Die Casting Tolerance Guide (ISO 8062)
A practical reference for specifying and interpreting dimensional tolerances on cast parts. Built around ISO 8062 general casting tolerance grades (GCTG).
What are General Casting Tolerance Grades (GCTG)?
ISO 8062 defines General Casting Tolerance Grades (GCTG) — a family of tolerance bands indexed to the nominal dimension of a feature. A finer grade (e.g. GCTG 4) gives a tighter allowable deviation; a coarser grade (e.g. GCTG 8) allows more variation. The required tolerance grows with the nominal size, because larger castings are harder to hold to absolute precision.
How to read the tolerance table
Find the nominal dimension range that contains your feature, then read the tolerance value for the selected grade. Example: a 50 mm feature at GCTG 6 allows roughly ±0.32 mm (i.e. 49.68 mm … 50.32 mm). Use the Die Casting Tolerance Calculator to resolve any dimension/grade pair instantly.
Choosing a tolerance grade (design for manufacturability)
- Start coarse. Specify the loosest grade that still satisfies function. Tighter grades raise scrap, tooling and machining cost.
- Reserve tight tolerances for critical features only. Localise precision (e.g. a bearing bore) rather than tightening the whole part.
- Mind wall thickness and draft. Very thin sections and deep ribs fight dimensional control regardless of grade.
- Confirm against the standard. The values here are representative; validate production numbers against the current ISO 8062 edition and your foundry's capability study.
GCTG quick reference
| Grade | Typical character | Use when… |
|---|---|---|
| GCTG 4 | Very fine | Critical fits, minimal post-machining |
| GCTG 5 | Fine | Precision features, tight assemblies |
| GCTG 6 | Standard | Default for most die cast features |
| GCTG 7 | Relaxed | Non-critical, cosmetic or large parts |
| GCTG 8 | Coarse | Generous allowances, cost-sensitive runs |