Brass Tolerances Guide

1. Why tolerances matter (and why over-specifying is the #1 cost driver)

Every brass component drawing has two costs hidden in it — the cost of the metal and the cost of the tolerances. Material is roughly proportional to mass × LME copper price, and changes slowly. Tolerance cost is geometric in the achievable band and changes dramatically depending on how tight you specify. A part dimensioned Ø10.0 ± 0.3 mm can run on a high-speed multi-spindle lathe in 8 seconds. The same part dimensioned Ø10.0 ± 0.005 mm typically needs a Swiss-type sliding-headstock CNC with a special boring tool and an in-process probe — same part, ~6× the cycle time.

Over-specifying is the most common mistake on brass component drawings. A designer who hasn't done this before will mark every dimension on every feature with a tight tolerance "just in case", driving the part to a cost band 3–5× higher than the application actually needs. This guide is the antidote: it tells you what the standards say, what a brass-component manufacturer can actually hold, and how to specify only the tolerances you actually need.

2. ISO 2768 — general tolerances (the workhorse)

ISO 2768 is the most-cited general-tolerances standard in Europe and on most international brass drawings. It is published in two parts:

• ISO 2768-1 — General linear and angular tolerances. Four classes: f (fine), m (medium), c (coarse), v (very coarse).
• ISO 2768-2 — Geometrical tolerances for features without individual tolerance indications. Three classes: H, K, L.

The shorthand used on drawings combines these — e.g. ISO 2768-mK means medium linear/angular tolerance plus K-class geometric tolerance. It is the default Brassland holds on production CNC parts when no other tolerance is specified.

ISO 2768-1 linear tolerances (per part size range)

ISO 2768-1 angular tolerances

3. ISO 286 — hole and shaft fits

For features that mate with another part (a brass insert pressed into a plastic boss, a brass shaft into a bronze bushing), ISO 286 defines fit codes that combine a tolerance grade (IT5 to IT18) with a fundamental deviation letter. The result is a clean code like H7 (hole) or h6 (shaft) that specifies a band of acceptable sizes.

Common fits used on brass components:

For a Ø10 mm feature on a brass part, the actual tolerance band of H7 is +0.000 / +0.015 mm (so 10.000 – 10.015 mm hole), and h6 is 0.000 / –0.009 mm (so 9.991 – 10.000 mm shaft). These bands hold for sizes up to ~50 mm in IT5–IT7; for larger features the band widens proportionally per ISO 286.

4. EN 12164 — brass rod dimensional tolerances

Before the brass becomes a part it is rod stock. EN 12164:2016 defines what dimensional tolerances the raw brass rod must hold before any machining. This matters because tighter rod tolerance reduces machining stock allowance and lowers cost.

EN 12164 rod diameter tolerances (peeled / centerless-ground)

Brassland stocks both standard and close-tolerance rod for the 19 alloys in the materials library; close-tolerance adds a small premium per kilogram but eliminates a finishing pass on the lathe.

5. EN 12420 — hot-forged brass dimensional tolerances

For hot-forged brass parts (valve bodies, complex fittings, near-net-shape components), EN 12420:2014 defines achievable dimensional tolerances on the as-forged surface — before any CNC finishing. The standard categorises tolerances by feature size and forging class (F1, F2, F3 from tightest to loosest).

This is why brass valve bodies that need a sealing-surface tolerance of ± 0.05 mm are always specified as "forged plus CNC finished" — the forging gets the part to near-net-shape, then a CNC operation re-cuts the sealing diameter to the tighter tolerance.

6. ISO 1101 — GD&T (geometric tolerancing)

ISO 1101 defines symbols for geometric tolerances — properties of a feature that cannot be captured by a plus/minus dimension alone. The 14 symbols cover form, orientation, location and run-out. The most commonly used on brass parts:

A well-tolerated brass valve body drawing might mark the sealing-bore roundness as ◯ 0.005 mm with reference datum A on the threaded interface — telling the manufacturer that this one feature is critical and the rest of the part can run on ISO 2768-m.

7. What Brassland can actually hold in production

Quoted tolerances depend on which machine type runs the part. The hierarchy from coarsest to tightest on a typical Brassland order:

8. How tolerance choice affects cost

A useful rule-of-thumb pricing index. All values relative to "ISO 2768-c on a multi-spindle screw machine" = 1.0:

9. How to write a clean tolerance block

A drawing's tolerance block answers four questions in a few lines. A well-written brass-part block:

UNLESS OTHERWISE STATED:
  Linear / angular tolerances:  ISO 2768-m
  Geometric tolerances:         ISO 2768-K
  Surface finish:               Ra 1.6 µm
  Edges:                        deburr / break ≤ 0.3 mm
  Thread:                       per ISO 261 / ISO 965 (medium fit class 6H/6g)
  Material:                     CW617N (CuZn40Pb2) per EN 12164
  Plating:                      none / specify on PO
  Note:                         All dimensions in mm

Critical features that need tighter tolerance are then called out individually on the drawing — e.g. a sealing-bore diameter marked Ø10 H7 with concentricity ◯ 0.01 mm to datum A. This keeps the bulk of the part on ISO 2768-m (cheap) while the one critical feature gets the precision it needs.

10. Inspection methods that match each tolerance class

Brassland's QC lab carries Mitutoyo equipment at every level of this list. Inspection reports against any drawing-specified tolerance class are supplied on request with the EN 10204 Type 3.1 mill certificate that accompanies each shipment.

Frequently asked questions

11. Sources & references

Last reviewed: June 2026. Standards are revised periodically by their issuing bodies; for procurement-critical decisions verify against the current published edition. This guide is general engineering reference only and is not a substitute for the published standard.