BS 4518 is the British Standard specification for metric O-ring dimensions. Published by the British Standards Institution (BSI), it defines inside diameters and cross-sectional diameters in millimeters, together with the tolerances needed for general industrial, hydraulic, pneumatic, and process sealing applications. Although British in origin, BS 4518 is dimensionally aligned with ISO 3601-1 and DIN 3771 Part 1, so a BS 4518 O-ring is generally interchangeable with the same metric size under those standards.
This guide explains the BS 4518 size system, cross-section series, tolerances, groove design, and how it compares to ISO 3601 and AS568.
Short Answer: BS 4518 at a Glance
| Cross-Section (mm) | Common Inside Diameter Range | Typical Application |
|---|---|---|
| 1.5 mm | 3.0 mm to 50 mm | Small static seals, instrumentation, pneumatic fittings |
| 2.0 mm | 4.0 mm to 100 mm | General static seals, valves and pumps |
| 2.5 mm | 5.0 mm to 200 mm | Medium static seals, hydraulic flanges |
| 3.0 mm | 6.0 mm to 400 mm | Heavy static seals, process equipment |
| 4.0 mm | 8.0 mm to 400 mm | Large static seals, pressure vessels |
| 5.0 mm | 10.0 mm to 400 mm | Very large static seals, tank manways |
| 7.0 mm | 14.0 mm to 670 mm | Extra-large static seals, shipbuilding and heavy industry |
BS 4518 designates each O-ring by Inside Diameter × Cross-Section in millimeters (for example, 25.00 × 3.00 mm). The standard covers dimensions only; material and hardness are selected separately.
What Is BS 4518?
BS 4518 is the British Standard for metric O-rings. It was developed to provide uniform dimensions for seals used in British and Commonwealth machinery, and it remains referenced in UK engineering drawings, marine specifications, aerospace subcontract documentation, and legacy hydraulic equipment manuals.
The standard defines inside diameter (ID) and cross-sectional diameter (CS) in millimeters, plus dimensional tolerances for both. It also cross-references ISO 3601 and DIN 3771 for international alignment. BS 4518 does not specify elastomer material, hardness, color, or surface finish — those are added by the design engineer using standards such as ASTM D2000, ISO 1629, or customer compound specifications.
Scope and Size Designation
BS 4518 uses a simple designation: ID × CS in millimeters. A drawing callout might read:
> O-ring 25.00 × 3.00 mm, NBR 70 Shore A, per BS 4518
This single line communicates size, material, hardness, and dimensional standard. Primary applications include UK and Commonwealth industrial machinery, metric hydraulic and pneumatic systems, marine engineering, aerospace subcontractors, and legacy equipment originally designed to BS 4518 or the older inch-based BS 1806 standard.
BS 4518 Cross-Section Series
BS 4518 uses a fixed set of preferred cross-sections in millimeters. Each cross-section is paired with a practical range of inside diameters. Selection depends on pressure, groove geometry, and whether the seal is static, dynamic, or rotary.
| Cross-Section (mm) | Typical Use Case | Why It Is Chosen |
|---|---|---|
| 1.5 mm | Miniature fittings, sensors, pneumatic connectors | Small groove, low closure force, compact assemblies |
| 2.0 mm | Small valves, pumps, general static seals | Balance of seal compression and groove space |
| 2.5 mm | Hydraulic flanges, medium static covers | Larger contact area for moderate pressure |
| 3.0 mm | Process equipment, hydraulic cylinders, pipe flanges | Most common metric heavy-duty cross-section |
| 4.0 mm | Pressure vessels, large flanges, heavy machinery | Higher fill volume and pressure resistance |
| 5.0 mm | Tank manways, large-diameter covers, shipbuilding | Very large contact area for low-pressure, wide-flange seals |
| 7.0 mm | Extra-large static seals, heavy industry, marine | Maximum conformability on large, slightly irregular surfaces |
The 2.0 mm and 3.0 mm cross-sections are the most commonly stocked metric sizes for general engineering.
Dimensional Tolerances
BS 4518 specifies tolerances separately for cross-section and inside diameter. The tolerance band widens as the O-ring becomes larger, reflecting the practical limits of compression molding, mold thermal expansion, and measurement over larger diameters.
| Cross-Section (mm) | CS Tolerance | Inside Diameter Range | ID Tolerance |
|---|---|---|---|
| 1.5 mm | ±0.08 mm | ≤ 30 mm | ±0.14 mm |
| 2.0 mm | ±0.09 mm | ≤ 50 mm | ±0.16 mm |
| 2.5 mm | ±0.09 mm | 50–80 mm | ±0.19 mm |
| 3.0 mm | ±0.10 mm | 80–120 mm | ±0.22 mm |
| 4.0 mm | ±0.13 mm | 120–180 mm | ±0.28 mm |
| 5.0 mm | ±0.13 mm | 180–250 mm | ±0.35 mm |
| 7.0 mm | ±0.15 mm | 250–400 mm | ±0.45 mm |
For a size of 25.00 × 2.00 mm, the acceptable dimensions are ID 24.84–25.16 mm and CS 1.91–2.09 mm. For 150.00 × 5.00 mm, the acceptable range is ID 149.65–150.35 mm and CS 4.87–5.13 mm. These tolerances are functionally equivalent to ISO 3601-1 Grade N (normal). For precision hydraulic or aerospace applications, specify ISO 3601-1 Grade S or add a custom tolerance note.
