Guide

How do you tell core shift from mold mismatch in a casting, and how do you tolerance it?

Short answer

Core shift is a casting defect in which a core moves away from its intended position in the mold before or while the metal freezes, so the internal cavity it forms ends up off-center relative to the outside of the casting. The result is a wall that is too thick on one side and too thin on the opposite side, while the outside of the part can look perfect. Mold mismatch (also called mold shift or a cross-joint) is different: the two mold halves are offset from each other, so the outside of the casting steps at the parting line.

By Saola Manufacturing engineering team · Published October 20, 2026 · Updated October 20, 2026

Both defects end in the same complaint at receiving or at the machine: a bore cleans up on one side and leaves black, unmachined casting skin on the other, or a wall that should be uniform measures thin on one side. The fix depends on which one you have. Core shift lives inside the mold and is controlled by how the core is supported. Mismatch lives at the parting line and is controlled by how the mold or die halves are aligned. This guide is for the design, quality or sourcing engineer who has a casting with uneven walls or a machined feature with no stock left. It covers how to tell the two apart, the ranked causes, how to measure them, which fix fits which cause, and how to put core position and wall thickness on the drawing so the supplier cannot misread the requirement.

On this page

How do you recognize core shift, and how is it different from mold mismatch?

Core shift shows up inside the part: internal walls are thick on one side and thin on the other, but the outside surfaces line up across the parting line. Mismatch shows up outside: a visible step or offset where the two mold or die halves meet.

Look at three things before anyone argues about the cause.

The parting line. Run a fingertip or a straightedge across the parting line on a round or flat outside feature. A step that lines up with the parting line, and is the same on several parts, points to mismatch between the cope and drag in a sand mold or between the two halves of a die. No step, but uneven walls inside, points to the core.
The pattern of wall variation. Core shift moves the whole cavity, so one wall gets thinner by roughly the same amount the opposite wall gets thicker. Half the difference between a matched pair of opposite walls is how far the core sat off center in that direction, so record both walls of each pair, not just the thin one. If the pair adds up to much more or less than the nominal total, the core or the outside is the wrong size as well, which is a different problem. Mismatch moves the features on one side of the parting line relative to the other side, so the error appears only on features that cross the parting line.
The direction. A core that floats in the liquid metal moves upward in the mold, toward the cope. A mismatch moves sideways, in the plane of the parting line. Marking which way the thin wall faces relative to the mold orientation often settles the question.

Die castings have their own version of both. The die halves are guided by pins and bushings, and the internal shapes are formed by steel core pins and slides rather than sand cores. A die-cast part can show a step across the die parting line, a hole that is offset because a slide is out of position, or a long hole that wanders because a slender core pin bent under the injected metal. The diagnostic logic is the same: check whether the error follows the parting line or follows one core.

What causes core shift and mold mismatch?

Core shift is usually caused by a core that is not held firmly enough against the lift and flow of the liquid metal. Mismatch is caused by loose or worn alignment between the mold or die halves.

Ranked by how often they explain uneven walls in a sand or gravity casting:

Core buoyancy with too little support. A sand core is lighter than the liquid metal around it, so it tends to float upward. Core supports called chaplets bridge the gap between the mold and the core. They are made of the same or a similar metal to the casting and have to be sized so they fuse into it: too small and they melt away and let the core move, too large and they do not fuse fully. The upper ones matter more than the lower ones because the core pushes up, and their number is kept low because they can cause defects or weak spots (core and chaplet practice, after Degarmo, Black and Kohser).
Short, loose or worn core prints. Core prints are the extensions that seat the core in the mold. A print that is short for the length of the core, a loose fit between print and seat, or a worn core box lets the core sit off position or rotate before the metal arrives.
A long, slender core supported at one end. A cantilevered core bends under its own weight and under the lift of the metal. The far end shifts more than the supported end, so the wall error grows along the length of the bore.
Gating that hits the core. Metal that enters fast and strikes the core directly can push it sideways or erode the print.
Mold mismatch. Worn flask pins and bushings, a pattern plate that is not aligned with its mate, or a mold half that is handled roughly during closing all offset the cope from the drag. In die casting, wear in the guide pins and bushings or a die that is not seated squarely does the same thing across the die parting line.
Pattern or core box error. If every casting shows the same offset by the same amount in the same direction, the tooling itself may be wrong. Random variation from part to part points to core setting or support instead.

