DIN 3771 O-Ring Standard: Complete Guide to German Metric Sizes

DIN 3771 is the German national standard for metric O-ring dimensions and the dominant specification for European machinery, hydraulics, pneumatics, and automotive systems designed in millimeters. Maintained by the Deutsches Institut für Normung (DIN), it is technically aligned with ISO 3601 and groups sizes into three cross-section series: A, B, and C. This guide covers the three parts of DIN 3771, the A/B/C series, tolerances, groove design, and the relationship to ISO 3601.

Short Answer: What Is DIN 3771?

DIN 3771 is a metric O-ring standard divided into three parts:

• Part 1: Inside diameter and cross-section dimensions with tolerances
• Part 2: Groove (housing) dimensions for static, dynamic, and rotary seals
• Part 3: Quality specifications and acceptance criteria

O-rings are designated by Inside Diameter × Cross-Section (ID × CS) in millimeters and grouped by cross-section series:

For general industrial hydraulics and pneumatics, DIN 3771 and ISO 3601 are dimensionally aligned and interchangeable. The full DIN 3771 size chart is available on our standards page.

History and Scope of DIN 3771

DIN 3771 was developed to standardize metric O-ring dimensions for German machinery, hydraulics, and automotive assemblies. Before metric standardization spread across Europe, OEMs often relied on inch-based standards or proprietary dimensions.

Today, DIN 3771 is used throughout Germany, Austria, Switzerland, and much of Central and Eastern Europe. Because the dimensions are aligned with ISO 3601, a DIN 3771 O-ring will normally fit an ISO 3601 groove of the same nominal size. The standard defines dimensions, tolerances, groove geometry, and quality requirements, but does not specify material or hardness.

DIN 3771 Parts 1, 2, and 3

Part 1 — Dimensions and Tolerances

Part 1 defines nominal inside diameters and cross-sections. Sizes are written as ID × CS, for example 25.00 × 3.00 mm, covering IDs from miniature instrument sizes up to approximately 670 mm. Part 1 also specifies tolerance bands for inside diameter and cross-section, with tighter tolerances on smaller cross-sections.

Part 2 — Groove Dimensions (Housing Dimensions)

Part 2 provides groove width, depth, edge radius, and lead-in chamfer values for static, dynamic, and rotary seals. Groove depth controls squeeze and groove width provides volume for the compressed ring. Use the compression calculator with Part 2 dimensions to verify squeeze percentage.

Part 3 — Quality Specifications

Part 3 defines surface quality, visual defect limits, batch inspection rules, and acceptance criteria. It is the German counterpart to ISO 3601 quality requirements and is especially important for hydraulic and automotive applications where surface defects can create leakage paths.

The A/B/C Cross-Section Series

DIN 3771 uses letter designations rather than the dash-number series found in AS568.

Series A — Static Seals

Series A is intended for static seals such as flange covers, valve bonnets, filter housings, and pressure vessel closures.

Series B — Dynamic Seals

Series B covers dynamic seals in reciprocating hydraulic and pneumatic cylinders, pistons, and rods. Groove design differs from Series A because the ring must slide without excessive friction or rolling.

Series C — Compact General-Purpose Seals

Series C provides intermediate cross-sections commonly used in automotive connectors, compact hydraulic ports, and general-purpose applications.

Series C is often the practical choice when a port or groove has been designed around ISO 3601 dimensions.

Dimensional Tolerances

DIN 3771 tolerance bands depend on cross-section diameter. The inside-diameter tolerance is split into two ranges: one for smaller IDs and one for larger IDs. The table below is based directly on the DIN 3771 size chart.

The small-ID tolerance applies to smaller inside diameters; the large-ID tolerance applies to larger IDs where measurement uncertainty and mold thermal expansion become more significant. Check the current DIN 3771 table for the exact ID boundary, which depends on cross-section.

How Tolerances Affect Seal Design

Tolerances define the minimum and maximum possible squeeze in the groove. A 3.0 mm CS O-ring with CS tolerance ±0.10 mm and groove depth designed for 15% squeeze can produce an actual squeeze range of roughly 12–18%. Use the compression calculator to check worst-case squeeze at both tolerance extremes.

Groove Design Guidelines

DIN 3771 Part 2 gives groove dimensions for three service conditions.

Higher squeeze in static seals creates contact stress for pressure sealing without the friction penalty of dynamic service. Lower squeeze in dynamic seals reduces friction and wear. Rotary seals are the most demanding: squeeze must be minimized to limit heat generation, and surface speed is normally limited unless a specialized PTFE or spring-energized seal is used.

Squeeze formula:

> Squeeze (%) = (CS − Groove Depth) / CS × 100

For a 3.0 mm CS O-ring in a static groove with depth 2.55 mm, squeeze is (3.00 − 2.55) / 3.00 × 100 = 15%.

DIN 3771 vs ISO 3601

DIN 3771 and ISO 3601 are technically aligned. For common metric hydraulics and pneumatics sizes, an O-ring labeled to one standard normally fits a groove designed to the other.

