Bolt diameter looks simple until an RFQ, drawing, and actual part do not match. A buyer may write “M12 bolt,” but the supplier still needs to know the standard, thread pitch, shank style, tolerance, and whether the bolt is fully threaded or partially threaded.

For industrial purchasing, diameter is not one number. It is a set of related dimensions.

The Three Diameter Terms Buyers Should Know

For common catalog items, buyers can review standard fasteners and confirm the exact standard before ordering.

Nominal Diameter Is a Name, Not Always the Measured Value

Metric Bolts

For metric bolts, “M12” means the nominal thread diameter is 12 mm. The actual outside diameter of the thread is usually slightly below 12 mm because of manufacturing tolerances.

A typical metric callout may be:

• M12 x 1.75 x 60 mm
• M12 = nominal thread diameter
• 1.75 = thread pitch in millimeters
• 60 mm = bolt length

For metric standard comparison, this DIN and ISO fastener standards guide helps buyers avoid confusing similar standards.

Inch Bolts

For inch fasteners, a size such as 1/2-13 UNC means:

• 1/2 inch = nominal diameter
• 13 = threads per inch
• UNC = Unified coarse thread

UNF threads use more threads per inch and are not interchangeable with UNC.

Shank Diameter and Hole Fit

The shank is the unthreaded portion of a partially threaded bolt. It may be close to the nominal diameter, but the exact tolerance depends on the standard and product type.

Shank size matters when:

• The bolt passes through precision holes.
• The joint carries shear load.
• The assembly needs alignment.
• The drawing requires a shoulder or close body fit.

Do not assume a fully threaded bolt and a partially threaded bolt will behave the same in a shear joint.

Thread Diameter and Pitch

Thread size controls assembly with nuts or tapped holes. A bolt with the correct nominal diameter but wrong pitch will not fit.

Buyers should confirm:

1. Metric or inch thread.
2. Coarse or fine pitch.
3. Full thread or partial thread.
4. Thread tolerance class.
5. Matching nut standard.
6. Coating effect on thread fit.

For coated parts, confirm thread fit after finishing, especially for hot-dip galvanized fasteners.

RFQ Checklist

Before requesting a quote, provide:

• Standard or drawing.
• Nominal size.
• Length.
• Thread pitch.
• Full thread or partial thread.
• Shank requirement.
• Grade and material.
• Finish.
• Quantity and inspection needs.

For mixed bolt, nut, and washer orders, buyers can contact XZ Fastener with drawings or size lists to reduce diameter and thread specification errors.