BSP Washer Sizes: Nominal BSP vs Actual Washer Dimensions
Choose BSP sealing washers by checking thread designation, inside diameter, outside diameter, thickness and material against the fitting specification.
A BSP washer size is usually associated with a nominal pipe-thread designation. It does not mean the washer’s hole has that fractional-inch diameter. A washer described for 1/4-inch BSP, for example, needs to accommodate a fitting whose male thread diameter is approximately 13.16 mm, rather than 6.35 mm.
For a replacement sealing washer, establish how the joint seals and compare the actual inside diameter, outside diameter, thickness and material. The BSP label is a useful starting point, but it does not complete the specification.
Why BSP sizes do not match their inch conversion
British Standard Pipe uses nominal pipe sizes. Converting 1/4 inch directly to 6.35 mm therefore does not identify the outside diameter of a 1/4-inch BSP thread.
The following rounded male thread diameters can help explain the distinction. They are thread reference values, not recommended washer hole sizes or manufacturing tolerances.
| BSP designation | Male thread diameter | Pitch |
|---|---|---|
| 1/4 inch | 13.16 mm | 19 TPI |
| 3/8 inch | 16.66 mm | 19 TPI |
| 1/2 inch | 20.96 mm | 14 TPI |
| 3/4 inch | 26.44 mm | 14 TPI |
See Swagelok’s thread and end-connection identification guide for thread identification and sealing arrangements. Measure the fitting and check its documentation rather than identifying it from a single approximate dimension.
Check whether the fitting seals against a washer
BSPP denotes a parallel thread; BSPT denotes a tapered thread. With a parallel connection, the seal may use a washer, an O-ring or a mating seat, depending on the fitting design. A parallel thread alone does not tell you which sealing component belongs there.
Check the shoulder and mating surface as well as the thread. A fitting intended to seal at a cone or another internal seat should not be converted into a washer-sealed joint merely because a washer will slide over it.
The three washer dimensions to record
- Inside diameter (ID): the opening that must accommodate the fitting as designed.
- Outside diameter (OD): the overall width that must suit the available sealing face and surrounding clearance.
- Thickness: part of the assembled geometry and seal specification.
Write these as separate measurements, with units. Also record the nominal BSP designation so you can compare a supplier’s application label with its actual dimensions.
A 1/4-inch BSP copper washer example
One published Duncan Rogers copper washer range lists its 1/4-inch BSP washer as 13.5 mm ID, 19 mm OD and 1.5 mm thick. This illustrates why the nominal label and actual measurements must be kept separate.
Those dimensions describe that supplier’s particular product. They are not a universal definition of every 1/4-inch BSP washer, and do not establish suitability for your fitting. Compare the fitting manufacturer’s requirements before buying a replacement.
BSP fibre washers vs copper washers
Choose the material specified for the joint. Fibre and copper washers are different sealing products, even if their ID and OD look similar. Check the actual material grade, service compatibility and installation requirements.
A generic material name does not establish compatibility with a fluid or an operating temperature and pressure. If you are replacing an old seal, use the equipment specification rather than treating its colour or worn dimensions as the whole answer.
Measuring and ordering a replacement
Record the fitting designation first, then the required ID, OD, thickness and material. If measuring a removed washer, recognise that compression or damage may have changed its thickness and shape. Prefer a specification or an unused correct sample where available.
For an assortment, compare every listed size and the quantity supplied. Keep the labelled compartments intact; a mixed drawer of copper and fibre washers can make a repeat repair harder than the first one.
If the seal face is damaged or the fitting cannot be identified, resolve that before selecting a washer. Increasing thickness or tightening harder is not a reliable substitute for the correct joint components.
Our workshop consumables checklist offers a practical way to organise replacement items. For adjustment material used elsewhere in an assembly, read brass shim thickness in imperial and metric sizes; shim stock and sealing washers serve different purposes and should be selected accordingly.