General tolerances and AS9102 Form 3
Most drawings do not tolerance every dimension. A title-block note sets a default, and each untoleranced dimension inherits it. That inheritance still has to appear on Form 3, and the number that goes in the tolerance columns depends on how the note is written. This page covers the three common forms of the note and where it stops applying.
What the note actually does
A block such as UNLESS OTHERWISE SPECIFIED: X.X ±0.5, X.XX ±0.25, X.XXX ±0.125 is a set of defaults. A dimension printed as 12.7 with no tolerance of its own takes the first line. A dimension printed as 12.70 takes the second. The number of decimal places is the selector, not the size of the part.
The dimension is still a characteristic. It still gets its own row on Form 3, its own measurement and its own verdict. The note only supplies the limits that were missing from the drawing.
Three forms of the note
| Form of the note | How the tolerance resolves | What to record |
|---|---|---|
| Decimal-place block | Selected by the number of decimal places printed on the dimension: 12.7, 12.70 and 12.700 can each take a different tolerance. | The value from the matching line, split into plus and minus. |
| Size-banded block | Selected by the nominal size: for example a tolerance for 0.5 to 3 mm and a different one for 3 to 6 mm. | The value for the band the nominal falls in. The edge cases are where errors happen. |
| Referenced standard class | The note names a class, such as a general tolerance standard at a stated grade. The grade and the nominal size together give the limits. | The limits from that class table for that nominal, in the drawing’s units. |
Which form your drawing uses is a property of the drawing, not of AS9102. Read the note block as written and take the class or grade it names; do not substitute a different default.
Where the general note does not apply
Three cases where inherited tolerance is the wrong answer:
- A dimension with its own tolerance. The specific tolerance wins over the general note. Recording both is worse than recording either, because the reviewer cannot tell which one was applied.
- A basic dimension. A boxed, theoretically exact dimension carries no tolerance of its own by design. Its limits come from the feature control frame that references it, so it is accounted for through that geometric control, not through the default block.
- A characteristic governed by a referenced specification. A finish, a thread class or a heat-treat requirement defined by a cited standard is judged against that standard, not against a size tolerance.
Recording an asymmetric or one-sided tolerance
Plenty of general notes are not bilateral. A default of plus 0.2 over minus 0.1, or a hole callout of plus 0.05 over zero, has two different numbers and the form has two columns to hold them. Writing the larger number in both columns turns a one-sided limit into a symmetric one and changes which parts pass.
A worked row, for a dimension printed as 12.70 with a general note of X.XX ±0.25 and a measured value of 12.66:
| Nominal | Tol + | Tol − | Actual | Result |
|---|---|---|---|---|
| 12.70 | 0.25 | 0.25 | 12.66 | Pass, inside 12.45 to 12.95 |
The band-edge trap
With a size-banded note, the tolerance changes at a boundary. A nominal of exactly 3.00 may belong to the lower band or the upper band depending on whether the band is written as inclusive at the top or at the bottom. Two rows either side of the boundary can correctly carry different tolerances, and a reviewer comparing them will assume one is wrong unless the drawing settles it. When a dimension sits exactly on a boundary, record the limits you applied and be prepared to show the note that produced them.
Why this matters more than it looks
A general tolerance applied one band too wide can turn a real nonconformance into a pass. That is not a formatting error; it is a wrong acceptance decision that is now in the product record. Of everything on Form 3, the inherited tolerances are the easiest to get wrong and the hardest to notice afterwards, because every row still looks complete.
Frequently asked questions
Should an untoleranced dimension get its own Form 3 row?
Yes. It is a design characteristic and must be accounted for. The general note supplies the limits, not the exemption.
What if the drawing has no general tolerance note at all?
Then the drawing is missing something a dimension needs, and the question goes back to whoever controls the drawing. Do not invent a default; record what the drawing states.
Does the general note apply to GD&T characteristics?
No. Geometric tolerances come from the feature control frame. A flatness or position callout carries its own value and datums, and the title-block size tolerance is not a substitute for it.
How do I record a dimension measured in a different unit?
Convert to the drawing’s units before recording, so the nominal, the tolerance and the actual are all in the same system. A mixed-unit row cannot be checked without arithmetic.
Where can I see a filled example?
The free worked example on this site uses exactly this pattern, including inherited tolerances and one recorded failure: Form 3 Excel template.
Sources
- IAQG — AS9102 forms and supporting form instructions
- SAE International — AS9102C, Aerospace First Article Inspection
- Net-Inspect — Update Form 3 data with an Excel template
Standards are revised. Always work from the current published edition of AS9102 from SAE International. This page is orientation only and is not a substitute for the standard.