ISO Fit Class G6 Tolerance Limits: How To Read Them
When a drawing callout says "Ø9.0 G6," it looks simple. Nine millimeters, G6 tolerance. But to manufacture it, inspect it, and report it on an FAI, you need the actual limits: the minimum and maximum acceptable bore diameter in millimeters, to the micron.
Getting those limits requires looking up two things: the fundamental deviation for the "G" letter code, and the tolerance grade for the "6" number. Then you combine them to get upper and lower limits.
This guide gives you the complete ISO fit class G6 tolerance limits for every bore size you are likely to encounter.
How ISO Fit Classes Work
An ISO fit class has two components:
The letter (G in this case) defines the fundamental deviation, which is the position of the tolerance zone relative to the nominal size. For holes (internal features), capital letters are used. "G" places the tolerance zone slightly above the nominal size, meaning the bore will always be slightly larger than the nominal dimension.
The number (6 in this case) defines the tolerance grade (IT grade), which is the width of the tolerance zone. IT6 is a precision grade commonly used for bearings, bushings, and close-fitting aerospace components.
ISO Fit Class G6 Tolerance Limits for Common Bore Sizes
Here are the actual limits for G6 bores in the diameter ranges most common in aerospace:
| Nominal Bore | Lower Deviation | Upper Deviation | IT6 Grade | Lower Limit | Upper Limit |
| Ø3 mm | +0.002 | +0.010 | 0.008 | 3.002 | 3.010 |
| Ø5 mm | +0.004 | +0.012 | 0.008 | 5.004 | 5.012 |
| Ø6 mm | +0.004 | +0.012 | 0.008 | 6.004 | 6.012 |
| Ø8 mm | +0.005 | +0.014 | 0.009 | 8.005 | 8.014 |
| Ø9 mm | +0.005 | +0.014 | 0.009 | 9.005 | 9.014 |
| Ø10 mm | +0.005 | +0.014 | 0.009 | 10.005 | 10.014 |
| Ø12 mm | +0.006 | +0.017 | 0.011 | 12.006 | 12.017 |
| Ø15 mm | +0.006 | +0.017 | 0.011 | 15.006 | 15.017 |
| Ø18 mm | +0.006 | +0.017 | 0.011 | 18.006 | 18.017 |
| Ø20 mm | +0.007 | +0.020 | 0.013 | 20.007 | 20.020 |
| Ø25 mm | +0.007 | +0.020 | 0.013 | 25.007 | 25.020 |
Critical note: Both the lower and upper deviations are positive for G-class holes. This means the bore is always larger than nominal. A Ø9.0 G6 bore must measure between 9.005 and 9.014mm. A bore that measures exactly 9.000mm is out of tolerance on the low side.
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Worked Example: Ø9.0 G6
Starting values:
- Nominal diameter: 9.000 mm
- Diameter falls in the "over 6 mm to 10 mm" range
- Fundamental deviation for G in this range: +0.005 mm (this is the lower deviation)
- IT6 tolerance for this range: 0.009 mm
- Upper deviation = lower deviation + IT6 = 0.005 + 0.009 = 0.014 mm
Final limits:
- Lower limit: 9.000 + 0.005 = 9.005 mm
- Upper limit: 9.000 + 0.014 = 9.014 mm
- Total tolerance band: 0.009 mm (9 microns)
How to Express G6 in an FAI
The AS9102 Form 3 row for a Ø9.0 G6 bore should show:
| Field | Value |
| Requirement | 9.0 G6 +0.014/+0.005 mm |
| Nominal | 9.0 |
| Tol + | 0.014 |
| Tol - | 0.005 |
| Upper Limit | 9.014 |
| Lower Limit | 9.005 |
| Inspection Method | Inside micrometer / Bore gauge / Air gauge |
| Designator | Minor (or Major if on a critical load path) |
| Notes | [ISO fit class G6 expanded] |
A common mistake: expressing the tolerance as "+0.014/-0.005." This implies the lower deviation is negative (below nominal), which would give a lower limit of 8.995mm. That is wrong. Both deviations are positive. The correct expression is "+0.014/+0.005" or simply showing the limits directly.
Manufacturing Implications of G6
A 9-micron total tolerance band on a bore is tight. Here is what it means for your shop:
Standard drilling will not achieve G6. A twist drill produces holes with typical size variation of ±0.05mm (50 microns), which is more than 5x the entire G6 tolerance band.
Reaming gets close. A precision reamer can hold ±0.01mm (10 microns) in ideal conditions, which is just barely within the 9-micron band. But tool wear, chip evacuation, and material springback can push you out.
Honing or lapping is the reliable method. For production work, most shops rough-drill, ream to near-size, and then hone to final dimension. Honing allows incremental material removal with real-time bore measurement, ensuring you land within the 9-micron window.
Inspection requires precision instruments. Calipers are not sufficient. Use an air gauge (resolution to 0.001mm) or a calibrated bore gauge. CMM with a bore probe is also acceptable but slower.
Where G6 Appears in Aerospace
G6 is most commonly found on:
- Bearing bores in structural brackets and fittings
- Hinge pin bores in flight control surfaces
- Bushing seats in landing gear and actuator components
- Alignment bores that mate with precision-ground shafts
On a European aerospace seat bracket we analyzed, two G6 bores appeared in different cross-sections of the same part, each with concentricity and parallelism GD&T callouts referencing multiple datums.
The bores needed to be coaxial to within 0.1mm while maintaining G6 size limits. This combination of size tolerance (9 microns) plus geometric tolerance (100 microns concentricity) requires a carefully planned machining sequence: establish datums first, then rough-bore, then finish-bore in the same setup to maintain concentricity.
Mavlon Expands Fit Classes to Actual Limits
Mavlon recognizes ISO fit class callouts (G6, H7, H6, f7, g6, etc.) in drawing extractions and automatically expands them to actual upper and lower limits based on the nominal diameter and the ISO 286 tolerance tables.
The FAI row for a Ø9.0 G6 bore shows the expanded limits (9.005 to 9.014), the correct inspection method (bore gauge or air gauge), and a note indicating the tolerance was derived from the ISO fit class.
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