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

对于常见的目录商品，买家可以查看 标准紧固件 并在订购前确认确切的标准。

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和ISO紧固件标准指南 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.

买家应确认：

1. 公制或英制螺纹。
2. Coarse or fine pitch.
3. 全螺纹或部分螺纹。
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.

询价清单

在请求报价之前，请提供：

• Standard or drawing.
• Nominal size.
• Length.
• Thread pitch.
• 全螺纹或部分螺纹。
• Shank requirement.
• Grade and material.
• Finish.
• Quantity and inspection needs.

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