Precision Tube Cluster
DIN 2391 Tolerance Class Table — T1/T2/T3/T4 + E/H8/H9 bore
Full tolerance class reference for DIN 2391 precision tubes: OD classes T1-T4, wall thickness, and bore classes E, H8, H9 for hydraulic and mechanical applications.
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DIN 2391 defines four OD tolerance classes (T1-T4) and three ID tolerance classes (E, H8, H9). T3/T4 with bore H8 is the hydraulic-cylinder spec; T2 with bore H9 is general precision; T1 is the coarsest class. Choose the class that matches your seal type and piston-to-bore clearance budget.
What does DIN 2391 cover beyond material grades?
DIN 2391 covers seamless cold-drawn precision tubes in ST35, ST45, and ST52 grades with multiple delivery conditions (NBK, BK, BK+S, GBK). It defines dimensional tolerances for outside diameter, wall thickness, and inside diameter that control fit, clearance, and seal performance in precision applications.
DIN 2391 is the original German standard for cold-drawn precision seamless tubes, now largely superseded by EN 10305-1 for new European specifications. The standard covers three material grades: ST35 (equivalent to E235), ST45 (equivalent to E275), and ST52 (equivalent to E355). Each grade has specific tensile and yield strength requirements that appear on the mill test certificate.
Beyond material properties, DIN 2391 defines dimensional tolerance classes that buyers must specify correctly. These are split into three categories: outside diameter (OD) tolerance classes labeled T1 through T4, wall thickness tolerance, and inside diameter (ID) or bore tolerance classes labeled E, H8, and H9. Understanding these classes is essential because the wrong tolerances cause assembly problems, seal damage, or rejected parts.
The delivery condition represents how the tube is produced and whether it undergoes heat treatment after cold drawing. NBK means normalized after drawing, providing a softer, more ductile tube suitable for welding and forming. BK means hard as drawn with the highest strength but potentially unstable for machining. BK+S is stress-relieved after drawing, giving a balance of strength and dimensional stability. GBK is fully annealed, producing the softest condition for severe forming operations.
EN 10305 effectively replaced DIN 2391-1 for new specifications around 2010, but the tolerance class terminology (T1-T4, E, H8, H9) remains in common use across both standards. Buyers should write both standards on RFQs when equivalence matters: "DIN 2391 ST52 or EN 10305 E355 subject to MTC" clarifies acceptable substitution. The key is that tolerance class requirements must be clearly stated regardless of which standard is cited.
How do OD tolerance classes T1 through T4 differ in practice?
T1 offers ±0.4% OD tolerance (minimum ±0.10 mm) for general mechanical use. T2 at ±0.2% (min ±0.05 mm) suits precision shafting. T3 at ±0.1% (min ±0.03 mm) is the hydraulic cylinder standard. T4 uses DIN ISO 286 h9 for tight-tolerance OEM work.
The OD tolerance class determines how tightly the outside diameter is controlled. T1 is the coarsest class with ±0.4% tolerance subject to a minimum of 0.10 mm. This is suitable for general mechanical components where the OD is not a locating or sealing surface. It offers the best availability and lowest cost among the tolerance classes.
T2 reduces tolerance to ±0.2% with a 0.05 mm minimum. This class is specified for precision shafting where the tube must fit into bearings, sleeves, or other precision features. Many stock sizes are available in T2, making it the common choice for mechanical power transmission components.
T3 tightens to ±0.1% with a 0.03 mm minimum. This is the standard class for hydraulic cylinder barrels where the OD may be clamped, centered, or used for mounting. T3 provides enough accuracy for most cylinder fabrications while remaining commercially available.
T4 uses the ISO 286 h9 tolerance for the closest outside diameter control. This class is reserved for tight-tolerance OEM applications where the tube must mate with designed fits without additional machining. T4 is less commonly stocked and may require mill confirmation before ordering.
What wall thickness tolerance applies to DIN 2391 precision tubes?
Wall thickness tolerance in DIN 2391 is separate from the OD class. It is typically specified as a percentage of the nominal wall, commonly ±10% for the E class bore condition and tighter for H8/H9 finished bores. The tolerance must be stated explicitly on the purchase order.