Key Differences Between DIN 3771 and ISO 3601

1. Scope: DIN 3771 is the German national standard; ISO 3601 is the international standard. DIN 3771 references ISO 3601 for many dimensions.
2. Cross-section series: DIN 3771 explicitly labels A (static), B (dynamic), and C (general) series. ISO 3601 does not use A/B/C letters but includes the same nominal cross-sections.
3. Quality terminology: DIN 3771 Part 3 quality rules are closely aligned with ISO 3601 quality classes, but documentation and inspection protocols may differ slightly between suppliers.
4. Documentation: German OEMs often call out "DIN 3771" on drawings; global OEMs more commonly call out "ISO 3601-1" even when the dimensions are identical.

In practice, a 25.00 × 3.00 mm O-ring to DIN 3771 is normally dimensionally interchangeable with the same size manufactured to ISO 3601-1.

Common DIN 3771 Size Examples

The table below shows representative DIN 3771 / ISO 3601 metric sizes used in hydraulic and pneumatic equipment.

Custom sizes outside the standard tables are available by molded tooling or vulcanized cord for very large diameters.

Material and Hardness Pairing

DIN 3771 defines dimensions and tolerances only; the engineer selects the elastomer. Common material-hardness combinations are listed below.

For hydraulic systems, 70 or 90 Shore A NBR is the most common starting point. FKM is used above 120 °C or with fuels and esters; EPDM is standard for steam and hot water. Validate material selection by compatibility testing for aggressive or unusual conditions.

Inspection and Acceptance Criteria

DIN 3771 Part 3 sets quality rules for accepting or rejecting a batch of O-rings.

Dimensional inspection:

• Inside diameter measured at least at three points around the circumference
• Cross-section measured at multiple points along the ring
• Tolerances verified against the applicable CS band and ID range

Visual inspection:

• No surface cracks, tears, or cuts
• No porosity, blisters, or voids visible at the surface
• No foreign inclusions embedded at the surface
• Parting-line flash within the limits specified by the quality class
• Mold marks acceptable if they do not exceed flash limits or create leakage paths

Batch testing for critical applications:

• Hardness per ASTM D2240 / DIN 53505 (Shore A)
• Tensile strength and elongation per ASTM D412 / DIN 53504
• Compression set per ASTM D395 / ISO 815
• Fluid swell per ASTM D471 / ISO 1817 (when required)

For automotive and hydraulic safety applications, suppliers often provide a Certificate of Conformance stating that parts meet DIN 3771 dimensions and the requested material specification.

FAQ

Q1: Is DIN 3771 the same as ISO 3601?

DIN 3771 is technically aligned with ISO 3601 and uses the same metric dimensions for most common sizes, so O-rings are normally interchangeable. DIN 3771 Part 3 adds German quality conventions, and some German OEMs still prefer a DIN callout on drawings.

Q2: What do DIN 3771 Series A, B, and C mean?

Series A is for static seals, Series B is for dynamic seals, and Series C provides intermediate cross-sections for automotive and general-purpose use. The same cross-section can appear in multiple series, but groove design differs between static and dynamic service.

Q3: How do I read a DIN 3771 O-ring size?

DIN 3771 sizes are written as Inside Diameter × Cross-Section in millimeters. For example, "25.00 × 3.00 mm" means ID 25.00 mm and CS 3.00 mm. The outside diameter is ID + 2 × CS, so this ring has an OD of 31.00 mm.

Q4: Can I use a DIN 3771 O-ring in an AS568 gland?

Metric and inch dimensions are close for some sizes but not identical. Always match the O-ring to the gland standard in critical applications. For low-pressure static seals, some sizes are practically interchangeable, but verify gland dimensions and squeeze before substituting.

Q5: What is the largest standard DIN 3771 size?

The standard tables cover inside diameters up to approximately 670 mm. Larger sizes are manufactured as custom-molded rings or as spliced/vulcanized rings for diameters beyond standard mold capabilities.

Q6: What groove depth should I use for a 3.0 mm DIN 3771 O-ring?

For a static seal, target 15–20% squeeze, or a groove depth of roughly 2.40–2.55 mm. For dynamic service, target 8–15% squeeze, or roughly 2.55–2.76 mm. Confirm the final depth with the compression calculator.

Q7: What hardness should I specify for a DIN 3771 hydraulic O-ring?

For general hydraulic oil, 70 Shore A NBR is the default. For systems above 100 bar, 90 Shore A NBR or polyurethane reduces extrusion. For high temperature or aggressive fluids, FKM 75 or 90 Shore A is common.

Q8: Are DIN 3771 O-ring kits available?

Yes. Metric kits organized to DIN 3771 / ISO 3601 dimensions are popular for servicing European machinery. Custom kits with specific sizes, materials, and hardness values are also available.

---

Need DIN 3771 metric O-rings? Send us your size list, material, and quantity for a fast quote through our request quote page or contact us directly. We supply standard DIN 3771 sizes from stock and custom-molded or vulcanized metric rings with material certification and DIN/ISO dimensional conformance.