Boiler gasket sizes —
common UK dimensions
and what fits where
This page is a sizing reference — dimensions for the most common boiler gasket positions, what fits where, and which packs cover standard service requirements.
Safety scope: This article is for identifying likely water-side sealing and pressure-system symptoms. Do not open combustion chambers, gas-side components, flues or any appliance you suspect is unsafe. Gas work, boiler safety checks and unsafe appliance decisions belong with a competent Gas Safe registered engineer.
The most important size — 3/4 inch BSP
If you are standing in front of a UK domestic boiler, 3/4 inch BSP is often the first size to check. It covers pump unions on most domestic combi and system boilers, many heat exchanger connections and a range of valve body connections in 22 mm pipework systems.
3/4 inch BSP — one of the most common boiler gasket sizes in UK domestic systems
Covers most pump union connections, many primary heat exchanger connections, 22 mm system pipework valve bodies and various boiler component connections.
The second most common size is 1/2 inch BSP — filling loops, smaller heat exchanger connections, some valve bodies and 15 mm pipework connections. Between 3/4 inch and 1/2 inch, you cover a large share of standard domestic boiler servicing.
Common sizes — dimensions and applications
3/8 inch BSP
1/2 inch BSP — frequently encountered
3/4 inch BSP — frequently encountered
1 inch BSP
Common market examples — not a BSP gasket-size standard
| BSP size | Gasket OD | Gasket ID | Thickness | Thread OD | TPI | Common in boilers |
|---|---|---|---|---|---|---|
| 3/8″ | 14.5 mm | 8–12 mm | 2 or 3 mm | 16.66 mm | 19 | Instrument fittings, small valves |
| 1/2″ | 18.5 mm | 10–14 mm | 2 or 3 mm | 20.95 mm | 14 | Filling loop, small heat exchanger, PRV |
| 3/4″ | 24 mm | 14 or 16 mm | 2 or 2.5 mm | 26.44 mm | 14 | Pump unions, heat exchanger, valves |
| 1″ | 30 mm | 21–24 mm | 2 mm | 33.25 mm | 11 | Larger pumps, system boilers, 28 mm pipe |
| 11/4″ | 39 mm | 30 mm | 2 mm | 41.91 mm | 11 | Commercial, large system connections |
| 11/2″ | 44 mm | 36 mm | 2 mm | 47.80 mm | 11 | Commercial, large pump unions |
These are examples found in parts ranges, not dimensions created by the BSP thread standard. BSP identifies the connection thread; the equipment maker and sealing recess determine gasket OD, ID and thickness. Measure the actual recess and confirm the appliance or fitting data before ordering.
What size goes where
Pump unions
Domestic combi and system boiler pump unions are most commonly 3/4 inch BSP. Older or larger output boilers may use 1 inch. Measure the union face recess — do not assume from pipe size alone.
Primary heat exchanger connections
The flow and return connections to the primary heat exchanger are typically 3/4 inch BSP on most domestic boilers. Compact units may use 1/2 inch on one or both connections.
Filling loop
The temporary filling loop connection is almost always 1/2 inch BSP. The gasket at each end is 18.5 mm OD. A common source of slow drips that get blamed on the boiler itself.
Pressure relief valve tail
PRV discharge connections are typically 1/2 inch BSP. Note: a dripping PRV is more commonly a valve seat or system pressure issue than a gasket failure — diagnose before replacing the gasket.
Expansion vessel connection
The threaded connection where the expansion vessel connects to the heating circuit is typically 3/4 inch BSP. Often overlooked as a gasket location — it sees the same thermal cycling as every other connection.
DHW (hot water) connections
The cold inlet and hot water outlet on combi boilers are commonly 1/2 inch BSP. Some larger output models use 3/4 inch. These are potable water connections — specify a food-contact compliant grade where required.
Which pack covers standard boiler servicing
For general boiler servicing and maintenance, a mixed pack covering 3/8 inch through to 3/4 inch handles the vast majority of domestic system connections. The 3/4 inch size should be the most represented in any service kit — it is the size most likely to be needed on any given call.
A standard Kinetics Line box set covers 3/8 inch through 3/4 inch in multiple thickness variants, with the 3/4 inch size represented in all three standard dimensions. The cellulose fibre with NBR binder material is appropriate for standard heating water connections within the operating range of domestic and most light commercial heating systems.
If you carry one gasket set on a service van: 3/4 inch BSP in 2 mm and 2.5 mm thickness covers most pump union and heat exchanger call-outs. Add 1/2 inch BSP for filling loops and PRV connections. Those two sizes cover the majority of standard domestic boiler gasket requirements.
Measuring when you do not know the size
When the original gasket is damaged beyond measurement, or when the OEM part number is obsolete and documentation is unavailable, measure the fitting recess directly:
- Recess OD — the inside diameter of the raised face or recess where the gasket seats. The gasket OD should be 0.5–1 mm smaller than this measurement.
- Bore ID — the through-hole diameter. The gasket ID must clear this without restricting flow.
- Recess depth — a rough guide to correct gasket thickness. A 2 mm deep recess is a rough starting check for gasket thickness — use it as an initial guide only, not the sole criterion. Fitting design, compression behaviour and manufacturer specification all affect the correct thickness choice.
Cross-reference your measurements against the table above. A fitting recess of approximately 25 mm OD corresponds to a 3/4 inch BSP connection. A recess of approximately 19–20 mm corresponds to 1/2 inch BSP.
The 3/4 inch BSP gasket covers most domestic boiler call-outs. The 1/2 inch covers most of the rest.
Carry 3/4 inch in both 2 mm and 2.5 mm variants — three standard dimensions cover most pump union and heat exchanger recesses. Add 1/2 inch in 2 mm and 3 mm for filling loops and smaller connections. Those cover the majority of standard domestic boiler service requirements. For larger systems, add 1 inch. For older continental imports or unusual fittings, measure the recess and match to the chart — the BSP size is a label, the millimetre measurement is what matters.
FAQ
What size gasket do I need for a pump union?
In many domestic boiler installations, pump unions commonly use 3/4 inch BSP connections, taking a gasket with approximately 24 mm outside diameter. ID varies by fitting — 14 mm or 16 mm are both standard. Some older or larger systems use 1 inch BSP. Always measure the actual fitting recess before ordering — fitting designs vary between manufacturers.
What is the most common boiler gasket size in the UK?
The 3/4 inch BSP gasket is the most commonly needed size in UK domestic boiler servicing. It covers most pump union connections, many heat exchanger connections and various valve body connections. The 1/2 inch BSP gasket is the second most common, covering filling loops, smaller heat exchanger connections and some valve connections.
Can I use a standard flat gasket instead of the OEM boiler gasket?
For a confirmed standard external water-side flat-face union, a non-OEM gasket may be suitable when profile, outside diameter, inside diameter, uncompressed thickness, material and service conditions all match. Internal appliance seals and proprietary joints must follow the manufacturer's service instructions or a documented equivalent; dimensions alone are not enough.