Why bore tolerance shows up on the invoice twice
A cylinder bore tolerance is one of the few numbers on a drawing that a buyer pays for twice. The first payment is the machining cost of holding the band. The second payment arrives later, in the form of seal life, or the absence of it. Specifying H8 when the seal package needs H9 wastes money on the first invoice and buys nothing on the second. Specifying H9 where a servo-grade piston needs H7 saves money once and then costs a warranty claim.
The tolerance grade sets how much the bore diameter is allowed to vary along the length and around the circumference. The piston seal rides in that bore. Its compression ratio is a function of the actual clearance between seal and wall at every point of the stroke. A wide tolerance band means the compression ratio varies from one end of the stroke to the other, and the seal is over-compressed at the tight end while it leaks at the loose end.
ISO 286-2 limit deviations for cylinder bores
Hole-basis tolerances use a capital letter. The letter H carries a lower deviation of zero, so an H-class bore can be at nominal size or larger, never smaller. The number is the IT grade, which sets the width of the band. All values below are upper deviations in micrometres, with the lower deviation at zero.
| Bore range (mm) | H7 | H8 | H9 | H10 | H11 |
|---|---|---|---|---|---|
| 30 – 50 | +25 / 0 | +39 / 0 | +62 / 0 | +100 / 0 | +160 / 0 |
| 50 – 80 | +30 / 0 | +46 / 0 | +74 / 0 | +120 / 0 | +190 / 0 |
| 80 – 120 | +35 / 0 | +54 / 0 | +87 / 0 | +140 / 0 | +220 / 0 |
| 120 – 180 | +40 / 0 | +63 / 0 | +100 / 0 | +160 / 0 | +250 / 0 |
| 180 – 250 | +46 / 0 | +72 / 0 | +115 / 0 | +185 / 0 | +290 / 0 |
| 250 – 315 | +52 / 0 | +81 / 0 | +130 / 0 | +210 / 0 | +320 / 0 |
| 315 – 400 | +57 / 0 | +89 / 0 | +140 / 0 | +230 / 0 | +360 / 0 |
Read one line aloud. A 100 mm bore at H8 accepts anything from 100.000 to 100.054 mm. At H9 the same bore accepts 100.000 to 100.087 mm. The difference between those two lines is 33 micrometres, and it is decided before a single chip is cut.
What each grade is actually used for
| Grade | Typical service | How it is reached |
|---|---|---|
| H7 | Servo cylinders, high-frequency cylinders, tight-clearance piston guidance | Honing with in-process gauging, often two-pass |
| H8 | Standard industrial and mobile hydraulic cylinders, the default for honed tube | Single-pass honing or skiving and roller burnishing |
| H9 | Low-pressure cylinders, agricultural rams, long-stroke telescopic stages | Honing or SRB at higher feed |
| H10 / H11 | Clearance bores, non-sealing sections, structural tube | Cold drawing alone |
The tolerance band is only half the requirement
A bore can sit inside H8 at every measured point and still destroy seals, because ISO 286 governs two-point diameter while the seal responds to form. Three geometric errors travel alongside the diameter number on a serious drawing.
- Roundness. An oval or three-lobed bore keeps the two-point diameter inside tolerance while the seal experiences alternating compression twice per revolution of measurement. A common shop rule holds roundness to one third of the diameter tolerance band.
- Straightness. A bowed barrel puts a continuous side load on the rod, the guide bushing and the piston. Published practice for honed tube commonly uses 0.3 mm per 1000 mm as an acceptance limit, with tighter figures for long-stroke cylinders.
- Cylindricity and taper. Taper along the stroke produces exactly the variable compression described above, and it is the error most often missed because calipers read fine at both ends.
Honing is unusual among finishing processes because it corrects form rather than reproducing it. The abrasive stones stay in contact with the whole circumference and cut preferentially where the bore is tight, so the process averages out lobing and taper. This is why honing regularly produces rounder bores than the machine tool that preceded it.
Where the cost curve bends
H8 is the economical band for most honed tube because a single honing pass reaches it from a cold-drawn or skived starting bore. H7 usually needs a second pass, finer abrasive, in-process air or laser gauging, and more frequent stone dressing. On a long heavy-wall barrel that second pass adds real spindle hours.
The jump from H9 to H8 is comparatively cheap. The jump from H8 to H7 costs proportionally more, and the jump from H7 to H6 enters grinding territory. If a drawing calls for H7 across the full length of a 3 m tube, ask whether the tight band is genuinely needed for the whole stroke. Specifying critical zones at the seal travel ends and a relaxed band through the mid-stroke is a legitimate way to hold function without paying for precision where the piston never seats.
Specifying it on the drawing
A complete bore callout names four things, and the diameter tolerance is only the first.
- Diameter with tolerance class, for example 100 H8 per ISO 286-2.
- Surface roughness, normally Ra 0.2 to 0.4 µm for hydraulic service, with a cross-hatch angle requirement if seal life matters.
- Geometric tolerances for roundness, straightness and cylindricity, referenced to the datum you actually assemble against.
- Material and delivery condition, for example E355 +SR or 27SiMn quenched and tempered, because surface hardness interacts with the finish you can hold.
When you send the enquiry, state the annual volume alongside the tolerance. Tolerance cost per piece drops sharply once a honing line is set up for your size and the stones are broken in, and a supplier quoting a single-piece price without knowing volume has priced the setup into one unit.
What we hold, and what we do not publish
Our standard honed and skived tube is produced to H8 as the default, with H7 and H9 available on request. Our published bore range is [ORISE: confirm current ID range, for example 40 to 500 mm] and our published length capacity is [ORISE: confirm current max length]. Values outside a confirmed measured range are marked not published rather than estimated.
See our cylinder tubes and barrels page for the current product scope, and our deep hole boring and honing page for process detail. Inspection equipment used for bore verification is listed under inspection equipment. If your drawing needs a grade we have not confirmed, send it through the RFQ page and ask for a capability statement rather than a catalogue claim.