Understanding Bolt and Nut Naming in Automotive Terminology

Bolts and nuts are fundamental components in automotive engineering, responsible for the structural integrity and safety of virtually every mechanical system. Their naming is more than just a label—it’s a precise language that conveys critical information about size, shape, thread type, material, and strength. Understanding this nomenclature is essential for mechanics, engineers, and enthusiasts alike.

1. Basic Definitions
Bolt: A threaded fastener typically used with a nut to join materials. Bolts have external threads and a head.

Nut: A fastener with an internal thread that mates with a bolt.

2. Key Naming Parameters
Automotive bolts and nuts are identified using a structured nomenclature based on the following parameters:

A. Thread Size and Pitch
This is the most fundamental part of naming.

Metric Example:
M10 x 1.5

M = Metric thread

10 = Major diameter (in mm)

1.5 = Thread pitch (distance between threads in mm)

Imperial (Unified Thread Standard - UTS) Example:
1/2"-13 UNC

1/2" = Major diameter (in inches)

13 = Threads per inch (TPI)

UNC = Unified National Coarse thread

B. Length
The bolt length is measured from the bottom of the head to the end of the threads (not including the head unless it’s a countersunk bolt).

Example: M10 x 1.5 x 50 = 50 mm long

C. Strength Grade
Metric Bolts (ISO 898-1):
Common grades: 8.8, 10.9, 12.9

Meaning of 8.8:

First number (8) × 100 = 800 MPa tensile strength

Second number (.8) = 80% of tensile strength is yield strength → 640 MPa

Imperial Bolts (SAE J429):
Grades: Grade 2, Grade 5, Grade 8

Tensile strengths:

Grade 2: ~400 MPa

Grade 5: ~800 MPa

Grade 8: ~1200 MPa

Nuts also have strength grades and must match or exceed the strength of the mating bolt.

3. Head Type
The head shape is often part of the bolt’s name or specified in technical drawings.

4. Thread Type and Class
Metric Threads:
Coarse (default) or Fine (designated with pitch):

M10 (coarse) vs. M10 x 1.25 (fine)

Imperial Threads:
Unified National Coarse (UNC)

Unified National Fine (UNF)

Unified National Extra Fine (UNEF)

Thread Fit Class (Imperial):
1A/1B = Loose fit

2A/2B = General-purpose

3A/3B = Tight tolerance, precision fit

5. Surface Finish and Coating
Surface finish improves corrosion resistance and can be included in descriptions or part numbers.

Zinc-plated (Zn)

Black oxide

Phosphate-coated

Hot-dip galvanized

Example: M8 x 1.25 x 40 10.9 Zn – indicates a zinc-coated bolt.

6. Automotive Industry Naming Practices
OEM Part Numbers:
Original Equipment Manufacturers (OEMs) often use internal part numbers that encode bolt size, length, material, and application without following public standards. These numbers are cross-referenced in service manuals.

Standard Designation Systems Used:
ISO (Europe/Global)

DIN (German, legacy)

SAE/ASTM/ASME (North America)

7. Examples of Bolt Naming in Automotive Context
Example 1: ISO Metric Designation
M12 x 1.5 x 75 – 10.9 – Hex – Zn

Metric thread, 12 mm major diameter

Fine pitch 1.5 mm

75 mm long

Strength grade 10.9

Hex head

Zinc coated

Example 2: SAE Imperial Designation
3/8”-16 x 2” – Grade 5 – Hex – Black Oxide

3/8 inch major diameter

16 threads per inch (UNC)

2 inches long

Grade 5 steel

Hex head

Black oxide finish

8. Special Automotive Fasteners
Shoulder Bolts: Used in suspension and pivot joints

Stud Bolts: Threaded both ends, used in cylinder heads, manifolds

Self-tapping Screws: For plastic or sheet metal assemblies

Torque-to-Yield Bolts: Designed to stretch during torqueing (single use only)

9. Nut Naming Conventions
Nut designations follow a similar structure:

Example:
M10 – Hex – Class 10 – Flanged – Zn

10 mm thread

Hex shape

Strength class 10 (to match bolt)

Integrated flange

Zinc plated

Other types:

Nyloc nuts (with nylon insert)

Jam nuts (thin, used to lock standard nuts)

Cap nuts (protect exposed threads)

Castellated nuts (for use with cotter pins)

Conclusion
The naming of bolts and nuts in automotive applications is a precise, standardized process that ensures safety, compatibility, and serviceability. Whether you’re selecting a replacement fastener or engineering a new automotive system, fluency in this terminology is essential. Always refer to OEM specifications, torque requirements, and strength grades to ensure reliability in your fastening applications.