Common BS 4518 Size Examples
The following tables show representative BS 4518 sizes for each cross-section. Outside diameter (OD) = ID + (2 × CS).
1.5 mm Cross-Section Series
| Size (ID × CS) | ID (mm) | CS (mm) | OD (mm) | Application |
|---|---|---|---|---|
| 3.00 × 1.50 | 3.00 | 1.50 | 6.00 | Miniature pneumatic fitting |
| 10.00 × 1.50 | 10.00 | 1.50 | 13.00 | Small sensor housing |
| 20.00 × 1.50 | 20.00 | 1.50 | 23.00 | Valve stem seal |
| 50.00 × 1.50 | 50.00 | 1.50 | 53.00 | Large instrument flange |
2.0 mm Cross-Section Series
| Size (ID × CS) | ID (mm) | CS (mm) | OD (mm) | Application |
|---|---|---|---|---|
| 8.00 × 2.00 | 8.00 | 2.00 | 12.00 | Miniature hydraulic valve |
| 15.00 × 2.00 | 15.00 | 2.00 | 19.00 | General static seal |
| 25.00 × 2.00 | 25.00 | 2.00 | 29.00 | Medium pump body |
| 50.00 × 2.00 | 50.00 | 2.00 | 54.00 | Large valve cover |
3.0 mm Cross-Section Series
| Size (ID × CS) | ID (mm) | CS (mm) | OD (mm) | Application |
|---|---|---|---|---|
| 20.00 × 3.00 | 20.00 | 3.00 | 26.00 | Medium hydraulic cylinder |
| 32.00 × 3.00 | 32.00 | 3.00 | 38.00 | Common flange seal |
| 50.00 × 3.00 | 50.00 | 3.00 | 56.00 | Process pipe flange |
| 100.00 × 3.00 | 100.00 | 3.00 | 106.00 | Pressure vessel manway |
5.0 mm and 7.0 mm Cross-Section Series
| Size (ID × CS) | ID (mm) | CS (mm) | OD (mm) | Application |
|---|---|---|---|---|
| 50.00 × 5.00 | 50.00 | 5.00 | 60.00 | Tank access cover |
| 100.00 × 5.00 | 100.00 | 5.00 | 110.00 | Storage tank manway |
| 200.00 × 5.00 | 200.00 | 5.00 | 210.00 | Large process vessel |
| 150.00 × 7.00 | 150.00 | 7.00 | 164.00 | Large-diameter static cover |
| 250.00 × 7.00 | 250.00 | 7.00 | 264.00 | Shipbuilding hull seal |
The complete BS 4518 table contains many more intermediate sizes, particularly in the 2.0 mm and 3.0 mm series that dominate industrial use.
Groove Design for BS 4518 O-Rings
BS 4518 O-rings follow the same gland principles as ISO 3601-1 and DIN 3771 Part 2. The key variables are squeeze percentage, groove width, and edge radius or chamfer.
| Seal Type | Squeeze | Groove Width | Edge Detail |
|---|---|---|---|
| Static | 15–20% | 1.15–1.25 × CS | Minimum edge radius 0.1 mm |
| Dynamic (reciprocating) | 8–15% | 1.20–1.30 × CS | Lead-in chamfer 15–20° |
| Rotary | 5–8% | 1.25–1.35 × CS | Lead-in chamfer, surface speed < 0.5 m/s |
For a static 25.00 × 3.00 mm O-ring with 18% squeeze, groove depth = 2.46 mm and groove width ≈ 3.60 mm. For dynamic service at 12% squeeze, groove depth = 2.64 mm and groove width ≈ 3.75 mm. Lower squeeze reduces friction and heat buildup during reciprocating motion. Verify your exact squeeze percentage with our O-ring compression calculator.
BS 4518 vs ISO 3601 and AS568
BS 4518 is metric and dimensionally aligned with ISO 3601, while AS568 is a separate inch-based system.
BS 4518 vs ISO 3601-1
| Feature | BS 4518 | ISO 3601-1 |
|---|---|---|
| Origin | British Standards Institution | International Organization for Standardization |
| Units | Millimeters | Millimeters |
| Size designation | ID × CS in mm | ID × CS in mm |
| Common cross-sections | 1.5, 2.0, 2.5, 3.0, 4.0, 5.0, 7.0 mm | 1.5, 1.8, 2.0, 2.5, 3.0, 3.5, 4.0, 5.0, 7.0 mm |
| Tolerance approach | By CS and ID range | Grade N (normal) and Grade S (special) |
| Interchangeability | Dimensionally equivalent for common sizes | Dimensionally equivalent for common sizes |
For most industrial sizes, a BS 4518 O-ring and an ISO 3601-1 O-ring with the same ID × CS callout are interchangeable.