NADCA design guidance notes that dimensions measured across the parting line are altered by flash and parting line thickness, and recommends keeping critical dimensions within one die half to reduce the tolerance needed (NADCA parting line design). The same thinking applies to sand castings: a requirement that ties a feature on one side of the parting line to a feature on the other side carries the mismatch error inside it.

How do you measure core shift and wall thickness on a casting?

Measure the wall at several opposite points around each cored feature, measure the outside step at the parting line, and compare the results on several parts. Opposite walls that change by equal and opposite amounts confirm core shift; an outside step at the parting line confirms mismatch.

Work through this checklist with the castings, the drawing and the casting supplier.

Wall thickness at opposite points. Where a caliper cannot reach, use an ultrasonic thickness gauge from the outside. ASTM E797/E797M covers contact pulse-echo thickness measurement on materials in which sound travels at a constant velocity and gives a resolvable back-wall echo; its scope says it is mainly for flat parts with parallel surfaces and has limited applicability to non-parallel or concentric surfaces (ASTM E797/E797M-21). On a curved cored wall or a coarse-grained casting, confirm the gauge readings against a sectioned part before you trust them. Measure at matched pairs of points, for example at the top and bottom and both sides of a cored passage, at each end of the core, and mark the points on the part so the supplier can repeat them.
Core position on a CMM. Where the cored hole is open, probe it on a coordinate measuring machine and report its axis location relative to the casting datums, not just its size. The CMM inspection guide covers what the report should contain.
Outside step at the parting line. Measure across the parting line on a cylindrical or flat outside feature. Record the direction of the offset as well as its size.
Section or scan one part. For closed passages, cut one casting through the worst section, or use industrial CT where it is available. A section shows wall thickness all the way around the core at once.
Map the lot. Record the thin side on several castings. A constant offset points to the pattern or core box; a varying offset points to core setting, support or mold closing.
Check the machining setup before blaming the casting. If the casting walls are even but a machined bore still has no stock on one side, the machining fixture or datum scheme may be locating on a surface that carries the mismatch. The datums on castings guide explains how to pick datum targets that keep stock even.

How do you fix core shift or mismatch once you know which one it is?

Fix core shift by holding the core better: longer or keyed prints, correctly sized chaplets, and gating that does not strike the core. Fix mismatch by restoring the alignment of the mold or die halves, and fix the drawing so critical features do not depend on both halves.

Use the table to match the fix to the finding. Drawing fixes belong to the drawing owner; tooling and process fixes belong to the casting supplier but often need your approval because they change the casting.

Core shift and mismatch: finding, cause and fix · general industry guidance; drawing references: ISO 8062-3:2023 and ASME Y14.5-2018
What you measured	Likely cause	Supplier fix	Drawing fix
Thin wall always on the cope (upper) side	Core floating	Stronger upper chaplets, longer prints, core weight or anchoring	Allow chaplet locations; add core-print bosses or holes where function allows
Wall error grows along a long bore	Cantilevered or slender core bending	Support both ends; stiffer core; second print	Open the passage at both ends, or add a core-support hole that is plugged later
Same offset, same direction, every part	Pattern, core box or die error	Correct the tooling	None, once the tooling is proven by layout
Random offset from part to part	Core setting, loose prints, gating force	Tighter print fit, setting gauges, move or slow the gate	Wall and core position tolerances that the process can hold
Step at the parting line on outside features	Mold or die mismatch	Replace worn pins and bushings; check closing	Keep critical features in one mold half; state allowed mismatch
Walls even, but no stock on one side of a bore	Machining datums locate on a shifted surface	Re-fixture on agreed datum targets	Datum targets on stable surfaces in one half; machining allowance per ISO 8062-3

Design first where you can. A core supported at both ends shifts far less than one supported at one end. Opening a blind passage to the outside, or adding a hole that is plugged after casting, gives the foundry a second print. If the part cannot have such an opening, accept that the foundry will need chaplets and say where they are allowed, because a chaplet leaves a fused insert in the wall that may matter for pressure-tight parts.

Stock as a buffer. If a cored bore is finish-machined, the casting needs enough stock to clean up even with the core at the limit of its position tolerance. The machining allowance guide covers how the allowance is set; the point here is that the allowance and the core position tolerance have to be chosen together.

How should the drawing tolerance core position and wall thickness?

Tie the cored features to casting datums with a position or profile tolerance, give a minimum wall where function depends on it, and state how mismatch is controlled. A size tolerance on the wall alone does not say which side may be thin.

