
| G1 1/2 | 11 | 47.80 | 48.44 | 58.60 | 3.25 |
| G2 | 11 | 59.61 | 60.58 | 73.03 | 3.25 |
Within this series, seal ID and OD agree closely between published catalogues. Thickness varies more: a G1 seal is listed at 2.34 mm and at 3.25 mm, and a G1/2 at 2.34 mm and 2.50 mm. It only matters when the height of the joint is tight, so tell us if yours is. The dimensions of your seal are confirmed with the quote.
Self-Centering Bonded Seals
A self-centering bonded seal, also spelled self-centring, has a thin rubber lip reaching inward that grips the thread and holds the washer central while the fitting goes in. Choose it when the port faces sideways or downward, or when assembly is blind and a plain washer could slip off centre and be pinched.
| Thread | Seal ID | Seal OD | Centering lip bore | Thickness |
|---|---|---|---|---|
| G1/8 | 10.37 | 15.88 | 8.26 | 2.00 |
| G1/4 | 13.74 | 20.57 | 11.18 | 2.00 |
| G3/8 | 17.28 | 23.80 | 14.76 | 2.00 |
| G1/2 | 21.54 | 28.58 | 18.24 | 2.47 |
| G3/4 | 27.05 | 34.93 | 23.83 | 2.47 |
| G1 | 33.89 | 42.80 | 29.92 | 3.40 |
| G1 1/4 | 42.93 | 52.38 | 38.45 | 3.40 |
| G1 1/2 | 48.44 | 58.60 | 44.45 | 3.40 |
| G2 | 60.58 | 73.03 | 56.26 | 3.40 |
The Thinner Series for ISO 1179 Connections
A second, thinner bonded seal series is made for pipe connections and couplings to ISO 1179, and its outside diameter is smaller than the standard series. A G1 seal here is 39.50 mm across against 42.80 mm, so the two are not interchangeable where the seat is a recess or a narrow spotface. Measure the outside diameter of the old seal before you order.
| Thread | Seal ID | Seal OD | Thickness |
|---|---|---|---|
| G1/8 | 10.40 | 14.70 | 1.25 |
| G1/4 | 13.85 | 18.70 | 1.25 |
| G3/8 | 17.35 | 22.70 | 1.25 |
| G1/2 | 21.65 | 26.70 | 1.25 |
| G3/4 | 27.30 | 32.50 | 1.25 |
| G1 | 34.20 | 39.50 | 2.00 |
| G1 1/4 | 42.80 | 49.50 | 2.00 |
| G1 1/2 | 48.70 | 55.50 | 2.00 |
| G2 | 60.50 | 68.50 | 2.00 |
Why a Stainless Steel Ring and Peroxide-Cured EPDM
A stainless steel ring with peroxide-cured EPDM is the bonded seal for water, steam and coolant lines built in stainless. The everyday catalogue seal has a zinc-plated mild steel ring and nitrile rubber, which is made for hydraulic oil. On a 316 stainless fitting in wet or washed-down service, that ring becomes the first thing to corrode at the joint, and nitrile is a poor choice for hot water and steam.
EPDM is the rubber for water, steam and glycol, and the way it is cured decides how it behaves when hot. Peroxide curing links the rubber with carbon-to-carbon bonds where sulfur curing uses sulfur bonds, and those carbon bonds stand up better to heat. A peroxide-cured seal typically runs continuously to about 130 °C in hot water and steam, against about 120 °C for sulfur-cured EPDM. It costs more and has lower tear strength, which matters little here because the metal ring holds the rubber in place. The exact limit depends on the compound, so give us your temperature and medium.
| Service | EPDM | Note |
|---|---|---|
| Cold and hot water | Yes | The main use for this seal |
| Low-pressure steam | Yes | Within the temperature limit of the compound |
| Water and glycol coolant | Yes | Cooling loops and heat exchangers |
| Oil-free compressed air | Yes | Check that no compressor oil carries over |
| Dilute acids, alkalis, cleaning agents | Usually | Send the chemical and its concentration |
| Mineral hydraulic oil, diesel, petrol | No | EPDM swells. Ask for NBR or FKM rubber |
Keep mineral grease off an EPDM seal during assembly. A smear of ordinary oil or grease on the rubber can make it swell, so use water or a lubricant rated for EPDM if the fitting needs help going in.
