For hydraulic and pneumatic cylinder barrels, the bore is the product. Honed tubing (often called SRB — skived and roller-burnished — when produced that way) arrives with the inside diameter already finished to a running tolerance. But "H8" and "H9" are often quoted without the buyer knowing exactly what is being promised. Here is what those letters mean and how to specify them correctly.
H8 vs H9: the numbers
"H" means the tolerance zone starts at the nominal bore diameter and runs positive (material can only be over, never under). The number is the tolerance width, and it scales with diameter:
- H9 — for a Ø100 mm bore, roughly +0 / +87 µm
- H8 — for a Ø100 mm bore, roughly +0 / +54 µm
For a sealing system designed around a standard honed finish, H9 is common for pneumatic cylinders. For hydraulic cylinders with pressure-packed seals, H8 is the safer specification — and some critical spools and sleeves need even finer.
Tolerance is not the whole story: surface finish
Seal life depends on the bore's surface texture. Typical specifications:
- Skived & roller-burnished: Ra 0.2–0.8 µm, fast production, ideal for long strokes
- Full honing: Ra 0.1–0.4 µm with a controlled cross-hatch pattern that holds lubricant — preferred where seal wear must be minimal
If your drawing only says "H8 tube", ask yourself whether the seal supplier also needs a specified Ra and cross-hatch. We can supply both, and we record measured values on the inspection report.
Wall allowance: order the right pre-hone size
Honing removes material. A common mistake is ordering a tube whose wall is exactly nominal, leaving nothing for the honing stock. The rule: the pre-honed ID must be smaller than final by the total stock (typically 0.3–0.8 mm on diameter depending on bore size and prior condition). Specify "honed to H8, final ID Ø100 H8" and let us select the pre-machined size — that is part of the job.
What ships with every order
- EN 10204 3.1 certificate with heat-number traceability
- Measured ID values and surface finish records on the inspection report
- Cut-to-length, end-facing and chamfering to your drawing
Whether you need a single length for a prototype cylinder or mixed diameters consolidated in one container, send us the bore sizes and seal system — we will confirm tolerance, stock allowance and finish in the quotation.
Request honed tubing quotation
Need these parts made to your drawing? Send your specification — clear quotation within 24 hours, EN 10204 3.1 certificates as standard.
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