BS 4518 vs AS568
| Feature | BS 4518 | AS568 |
|---|---|---|
| Origin | British Standards Institution | SAE International |
| Units | Metric (mm) | Imperial (inch) |
| Size designation | ID × CS in mm | Dash number |
| Common cross-sections | 1.5, 2.0, 2.5, 3.0, 4.0, 5.0, 7.0 mm | 0.070, 0.103, 0.139, 0.210, 0.275 inch |
| Primary regions | UK, Commonwealth, Europe | North America, aerospace, global hydraulics |
There is no exact one-to-one mapping between BS 4518 and AS568. When converting between standards, always verify groove fill, squeeze percentage, and application pressure rather than relying on approximate dimensional similarity.
Material Selection for BS 4518 O-Rings
BS 4518 defines dimensions only; the design engineer must select the elastomer.
| Material | Hardness | Typical Application |
|---|---|---|
| NBR (Nitrile) | 70 Shore A | General hydraulic oil, pneumatic systems, mineral-oil-based fluids |
| NBR (Nitrile) | 90 Shore A | High-pressure hydraulic systems, abrasive environments |
| FKM (Fluorocarbon) | 75 Shore A | High temperature, fuel systems, aggressive chemicals |
| EPDM | 70 Shore A | Water, steam, phosphate ester hydraulic fluids, outdoor weathering |
| VMQ (Silicone) | 70 Shore A | Low temperature, food-grade, medical devices |
| HNBR | 70–90 Shore A | Elevated temperature, ozone resistance, sour gas service |
| FFKM | 75 Shore A | Extreme chemical resistance, semiconductor, aerospace |
| PTFE / FEP Encapsulated | — | High chemical resistance, cryogenic or high-temperature duty |
Material selection should consider fluid compatibility, temperature range, pressure, movement type, and regulatory requirements such as FDA, WRAS, or aerospace specifications.
Inspection and Acceptance Criteria
O-rings supplied to BS 4518 should be inspected for both dimensions and visual quality.
Dimensional inspection:
- Inside diameter measured with cone gauges, optical comparator, or precision pins.
- Cross-section measured at three points around the circumference.
- Outside diameter calculated or measured as a secondary check (OD = ID + 2 × CS).
Visual acceptance:
- Parting line flash not exceeding 0.08 mm for commercial grades.
- No porosity, blisters, voids, cracks, tears, cuts, or foreign inclusions.
- Mold marks acceptable if within flash limits and not penetrating the surface.
Batch testing for critical applications: hardness per ASTM D2240 or ISO 48; tensile strength and elongation per ASTM D412 or ISO 37; compression set per ASTM D395 Method B or ISO 815; fluid swell per ASTM D471 or ISO 1817 when required.
FAQ
Q1: Is BS 4518 the same as ISO 3601?
BS 4518 is dimensionally equivalent to ISO 3601-1 for most common metric sizes. An O-ring specified as 25.00 × 3.00 mm under BS 4518 will normally match the same size under ISO 3601-1. For precision applications, verify the exact tolerance grade.
Q2: What is the difference between BS 4518 and BS 1806?
BS 4518 is the metric British standard with dimensions in millimeters. BS 1806 was the older British inch-based standard, largely superseded by AS568 for inch sizes.
Q3: Can I use BS 4518 O-rings in DIN 3771 grooves?
Yes. BS 4518 and DIN 3771 are dimensionally aligned for general industrial use. The groove dimensions in DIN 3771 Part 2 are compatible with BS 4518 O-rings of the same ID × CS. Verify squeeze percentage and groove fill for your specific design.
Q4: What industries commonly reference BS 4518?
BS 4518 is commonly referenced in British and Commonwealth machinery, marine engineering, aerospace subcontractors, hydraulic equipment servicing, and process plant maintenance.
Q5: Are BS 4518 O-rings available in standard kits?
Yes. Metric O-ring kits are typically organized to BS 4518 / ISO 3601 / DIN 3771 dimensions and are useful for maintenance departments servicing British and European equipment.
Q6: Does BS 4518 specify material or hardness?
No. BS 4518 defines dimensions and tolerances only. Material, hardness, color, and surface finish must be specified separately using standards such as ASTM D2000, ISO 1629, or customer-specific compound specifications.
Q7: How do I convert an AS568 dash number to a BS 4518 metric size?
Convert the AS568 ID and CS from inches to millimeters, then select the nearest BS 4518 metric size. Because the two standards use different base cross-sections, always verify groove compatibility before substituting.
Q8: What tolerance should I specify for precision hydraulic seals?
For precision hydraulic seals, specify tighter tolerances than standard BS 4518 commercial grades. Reference ISO 3601-1 Grade S (special), add custom drawing tolerances, or work with your supplier to define a precision molding grade.
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Need BS 4518 metric O-rings for British or European equipment? Request a quote or contact our engineering team with your size (ID × CS in mm), material, hardness, quantity, and any certification requirements. We supply BS 4518 / ISO 3601 / DIN 3771 metric O-rings from stock in 3–7 days and can manufacture custom sizes with full batch traceability on request.