Bonded Seal or O-Ring on a BSP to NPT Adapter
A BSP to NPT adapter has two ends that seal in two different ways, and only the BSP end needs a bonded seal or an O-ring. Take a 316 stainless adapter with a 1-1/4 NPT female end and a G1 male end. The NPT end seals on its taper with sealant. On the G1 male end, a 33.89 × 42.80 mm bonded seal slips over the thread and is clamped between the hex shoulder and the port face.
If that same adapter has a G1 female end, the seal goes on the male part that screws into it, and the end face of the adapter becomes the seat. That face has to be flat and wider than the 42.80 mm seal, which sets the minimum body diameter of the adapter.
| Point | Bonded seal | O-ring in a groove |
|---|---|---|
| Machining on the adapter | Flat shoulder only | Groove cut under the hex |
| In the joint | Adds 1.25 to 3.40 mm between the faces, by series | Metal to metal, no added height |
| Before assembly | Loose washer, or self-centering on the thread | Held in its groove |
| Good choice when | The port face is flat and the adapter has no groove | The adapter is new and the groove can be designed in |
With either seal the seat decides whether the joint holds. The shoulder must be flat, square to the thread and at least as wide as the seal, with no thread run-out or large chamfer under the rubber. We cut those faces on the adapter itself; the other sealing forms are on custom sealed metal parts, and the thread difference is explained in BSPP vs BSPT.
Catalogue Seal or a Seal Made for Your Adapter
A standard steel and nitrile bonded seal is on the shelf at any hydraulic parts dealer, and if that is what your line needs, buying one locally today is the fastest route. We are the better fit in these cases:
| What you need | Local dealer | EKINSUN |
|---|---|---|
| Steel ring and nitrile rubber, a few pieces today | Yes | Buy locally |
| Stainless steel ring with peroxide-cured EPDM, G1/8 to G2 | Rarely stocked | Made to order |
| Seal sized to an adapter made for you | No | Packed with the adapter, seat cut to suit |
| Another size or a thread outside this chart | Rarely | Quoted individually |
What to Send
Send the thread size, the quantity, the medium, the temperature and the pressure. For a replacement, add the ID, OD and thickness of the old seal, and say whether it has a self-centering lip. If the seal goes on an adapter, give both thread ends and we quote the two together within 12 hours. If the line needs a drinking-water or food-contact approval, say so at the start, because that decides which rubber compound can be used.
Bonded Seal FAQ
A Dowty seal is the same part as a bonded seal: a metal ring with a rubber seal bonded to its inner edge. The name comes from the company that made the design well known, and Dowty washer, bonded washer and U-seal all describe this one part.
A 1 inch BSP thread, written G1, takes a G1 bonded seal with an inside diameter of 33.89 mm and an outside diameter of 42.80 mm. The thread itself measures 33.25 mm over the crests, which is why the seal slips over it.
No, a bonded seal only works on parallel threads such as G (BSPP) and metric. NPT and BSPT are tapered and seal on the thread itself, so they tighten into the port without a flat shoulder closing on a washer.
A bonded seal closes with less tightening force and copes better with a lightly marked face, because the rubber does the sealing. A copper washer is solid metal, so it takes higher temperatures than any rubber but needs more torque and clean, flat faces.
No, EPDM swells in mineral oil, so an EPDM bonded seal should not be used on hydraulic oil, diesel or petrol. For those fluids ask for NBR or FKM rubber in the same ring.
Peroxide-cured EPDM is typically used from about −40 °C to about +130 °C in continuous service, including hot water and low-pressure steam. The exact limit depends on the rubber compound, so state your temperature when you ask for a quote.
Often yes, if the rubber is undamaged and the ring is not bent. On a joint that must not leak, fit a new one when the joint is remade, because it is a low-cost part next to the time spent on a leak.
Tighten it until the metal ring is clamped between the two faces, then stop at the torque given for the fitting. The ring limits how far the rubber can be squeezed, so extra torque adds load to the thread and does nothing for the seal.
Yes, bonded seals can be ordered on their own or packed with the adapter they go on. Send the thread size, quantity and medium and we quote within 12 hours.
Get Bonded Seals Quoted
State the thread size, quantity and medium. Engineers reply within 12h.
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