Bolt Clearance Hole Sizes, Inch (ASME B18.2.8) — Normal, Close, Loose
ASME B18.2.8 clearance hole chart for inch fasteners: normal, close and loose fit classes with nominal drill size and hole diameter min/max per screw size from #0 up the inch ladder.
Last updated: 2026-11
Specifications
Clearance holes are where tolerance discipline pays its rent: too tight and the bolt gets forced (and the hole drills oversized anyway), too loose and the joint squirms. The ASME B18.2.8 chart collected here tabulates the three fit classes per inch screw size — Normal (the default for standard assembly), Close (where alignment matters and drilled tooling is accurate), Loose (over punched holes, dirty work, insulation layers) — each with the nominal drill size and the hole diameter min/max band. The ladder opens at #0 (normal: #48 drill, 0.076-0.082 in hole) and runs up the inch series; a 1/2-in fastener takes a 17/32 normal drill with a 0.562/0.578 band. The drill-size column is the practical takeaway: it names the bit, the min/max band is the acceptance check on the drilled result.
| Standard / scope | ASME B18.2.8 clearance holes for inch fasteners — fit classes Normal, Close, Loose; per class nominal drill size and hole diameter min/max |
|---|---|
| Fit classes | Normal (standard assembly default), Close (accurate alignment, drilled tooling), Loose (punched/oversize conditions, multi-layer stacks) |
| Reading the columns | Per size: Nominal Drill Size (the bit to use) + Hole Diameter Min./Max. (the acceptance band) per fit class — stacked header rows carried as printed |
| Ladder start | #0: normal #48 (0.076/0.082), close #51 (0.067/0.071), loose 3/32 (0.094/0.104) — the ladder continues up the inch series as tabulated |
| Not tabulated | Metric fastener chart (separate page), csk/counterbore combinations — per RFQ |
Materials
- The fit class is a joint decision, not a drill preference: specify it on the fabrication drawing and hold the drilled part to the min/max band here — a hole out of the loose band's max is a rejected hole, not a loose bolt. Punching produces oversize, drifted holes (loose-class territory); CNC drilling holds close-class. When the drawing is silent, normal fit is the standard's default — and stating "ASME B18.2.8 normal fit" on the RFQ removes the ambiguity that turns into reamed holes and shims on site
Ordering & documentation
Documents
Shipments are backed by Mill Test Certificate (MTC), Dimensional Report, Salt Spray Test Report on request, with full batch traceability from raw material to finished product. Quality management follows the owner-confirmed compliance list: ISO 9001:2015, RoHS, REACH. Exact document sets are confirmed on the order.
Quotation
- The fastener family (Bolt Clearance Hole Sizes, Inch (ASME B18.2.8) — Normal, Close, Loose) plus the standard designation from your drawing — DIN / ISO / JIS / ASTM / ANSI, or a drawing for custom parts.
- Size (diameter and length), thread type (metric or inch), property class or grade, and coating or finish.
- Quantity per size and your target delivery date.
- Destination country, and any required documents (for example a mill test certificate) so the quote includes them.
Sample & supply
Minimum order quantity is 10,000 pieces. Stock items run 3–7 days. Quantities or variants outside the standard range are confirmed at quotation.
Storage & shipping
Parts are packed for export and shipped factory-direct to buyers in Europe, Asia, the Americas and the Middle East. Store fasteners dry and protected until installation, and keep coated parts in their packaging until use; specific packing and preservation requirements can be stated on the order and confirmed at quotation.
Frequently asked questions
How do I choose between normal, close and loose fit for a clearance hole?
Start from how the hole is made and what the joint needs. Normal fit is the default: enough clearance for standard drilling across the tolerance stack of bolt and hole. Close fit (a drill only a few sizes up from the screw) is for jigs, fixtures and aligned flanges where play would show as misalignment — but it demands accurately drilled, not punched, holes. Loose fit (several sizes up) absorbs punched, drifted or multi-layer holes and thermal movement — structural work on oversized holes lives here, usually with washers to keep the bearing seat. The cost of guessing wrong runs both ways: close-fit ambitions on punched steel get you reaming, loose-fit habits on precision assemblies get you slop.
Why does the chart give both a drill size and a min/max hole diameter?
Because the drill size is the intent and the band is the acceptance. A jobber drill in a hand-held gun will not land exactly nominal — drill point wander, wear and wobble all cut oversize or off-axis — so the standard bounds what the resulting hole may measure: the min/max hole diameter per fit class. Fabrication QC checks the drilled hole (pin gauge or caliper) against that band, not against the drill number. The same logic is why a worn 17/32 bit drilling 1/2-in normal holes is a real finding: the hole can pass the drill number and still fail the band.
Related knowledge
- Thread-Forming vs Thread-Cutting vs Thread-Rolling Self-Tapping Screws: What the Evidence Settles on Base Material
- Screw Family Selection by Base Material: Wood, Machine and Self-Tapping Threads, Engagement and Coating
- Hex Bolts vs Socket Head Cap Screws: What the Evidence Settles on Strength, Clearance and Cost
Request a Quote
Get factory-direct pricing from the manufacturer.
What to include in your inquiry
- ✓ Standard and property class (e.g. DIN 912, ISO 4762, class 8.8) — or a drawing for custom parts.
- ✓ Sizes, lengths and thread type (metric or inch).
- ✓ Quantity per size and your target delivery date.
- ✓ Destination country, plus any coating or finish requirement.
- ✓ Any documents you need with the shipment (e.g. mill test certificate) so the quote includes them.
- ✓ Your inquiry goes straight to the factory team by email and gets a reply within 24 hours.
- ✓ Factory-direct pricing
- ✓ Standard and custom specifications
- ✓ Factory-direct export to buyers in Europe, Asia, the Americas and the Middle East
- ✓ Full fastener family support