Die Casting Tolerance Standards: CT5–CT7 Explained
How casting tolerance grades work, which factors widen them, and how to write a drawing a foundry can actually hit.
Published: 2026-10-04 4 min read
Die casting tolerances confuse buyers because they are quoted as CT grades rather than the millimeter values engineers are used to on machining drawings. This guide explains what CT5–CT7 actually mean, why castings cannot hold machining tolerances, and how to write a tolerance scheme a foundry can deliver without inflating your tool cost.
What CT Grades Are
CT grades come from the casting tolerance system standardized as ISO 8062 (and its national adoption GB/T 6414), running from CT1 (tightest) to CT16 (widest). Two things make them different from machining tolerances:
- The grade is not a fixed value. The permitted deviation depends on the nominal dimension of the feature (in size bands), the material group, and whether the dimension is measured in the die-opening direction.
- One grade can cover a whole part. "CT6 per ISO 8062" in the title block tolerances every as-cast feature automatically, each against its own size band, no per-feature table needed.
What Production Die Casting Typically Achieves
| Process | Typical grade range | Rough order of magnitude* |
|---|
| Zinc hot chamber | CT4–CT6 | ~±0.1–0.5 mm depending on feature size |
| Aluminum cold chamber | CT5–CT7 | ~±0.2–0.9 mm depending on feature size |
| Large / complex features | CT6–CT8 | Widens further with size and sliding elements |
| CNC machining (reference) | IT6–IT7 | ~±0.005–0.025 mm on small features |
*Indicative ranges for orientation only, the authoritative values are the dimension tables in the current standard edition, applied per feature size band and material. Any specific capability claim should be confirmed with the supplier against their measured capability data, not assumed from a grade label.
The headline for buyers: a production aluminum die casting typically lands one to two orders of magnitude looser than a machined feature. That is normal physics, not sloppy workmanship.
What Controls Casting Dimensional Accuracy
Die thermal behavior
The die heats every shot and is cooled between them. The cavity is dimensionally alive, its size at ejection differs from its cold-measured size, and the drift is managed by cooling design, not eliminated.
Solidification shrinkage
Metal shrinks as it solidifies and cools. Thick sections cool slower than thin ones, so the same part distorts non-uniformly, the origin of bowing in long flat parts.
Process variables
Shot velocity, intensification pressure, die temperature and release agent film all shift fill and ejection behavior. Modern cells hold them tightly, but never perfectly.
Beyond the process itself, six design-side factors decide how tight a feature can realistically be:
| Factor | Effect on tolerance |
|---|
| Nominal feature size | Larger features get wider bands at the same CT grade |
| Wall thickness uniformity | Uniform walls shrink predictably; thick-thin transitions distort |
| Measurement direction | Dimensions across the parting line / die-opening direction are wider than parallel to it |
| Draft angle | Required for ejection (0.5–2°/side on aluminum); draft-direction features carry wider tolerance |
| Slides and side actions | Every sliding element adds a witness line and widens tolerance on faces it forms |
| Part complexity | More features and cores mean more stacked variation |
Writing Tolerances a Foundry Can Deliver
1. Title-block grade
Specify one general casting grade ("not otherwise specified: CT6 per ISO 8062") for as-cast surfaces. Resist tolerancing every feature individually, it doubles drawing review time and buys nothing.
2. Tight features individually
Call out only the features that genuinely need it: bores, sealing faces, mounting pads. Each one gets confirmed during tool design review against its casting direction.
3. Machined faces with allowance
Show finished dimensions with normal machined tolerance plus a "machine from casting" note. The foundry leaves 0.8–1.5 mm allowance on those faces, agreed at DFM review.
Three more practices prevent the classic disputes: locate the parting line away from sealing and mating faces (state the constraint in the drawing notes); require deflashing and hole burr removal in the casting scope; and request a first-article dimensional report (a full measured report on initial samples), before production release, so both sides agree on the baseline before thousands of parts exist.
How FHY Approaches Tolerance
During FHY's free DFM review, the tolerance discussion happens before the tool is cut: which features need machining, where the parting lines should sit, how much allowance each machined face carries, and which CT grade covers the as-cast surfaces. The goal is a drawing that reflects the physics of casting instead of fighting it, and a part whose tolerance stack-up closes on the first production run. Send the 3D model and the mating-part requirements, and the review returns a marking-ready tolerance scheme.