Unlike the OD tolerance classes, wall thickness tolerance in DIN 2391 is not labeled T1-T4. Instead, it is typically stated as a percentage of the nominal wall thickness or as an absolute dimension. The wall thickness directly affects pressure rating, bending stiffness, and the ability to machine features.
For non-honed tubes (bore class E), a common wall thickness tolerance is ±10% of nominal wall. For honed or finishing tubes (bore class H8 or H9), the wall tolerance is typically tighter, often ±7.5% or ±5%, because the finished ID and wall must both meet functional requirements.
Buyers specifying hydraulic cylinder barrels should state both the OD tolerance class and the wall thickness tolerance separately. A common specification reads: "OD tolerance T3, wall thickness ±10% or minimum 0.5 mm after finishing." This prevents the supplier from quoting the most economical but potentially unsuitable combination.
Wall thickness variation also affects ovality and eccentricity. DIN 2391 controls these separately through straightness and eccentricity limits. For long-stroke cylinders, straightness often matters more than the tightest wall tolerance because a bowed tube cannot be straightened economically after honing.
When should a buyer specify bore class E, H8, or H9?
Specify bore class E (as-drawn, ±10% wall) for non-honed mechanical applications. Use H8 (ISO 286 H8) for honed piston-seal duty in hydraulic cylinders. Choose H9 (ISO 286 H9) when the bore is machined or sized to a specific dimension for general precision work.
The bore class controls the inside diameter tolerance. Class E represents the as-drawn condition with approximately ±10% wall thickness tolerance on the ID, meaning the tube is supplied in its cold-drawn state without additional ID finishing. This is suitable for non-honed applications such as bushings, sleeves, or mechanical components where the ID is not a sealing surface.
Class H8 refers to the ISO 286 hole-basis tolerance H8 applied to the inside diameter. At common hydraulic cylinder bores, H8 provides a bore tolerance range of roughly 0.02-0.04 mm depending on the nominal size. This is the standard choice for hydraulic cylinder barrels where the piston seal runs directly on the bore surface.
Class H9 uses the wider ISO 286 H9 tolerance, giving a larger bore tolerance window than H8. H9 is specified when the bore will be final-machined by the buyer, when the seal design permits wider clearance, or when shorter lead time is prioritized over the tightest fit.
For hydraulic cylinder barrels, H8 with OD class T3 is the most common combination. H9 may be acceptable for moderate-duty cylinders where the seal catalogue permits the wider bore tolerance. The piston seal supplier should confirm the acceptable bore window before specifying H8 or H9 on the RFQ.
Which tolerance class combination suits different applications?
For hydraulic cylinders, use T3 OD + H8 bore. For automotive driveshafts and CV joints, use T2 or T3 OD + H9 or E bore. For precision shafting with bearing fits, use T2 OD. For structural high-pressure applications, use T4 OD + NBK or BK condition.
Hydraulic cylinder barrels require the tightest dimensional control because the piston seal operates on the ID while the barrel may be mounted on the OD. The standard combination is T3 OD tolerance with H8 bore tolerance. This provides consistent seal compression, controlled leakage, and reliable mounting. Some high-performance cylinders specify T4 OD for tighter flange or adapter fits.
Automotive driveshaft and constant velocity (CV) joint tubes require precision OD control for spline engagement and torque transmission. The typical specification is T2 or T3 OD with E or H9 bore. These applications often use larger diameters and may involve forging or flanging at the ends. T3 provides extra security for as-drawn dimensions that will not be machined.
Precision shafting for bearings, linear guides, or hydraulic piston rods requires T2 OD tolerance at minimum. Many such applications locate components on the OD, so tolerance control affects alignment and running clearance. The ID may not be functionally critical in these applications.
Structural high-pressure applications such as accumulator bodies, hydraulic manifolds, or pressure vessels may use T4 OD combined with the tighter delivery conditions (NBK for welding or BK for maximum strength). These applications prioritize pressure containment and structural integrity over dimensional precision in the as-delivered condition.
What is the relationship between DIN 2391 and EN 10305 tolerance classes?
EN 10305-1 effectively replaced DIN 2391-1 around 2010 for new European specifications. DIN 2391 tolerance classes map to EN 10305 order codes: T1 maps to E, T2 maps to E1, T3 maps to E2, and T4 maps to E3 or h9. Both standards describe the same dimensional tolerances using different terminology.
