Metric Clearance Hole Sizes (ASME B18.2.8) — Close, Normal, Loose
Metric clearance hole chart M1.6 to M100: close, normal and loose fit classes with nominal drill size and hole diameter min/max per thread size — the metric side of the ASME B18.2.8 clearance system.
Last updated: 2026-11
Specifications
The metric half of the B18.2.8 clearance system runs M1.6 through M100: per thread size, three fit classes (close, normal, loose), each carrying the nominal drill size and the hole diameter min/max band. The ladder opens at M1.6 (normal: 1.8 drill, 1.8/1.94 band; close: 1.7, 1.7/1.8; loose: 2, 2/2.25) and climbs through the full metric series — the class deltas widen as sizes grow, which is why the loose fit matters most on big structural holes where punched and thermal-movement conditions dominate. Pairs with the inch chart as the same-named fit-class system; keep both pages in view on mixed-unit projects so a "normal fit" callout means the same clearance philosophy on both sides of the drawing.
| Standard / scope | ASME B18.2.8 clearance holes, metric fasteners M1.6-M100 — fit classes close, normal, loose; per class nominal drill size and hole diameter min/max |
|---|---|
| Ladder start | M1.6: close 1.7 drill (1.7/1.8), normal 1.8 (1.8/1.94), loose 2.0 (2/2.25) — continuing through M2 (2.2/2.4/2.6 drills) as tabulated |
| Reading the columns | Per size: Nominal Drill Size + Hole Diameter Min./Max. per fit class — stacked header rows carried as printed |
| Ladder end | Runs to M100 — the large-diameter rows are the structural and heavy-equipment range |
| Not tabulated | Countersink/counterbore combinations, positional tolerancing of the hole pattern — per RFQ / drawing |
Materials
- Metric fabrication drawings usually inherit the fit-class convention from this standard or the ISO drafting equivalents — the values here are the check. Fastener supply follows the drawing's hole, not the reverse: if the shop holds loose-fit practice, washers sized for the loose hole keep the bearing seat; if close-fit, expect bolt-and-hole selection gauging at assembly. State the fit class next to every hole callout on RFQ fabrication packages — it is the single most-missed note on outsourced plate work
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 (Metric Clearance Hole Sizes (ASME B18.2.8) — Close, Normal, 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
Do the fit classes mean the same thing as on the inch chart?
Yes — the philosophy is shared: close for aligned drilled work, normal as the default, loose for punched/drifted and movement-absorbing holes. The numbers differ per series because the bolt diameter ladders differ, but the relative spacing (a drill a few tenths up for close, several for loose) carries across. On mixed-unit projects this is the trap: a "normal" hole called out in inches and one in millimetres are both normal fit, but a shop translating between them by eye will drift — reference the chart per series and keep the class name literal on every drawing.
At what size does the loose fit become structurally necessary?
Not at a single size — it follows the hole-making method, but the stakes grow with diameter: large punched or flame-cut holes can exceed the normal band by more than the bolt's bearing width can forgive, and thick plate stacks accumulate misalignment that only the loose class absorbs. From the mid-ladder upward (where this chart's loose drills jump well clear of the bolt), expect loose fit plus a structural washer as the standard pairing on steelwork. The chart's min/max bands per class are the acceptance lines either way — measure the hole, not the intention.
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