Use casting datums in one mold half. Pick datum targets on stable surfaces that the foundry, the machinist and the inspector can all locate on. ASME Y14.5 sets the rules for datum targets and datum reference frames (ASME Y14.5-2018), and ASME Y14.8 adds conventions for castings and forgings. Keep the datum targets on one side of the parting line so mismatch does not move the datums.
Tolerance the core, not just the hole size. Give the cored passage or bore a position tolerance, or a profile of a surface tolerance for an irregular passage, relative to those datums. That tolerance is the real limit on core shift. The profile tolerance guide explains how to apply it.
State a minimum wall where it matters. For pressure walls and load paths, add a minimum wall thickness note at the critical section and say how it is checked, for example by ultrasonic gauge at marked points. Uniform wall tolerance does not protect you if the core may sit at one extreme.
Say how mismatch is controlled. ISO 8062-3 applies general casting tolerances and machining allowance grades through a note in or near the title block (ISO 8062-3:2023). If you use it, name the tolerance and machining allowance grades in the note, and check in the edition you cite how it treats mismatch and wall thickness. Its general geometrical tolerances cover straightness, flatness, roundness, parallelism, perpendicularity, symmetry and coaxiality, and the standard states that it does not cover other position tolerances, so a cored bore located from casting datums still needs its own position or profile tolerance. If a feature cannot accept the general mismatch, put a specific limit on that feature.
Mark where chaplets are allowed. On pressure-tight or cosmetic parts, show the zones where chaplets may be placed, or say that none are allowed in a zone.
Machining stock on cored bores. Show the stock on the casting drawing and make sure it covers the core position tolerance.

On the RFQ, send the measurement map from the problem lot, with the thin side marked, and photos of the parting line. A supplier who sees which way the error runs can propose a print, chaplet or alignment fix before the next lot is poured.

Who does what on a Saola order with a core shift problem?

Castings are made by partner plants we manage, and cored bores and faces are machined at our lead CNC plant, T.K Viet Nam Precision Company Limited in Vietnam. The quote names the production site and country of origin for each step.

Because the castings arrive at the lead CNC plant for machining, a bore with no stock on one side is caught at the machine. The plant measures it with the height gauge or the vision measuring machine where those fit the feature, and the findings go back to the casting partner and to you with photos and the thin-side location, so the decision on prints, chaplets or the drawing can be made with data. See die casting and sand casting for the partner-plant programs, and CNC milling and CNC turning for the machining. If your part needs CMM reports, ultrasonic wall checks or CT, say so on the RFQ and we will confirm before quoting whether it can be arranged and where. ISO 9001 certification is in progress, not held. Send the drawing, the wall thickness map and the parting line photos through the quote form.

Frequently asked

Is core shift the same as mold shift?

No. Core shift moves an internal cavity relative to the outside of the casting. Mold shift, or mismatch, offsets the two mold halves, so outside features step at the parting line. A casting can have both.

Can core shift happen in die casting?

Die casting uses steel core pins and slides rather than sand cores, so it does not float. But a slide can sit out of position and a long, thin core pin can bend under the injected metal, which gives the same offset hole or uneven wall.

Why does a bore clean up on one side and not the other?

Either the core shifted, so the cast hole is off-center, or the machining datums locate on a surface that carries mismatch, so the tool is off-center. Measure the casting walls first; if they are even, look at the datums.

Is a wall thickness tolerance enough to control core shift?

Not on its own. A wall tolerance limits each wall, but a position or profile tolerance on the cored feature relative to casting datums is what controls where the core sits. Add a minimum wall where function depends on it.

Can a casting with core shift be used?

Sometimes. If the thin wall still meets the minimum wall and the machined features clean up, the part may be acceptable as a deviation approved by the engineer who owns the function. If the wall is below the minimum or the bore has no stock, it is nonconforming.

Sources

ISO 8062-3:2023, GPS: Dimensional and geometrical tolerances for molded parts, Part 3: General dimensional and geometrical tolerances and machining allowances for castings (scope), ISO, 2023
ASME Y14.5-2018, Dimensioning and Tolerancing, ASME, 2018 (reaffirmed 2024)
ASME Y14.8-2022, Castings, Forgings, and Molded Parts, ASME, 2022
ASTM E797/E797M-21, Standard Practice for Measuring Thickness by Manual Ultrasonic Pulse-Echo Contact Method (scope), ASTM International, 2021
Parting line design guidance, NADCA, accessed October 2026
Core (manufacturing): core prints and chaplets, citing Degarmo, Black and Kohser, Materials and Processes in Manufacturing, 2003, accessed October 2026