Bolt-Hole Position Tolerance: Why Center Distances Aren't Enough
A bolt-hole pattern can pass every center-distance check and still sit in the wrong place. Pairwise spacing stays the same when the whole pattern shifts or rotates, but a position requirement controls where each hole sits relative to the datums on the drawing. To judge a hole-pattern inspection, match the position callout, the datum frame, the hole sizes and any modifiers to per-hole results. A bolt that goes in during trial assembly does not by itself show that the pattern conforms.
What a position callout controls
KEYENCE, a measurement-equipment maker, describes true position as finding how far a hole, slot or bore deviates from its blueprint position in the X, Y and Z axes. It says a position tolerance sets the range the feature can occupy and still assemble successfully, and that the tolerance zone may be square or circular depending on the feature. Its manual method names the ingredients: the part's blueprint, a fixture that holds the part, and two or three datums established for reference.
The datums are the point. A position result has meaning only against the datum frame the drawing names. ASME describes Y14.5-2018 (R2024) as the authoritative guideline for the GD&T design language, and lists datum references and degrees of freedom, and composite position tolerances, among its important changes. A composite callout can place a pattern as a group in one frame and control the holes' relationship to each other in another, so the callout's exact form decides what evidence is needed.
A shifted pattern with every spacing correct
This is a hypothetical illustration with assumed numbers, not a tolerance for any part. Take a four-hole rectangular pattern whose holes are drawn at X = ±50 and Y = ±40 about the datum origin. Suppose the holes were produced at X = −47 and X = +53, with Y unchanged.
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The distance between the two holes in each row is still 100.
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The distance between the two rows is still 80.
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Every diagonal is unchanged.
A check of center distances reports no difference. Against the datum frame, however, all four holes are displaced 3 in X. Whether that passes depends on the position tolerance and modifiers in the callout. The same logic applies to rotation: turn the pattern about its center and the spacings are untouched while each hole moves off its drawn location.
Center-distance checks still have a use. They show the internal relationship of the holes, which matters when a composite callout controls it separately. They do not replace the datum-based result.
Build a callout-to-per-hole coverage record
Use the record below to check a report against the drawing. It is a recommended review aid, not a form required by the standard.
| Field | What to confirm | If missing or unclear |
|---|---|---|
| Callout | The full position control, including modifiers and whether it is a single or composite callout | Ask the design owner for the drawing's meaning |
| Datum frame | Which datum features, in which order, define the coordinate system | Treat the result as unresolved |
| Controlled holes | Every hole the callout covers, identified by number or location | Missing holes mean incomplete coverage |
| Hole size | Measured size of each hole, kept as a separate result | Do not merge size and position |
| Per-hole results | The reported position result for each hole, with its coordinates or deviation | A single summary hides which hole is off |
| Datum establishment | How the part was located to build the frame in the report | A different frame is a different measurement |
| Tolerance zone and modifiers | The zone shape and any modifier applied in the evaluation | Do not assume extra tolerance is allowed |
| Result and uncertainty | The reported conclusion and stated uncertainty | Record gaps as open |
KEYENCE's CMM article is sales material. It does not establish every tolerance zone, automatic release of a part, or that no training is needed to interpret a result, and the drawing's callout governs. Do not use a generic position formula or assume bonus tolerance applies unless the callout and modifiers allow it.
Request pattern evidence
Ask for per-hole position results against the datum frame in the callout, with hole size reported separately and every controlled hole accounted for. Ask what datums were used to locate the part and how the reported frame matches the drawing. If the callout is ambiguous, send the question to the design owner before judging the result. General checks of a CMM report against a drawing are covered in reading a CMM report against the drawing. Torque and any corrective reaming are separate matters and outside this article.
For a part from KTSU, provide the drawing, revision and measurement report you hold when asking what documentation is available. The public excavator undercarriage parts collection identifies the product family. It does not establish a hole-position capability or a tolerance level for any part.
The request is complete when the callout, datum frame and each controlled hole are tied to a reported result, hole size is kept apart, and any unresolved datum interpretation is named.