A precision tube is a cold-drawn (or cold-rolled welded) tube manufactured to tighter OD/ID/wall tolerances than commodity ERW or hot-finished seamless. Tolerance classes are defined in DIN 2391 / EN 10305 — typically ±0.05 mm OD and ±0.075 mm wall. Honed-bore precision tubes (ST52 BK+S H8) are the norm for hydraulic cylinders. Use precision tubes for hydraulic and pneumatic systems, automotive components, and any application where assembly tolerances cannot accommodate commodity-tube variation.
Tolerance classes — what to ask for
Tolerance class is the single most important precision-tube specification after material grade. Class T1 / T2 / T3 (per DIN 2391) progressively tighten OD and wall tolerance. For hydraulic cylinder service, T2 with a honed bore (suffix H8 or H9) is standard.
The H-class refers to the inside diameter (bore) tolerance for hydraulic seals: H8 = ±0.027 mm at 50 mm bore, H9 = ±0.062 mm. Tighter is better but more expensive — match to seal manufacturer's recommendation.
Material conditions: BK, BK+S, NBK, GBK
- BK: hard, as-drawn — high tensile, low elongation, no further heat treatment
- BK+S: hard, stress-relieved — tensile retained but residual stresses removed
- NBK: normalised — soft, weldable, good for further forming
- GBK: annealed — softest condition, used for tight-radius bending
| Class | OD tolerance | Wall tolerance | Typical use |
|---|---|---|---|
| T1 | ±0.20 mm | ±10 % | General mechanical |
| T2 | ±0.10 mm | ±10 % | Hydraulic cylinders |
| T3 | ±0.05 mm | ±7.5 % | Hydraulic + tight-tolerance OEM |
| T4 | ±0.05 mm | ±7.5 % | Honed bore for piston seals |
| Outer diameter (OD) | 6 mm to 250 mm typical |
|---|---|
| Wall thickness | 0.5 mm to 25 mm |
| Standards | DIN 2391, EN 10305-1/2/4, ASTM A519 |
| Common grades | ST52, ST37.4, E235, E355, SAE 1018, SAE 1020 |
| Surface finish | Bright (BK), pickled, phosphated, honed (H8/H9) |
| Documentation | EN 10204 3.1 MTC, hardness/dimensional report |
- DIN 2391-1:1994 — Seamless precision steel tubes — Part 1: 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 10204:2004 — Metallic products — Types of inspection documents (3.1, 3.2 MTC) (European Committee for Standardization (CEN))
- ASTM A519/A519M-22 — Standard Specification for Seamless Carbon and Alloy Steel Mechanical Tubing (ASTM International)
- Precision Tubes & Hydraulic — product detail
- Precision Tubes for Hydraulic Applications
- Quality, MTC & TPI
- DIN 2391 ST52 honed-bore tubes
- EN 10305 cold-drawn precision tubes
- BK+S surface finish
- Hydraulic cylinder OD/ID tolerances
- Skived and roller-burnished tubes
- Phosphated vs galvanised precision tubes
- How to Read MTC
- Equivalent Grades Cross-reference
- Glossary: Honed Bore
- Glossary: Cold-drawn
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