EN 10305-1 is the current European standard for cold-drawn precision seamless tubes. It superseded DIN 2391-1 for new specifications, though DIN 2391 remains in widespread use for legacy drawings, stock lists, and procurement language. The dimensional requirements are fundamentally the same.
The tolerance class mapping between the two standards follows a pattern. DIN 2391 class T1 corresponds to EN 10305 order code E (the most economical tolerance). T2 maps to E1, T3 maps to E2, and T4 roughly corresponds to E3 or h9 (the tightest tolerance). The ISO 286 bore tolerance classes (E, H8, H9) are identical across both standards.
For procurement clarity, write the specification to accept either standard: "DIN 2391 ST52 or EN 10305 E355, tolerance class T3 / h8, BK+S condition, EN 10204 3.1 MTC." This allows suppliers to quote their available standard while ensuring the correct dimensional and material requirements are met.
Buyers should reject RFQ responses that cite only the old standard without confirming tolerance equivalence. A response reading "DIN 2391 ST52 BK" without tolerance class specification is incomplete. The correct response includes both the standard and the tolerance class together.
What does RP Sales normally stock, and what substitutions should buyers avoid?
RP Sales typically stocks T3 OD class precision tubes in ST52/E355 with BK+S condition and H8 or H9 bores where finished. Common buyer mistakes include substituting T2 for hydraulic cylinders (which leads to seal leakage), specifying T4 for general mechanical work (over-spec and higher cost), and omitting bore class on ID-critical applications.
Stock availability for DIN 2391 precision tubes follows a predictable pattern. The most commonly stocked combination is OD class T3, wall thickness with ±10% tolerance, ST52/E355 grade, and BK+S delivery condition. This covers the majority of hydraulic cylinder and general precision applications without requiring mill production.
A common buyer mistake is specifying T2 OD for hydraulic cylinder barrels to reduce cost. T2 offers ±0.2% tolerance, which is looser than the T3 ±0.1% requirement. Using T2 on a hydraulic cylinder can result in insufficient OD control for mounting, clamping, or flange fits. The saving on tube cost is lost in fabrication rework.
Another frequent error is over-specifying T4 for general mechanical applications. T4 uses the tightest tolerance and may require mill confirmation, longer lead times, and premium pricing. For non-sealing applications such as structural sleeves, bushings, or non-critical bearing supports, T2 or T3 provides sufficient accuracy at standard pricing.
The most critical substitution error is omitting the bore class when the ID is functional. Simply writing "100 x 90 tube" does not control the bore tolerance. For hydraulic applications, the RFQ must state H8 or H9 explicitly. Without it, the supplier may supply a ±10% wall tolerance (class E) bore that will not retain the piston seal.
Specifications
| OD tolerance classes | T1 (±0.4%), T2 (±0.2%), T3 (±0.1%), T4 (h9 per ISO 286) |
|---|---|
| Bore classes | E (as-drawn ±10% wall), H8 (ISO 286 H8), H9 (ISO 286 H9) |
| Wall thickness | Typically ±10% for E bore; tighter for H8/H9 finished bores |
| Material grades | ST35/E235, ST45/E275, ST52/E355 per DIN 2391 / EN 10305 |
| Delivery conditions | NBK (normalized), BK (hard drawn), BK+S (stress relieved), GBK (annealed) |
| Documentation | EN 10204 3.1 MTC standard; dimensional and roughness reports for finished bores |
Standards cited for DIN 2391 Tolerance Class Table
Reference standards cited on this page
- DIN 2391-1:1994 — Seamless precision steel tubes — Technical delivery conditions (Deutsches Institut für Normung)
- EN 10305-1:2016 — Steel tubes for precision applications — Seamless cold-drawn tubes (European Committee for Standardization (CEN))
- EN 10305-4:2016 — Seamless cold-drawn tubes for hydraulic and pneumatic power systems (European Committee for Standardization (CEN))
- ISO 286-1:2010 — Geometrical product specifications — ISO code system for tolerances on linear sizes (International Organization for Standardization)
- ISO 4287:1997 — Surface texture — Profile method — Terms, definitions and surface texture parameters (International Organization for Standardization)
- EN 10204:2004 — Metallic products — Types of inspection documents (European Committee for Standardization (CEN))
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