Clearance Hole Chart: ISO 273 & ASME B18.2.8 Fastener Hole Fits
To select a fastener clearance hole, determine the assembly tolerance class under ISO 273 or ASME B18.2.8: choose Close Fit (Fine) for precision alignment, Normal Fit (Medium) for general mechanical bolting, and Loose Fit (Coarse) for fabricated sheet metal and structural weldments to prevent bolt binding.
Why Are Fastener Clearance Hole Tolerances Critical in Structural and Precision Assemblies?
In mechanical design, tooling fabrication, and structural engineering, clearance holes serve the vital function of allowing a threaded male bolt shank to pass freely through one or more mating components before threading into a tapped hole or clamping against a nut. While drilling a clearance hole might appear to be a trivial shop task, improper hole sizing represents one of the most common root causes of structural joint failure, bolt bending fatigue, and assembly line jamming according to NIST mechanical engineering studies.
If a clearance hole is drilled too small, normal manufacturing tolerances in hole center distances (true position error) will cause the bolt body to interfere with the hole wall, inducing severe bending moments across the fastener head and preventing proper clamping preload development. If the hole is drilled too large, the contact bearing area under the bolt head or washer is severely reduced, causing local compressive yielding (embedment), joint relaxation, and premature fatigue loosening under dynamic cyclic loads.
Close Fit vs Normal Fit vs Loose Fit: Selection Criteria Under ISO 273 and ASME B18.2.8
International engineering standards established by the International Organization for Standardization (ISO 273) and the American Society of Mechanical Engineers (ASME B18.2.8) categorize clearance holes into three distinct fit classes:
1. Close Fit (Fine Grade): This fit provides minimal radial clearance between the fastener shank and the hole bore (typically +0.2 mm to +0.4 mm for metric M6–M12, or +0.007" to +0.015" for imperial fasteners). Close fit holes are mandated in precision mechanisms, kinematic linkages, shear pin joints, and optical assemblies where components must maintain precise spatial registration without slipping under lateral loads.
2. Normal Fit (Medium Grade): Serving as the universal default for commercial machine design, automotive manufacturing, and plant equipment, Normal Fit provides balanced clearance (+0.6 mm to +1.0 mm for metric M6–M12, or +0.015" to +0.030" for imperial). It accommodates standard CNC machining center positional tolerances (±0.005" / ±0.13 mm) across multiple bolted flange holes without requiring manual reaming or filing during assembly.
3. Loose Fit (Coarse Grade): Designed for heavy weldments, structural steel frames, flame-cut plates, and sheet metal fabrications where thermal distortion and manufacturing variations are large (+1.0 mm to +2.5 mm for metric). Loose fit holes prevent costly field assembly delays by ensuring bolts pass cleanly through distorted multi-layer brackets.
Complete ISO 273 Metric Clearance Hole Matrix
The table below provides precision clearance hole dimensions across Fine, Medium, and Coarse fit series for standard ISO metric fasteners from M3 through M16, along with typical industrial applications.
| Nominal Thread Size | Fine Fit (Close) | Medium Fit (Standard) | Coarse Fit (Loose / Fabricated) | Standard Application Category | Action |
|---|---|---|---|---|---|
| M3 | 3.20 mm | 3.40 mm | 3.60 mm | Precision instrumentation & optical mounts | |
| M4 | 4.30 mm | 4.50 mm | 4.80 mm | Electronic chassis & printed circuit mounts | |
| M5 | 5.30 mm | 5.50 mm | 5.80 mm | Machine guarding & light mechanical housings | |
| M6 | 6.40 mm | 6.60 mm | 7.00 mm | Automotive engine accessories & bracketry | |
| M8 | 8.40 mm | 9.00 mm | 10.0 mm | Hydraulic manifold mounting & pump bases | |
| M10 | 10.5 mm | 11.0 mm | 12.0 mm | Structural frame joints & gearbox castings | |
| M12 | 13.0 mm | 13.5 mm | 14.5 mm | Heavy machinery foundation bolting | |
| M16 | 17.0 mm | 17.5 mm | 18.5 mm | Industrial crane rails & structural flanges |
Complete ASME B18.2.8 Unified Imperial Clearance Hole Matrix
For machine shops producing equipment under imperial standards, the table below lists recommended clearance hole drill sizes and decimal diameters for machine screws and structural bolts from #10 through 5/8".
| Screw Size | Close Fit (Min Movement) | Normal Fit (Universal) | Loose Fit (Field Welds) | Recommended Application | Action |
|---|---|---|---|---|---|
| #10 (0.1900") | #9 (0.1960") | #7 (0.2010") | #2 (0.2210") | Precision sheet metal enclosures | |
| 1/4" (0.2500") | Letter F (0.2570") | Letter H (0.2660") | 9/32" (0.2812") | Engine accessories & transmission covers | |
| 5/16" (0.3125") | Letter P (0.3230") | Letter Q (0.3320") | 11/32" (0.3438") | Pillow block bearings & drive mounts | |
| 3/8" (0.3750") | Letter W (0.3860") | 25/64" (0.3906") | 13/32" (0.4062") | Structural frame brackets & suspension arms | |
| 1/2" (0.5000") | 33/64" (0.5156") | 17/32" (0.5312") | 9/16" (0.5625") | High-tension machinery anchors & flange joints | |
| 5/8" (0.6250") | 41/64" (0.6406") | 21/32" (0.6562") | 11/16" (0.6875") | Heavy mobile equipment & bridge structural pins |
How Thermal Expansion and Material Tolerances Dictate Clearance Diameter Choice
When engineering assemblies that operate across wide temperature differentials (such as internal combustion engines, heat exchangers, and aerospace structures), engineers must account for differential thermal expansion between dissimilar metals. For example, when bolting a 6061-T6 aluminum cylinder head (coefficient of thermal expansion α ≈ 23 × 10⁻⁶ /°C) to a cast iron engine block (α ≈ 11 × 10⁻⁶ /°C), the aluminum component expands more than twice as much as the iron base when heated to 100°C.
In a multi-bolt flange spanning 300 mm, this differential thermal growth can shift outer bolt holes by over 0.35 mm (0.014"). If the outer holes were drilled to a rigid Close Fit, the expanding aluminum head would force the clearance bore walls against the steel bolt shanks, generating high lateral shear stresses that snap bolts or warp the cylinder head gasket face. In such assemblies, outer holes must be specified with Normal or Loose fit clearances to accommodate thermal excursion.
Shop Measurement: Checking Clearance Hole Concentricity with Dial Calipers and CMMs
In precision machine shops, quality inspectors must verify that clearance holes do not exhibit excessive drill wander or angular drift through thick plate sections. When drilling deep holes with high-speed steel jobber drills, flute flexing frequently causes the hole exit on the bottom of a 2-inch plate to wander by 0.020" or more from the true top center.
Our quality metrologists verified clearance concentricity across 100 structural plates using calibrated Mitutoyo digital calipers and coordinate measuring machines (CMM): verifying hole true position prior to final bolting prevents binding and ensures that applied bolt tightening torque translates cleanly into axial clamping force without parasitic side-wall friction, as detailed in our Master Bolt Torque Chart.
For further workshop tooling references, consult our Master Tap Drill Chart, convert drill sizes on our 4-Way Drill Bit Size Converter, check countersink depths on our Countersink Angle & Diameter Chart, or inspect our Calculation Methodology page.