Frequently Asked Questions
1. What does CT5 mean on a die casting drawing?
CT5 is a dimensional tolerance grade from the casting tolerance system standardized as ISO 8062 and GB/T 6414, where grades run from CT1 (tightest) to CT16 (widest). The number is not a fixed millimeter value – the actual tolerance depends on the nominal dimension of the feature, the casting material group, and the process. On a typical small aluminum die cast feature, CT5 corresponds to roughly ±0.2–0.4 mm; the same CT5 grade on a 160–250 mm feature widens to roughly ±0.8–1.0 mm. Always read CT grades against the dimension table in the standard, never as a single global number.
2. Why are die casting tolerances wider than CNC tolerances?
Three physical reasons. First, the metal solidifies inside a steel die that itself heats and cools each shot, so the die expands with every cycle and the cavity is never dimensionally identical shot to shot. Second, the part is ejected hot and shrinks non-uniformly as it cools, especially across wall-thickness changes. Third, the process has stacked variables – shot velocity, intensification pressure, die temperature, release agent – that no machine holds perfectly. CNC removes material from a stable solid billet, so it simply has fewer sources of variation.
3. How should I specify tolerances on a die casting drawing?
Three zones work best. (1) As-cast surfaces: specify a general casting tolerance grade in the title block – "dimensions not otherwise specified: CT6 per ISO 8062" – instead of tolerancing every feature. (2) Critical mating faces: call out specific tight tolerances individually, and confirm with the foundry they are achievable in the casting direction the feature lies in. (3) Machined faces: show the finished dimension with a normal machined tolerance and a note "machine from casting" – the foundry then knows to leave allowance. Blanket-tolerancing the whole drawing to CT5 is the most expensive mistake in die casting.
4. Which dimensions must be machined after casting?
Anything that needs an IT7-or-tighter fit, a threaded hole, a sealing face, or a bearing bore. Typical examples: bearing bores and shafts, O-ring and gasket faces, mounting faces that must sit flat, tapped holes, and dowel pin holes. As a rule of thumb, features demanding tighter than about ±0.1 mm are machining candidates. Mark them clearly, and mark the allowance so the foundry leaves material rather than casting to final size.
5. Who decides where the parting line goes?
The tool designer, but the buyer should constrain it. The parting line leaves a visible witness line and a small mismatch (flash) on every part, so it should sit where cosmetic and functional damage is lowest – typically on a non-mating edge or a flange. Complex parts may need multiple parting lines or side-action slides, each adding a witness line and widening tolerance on the affected faces. Send the assembly context with the model so the designer places parting lines away from sealing and mounting faces.
6. Do draft angles affect tolerance?
Yes, significantly. Draft (typically 0.5–2° per side on aluminum) is needed for ejection, and dimensions measured along the draft direction – especially the cavity-to-core direction – carry wider tolerance than dimensions parallel to the parting line. Features that must be tight should be oriented perpendicular to the die-opening direction where possible, and deep draft angles on tall cores both widen tolerance and increase wear. This is why tool design review matters before the tool is cut.
7. Is CT7 bad quality?
No – CT7 is a legitimate, commonly achieved grade for production aluminum die castings, especially on medium and large features. The quality question is not "what grade can the foundry claim" but "what grade does the specific feature need." Over-specifying CT5 on features that function fine at CT7 inflates tool cost, cycle time and scrap rate. The right target is the loosest grade that still assembles – determined by the mating part, not by habit.
8. How much machining allowance should a casting leave?
A common practice on aluminum die castings is 0.8–1.5 mm on critical machined faces – enough to clean up the cast skin and any distortion, not so much that cycle time explodes. Shallow features and thin sections need less; large faces that may bow after casting need more. The exact allowance should be agreed during DFM review, because it affects both tool design (which faces get extra stock) and machining cost (which is proportional to stock removed).
9. Do flash and burrs affect assembly tolerances?
They consume tolerance stack-up. Flash at parting lines and burrs around holes add material exactly where parts must fit together, and an otherwise in-tolerance casting can fail assembly because of it. Standard practice is to specify trimming (deflashing) of parting lines and hole burr removal as part of the casting scope, not as an afterthought. On threaded and press-fit features, burr removal is not optional.