BSP flat gasket sizes —
measure the face,
not just the thread
A 3/4 inch BSP union and a 1" BSP union are obviously different, but even two 3/4 inch BSP unions can use different flat gaskets. The sealing size comes from the flat face recess — the step where the gasket sits — so the final check is always OD, ID and thickness at the actual fitting.
What BSP size actually tells you
BSP — British Standard Pipe — is a thread standard. The nominal size designation (1/2 inch, 3/4 inch, 1" etc.) comes from the pipe size the fitting was made for. It is not the thread outside diameter, and it is not the size of the flat gasket.
On a flat face union, pump connection or compression fitting, the gasket sits in a machined recess on the fitting face. The OD and ID of that recess are set by the fitting design. That is why two fittings with the same BSP thread can still ask for slightly different gaskets.
Use BSP size as a sanity check, not as the final order line. The table below helps you see the normal range and spot obvious mistakes. The order specification should come first from verified equipment data or a confirmed part reference, then from the clean fitting geometry. A removed gasket is a useful OD/ID cross-check, but service compression makes its thickness unreliable.
Typical flat gasket dimensions — BSP flat face connections
These are common market ranges for flat-face union patterns. They are not universal BSP gasket standards.
| BSP size | Thread OD (approx.) | Typical gasket OD × ID | Common applications |
|---|---|---|---|
| 3/8" | 16.66 mm | 16–18 × 10–11 mm | Small instrumentation connections, gauge ports, small valve connections |
| 1/2 inchCOMMON | 20.96 mm | 20–22 × 13–15 mm | Domestic heating, solar thermal, filling loop connections, small appliance unions |
| 3/4 inchCOMMON | 26.44 mm | 26–28 × 18–20 mm | Central heating pump unions, boiler connections, valve connections, heat pump connections |
| 1"COMMON | 33.25 mm | 33–36 × 24–26 mm | Larger pump unions, heating manifold connections, buffer tank connections |
| 1-1/4 inch | 41.91 mm | 42–45 × 32–34 mm | Larger heating system connections, commercial pump stations, solar primary circuits |
| 1-1/2 inch | 47.80 mm | 48–52 × 38–40 mm | Commercial heating, expansion vessel connections, industrial flat face unions |
| 2" | 59.61 mm | 60–65 × 48–50 mm | Larger commercial and light industrial connections, boiler primary circuits |
| 2-1/2 inch | 75.18 mm | 75–80 × 62–65 mm | Industrial flat face unions, larger process connections, commercial plant |
All dimensions are indicative ranges for flat face union connections in common UK and EU market fittings. Actual recess dimensions vary by fitting manufacturer, product range and country of origin. Treat these figures as a cross-check, not a specification. Measure the actual fitting or removed gasket before ordering.
What to measure before ordering
Measure the removed gasket — if it is still intact
Use the removed gasket to cross-check profile, OD and ID before discarding it. Its measured thickness may be below nominal after service, so confirm thickness from equipment data, a part record or the verified recess, stand-off and required compression.
If the gasket is missing or too damaged — measure the fitting face recess
With the union disconnected, look at the flat face. The gasket recess is the shallow step ring machined into the face. Measure the outer diameter of the recess (= gasket OD) and the inner diameter (= gasket ID). Recess depth is only one geometry input; confirm thickness from the equipment specification or the joint's stand-off and required compression rather than copying a compressed old gasket. Most domestic BSP union gaskets are commonly 1.5 mm or 2 mm.
Cross-check against the BSP reference
Check that your measured dimensions sit in the expected range for the BSP size marked on the fitting. If the numbers are far outside the table, stop and re-check the face you measured. The fitting may be a different pattern, a metric connection, or a thread form where the seal is not made by a flat gasket.
Confirm the thickness
Thickness matters. A gasket that is too thick for the recess will prevent the union from fully closing; one that is too thin may not be fully compressed when the fitting is correctly assembled. Standard domestic BSP union gaskets are typically 1.5 mm or 2 mm. Measure the old gasket or the recess depth to confirm.
Why the same BSP size gives different gasket dimensions
This variation is normal. It is not usually a catalogue error or a bad gasket supplier. The BSP thread can be standard while the face detail is still manufacturer-specific.
Several things affect the gasket dimension inside the same nominal BSP size:
- Fitting manufacturer and product family: different manufacturers machine the face recess to different dimensions, including imported fittings built to different house patterns. A 3/4 inch BSP union from one supplier and a 3/4 inch BSP union from another may present different sealing face recesses.
- Product range: a heavy-duty or high-pressure union in the same BSP size will typically have a different recess geometry than a standard-weight domestic union. The face design is a function of the product specification, not just the thread size.
- Age of installation: older fittings may be to obsolete standards with face dimensions that differ from current production. This is particularly relevant when replacing gaskets on older installations where the original fitting is not identifiable.
Common mistakes when ordering by BSP size alone
When the problem is not size — it is material, thickness or face condition. If a correctly sized gasket continues to leak, look at the medium, temperature, seating stress and the condition of the two faces. A different OD does not fix a scratched face, a loose union, a hardened gasket or the wrong material grade.
Quick answers
Measure the joint, not the label.
The BSP size on the fitting gets you into the right family. It does not tell you the gasket specification. For a flat face joint, the useful numbers are the recess OD, recess ID and gasket thickness. Use the table as a check, then order to the measured dimensions.