Hub Bolt Size Guide: Diameter, Thread Pitch, Length and Knurl Explained
Hub bolts are important fastening components used in truck, trailer and commercial vehicle wheel and hub assemblies. Although two hub bolts may appear similar, small differences in diameter, thread pitch, overall length, shoulder size or knurl dimensions can make them unsuitable for the same application.
For OEM buyers, automotive distributors, workshops and commercial vehicle spare-parts suppliers, identifying the complete hub bolt specification is therefore essential before ordering replacements or bulk quantities.
This guide explains the main hub bolt sizes and dimensions, what each measurement means and what buyers should provide when requesting standard or customised hub bolts.
For commercial vehicle requirements, buyers can explore the Hub Bolts manufactured by Benz Auto Corporation.
What Does Hub Bolt Size Mean?
Hub bolt size is not determined by a single measurement.
Depending on the design, a complete specification may include:
- Thread diameter
- Thread pitch
- Overall length
- Thread length
- Shoulder diameter
- Shoulder length
- Knurl diameter
- Knurl length
- Head diameter
- Head height
- Property class
- Surface finish
This is why simply requesting an M18 hub bolt may not provide enough information for accurate manufacturing.
Two M18 bolts can have completely different lengths, pitches and mounting profiles.
1. Hub Bolt Thread Diameter
The first measurement to identify is the thread diameter.
Metric hub bolts may use specifications such as:
- M14
- M16
- M18
- M20
- M22
depending on the vehicle and hub design.
For example:
M18 indicates a nominal metric thread diameter of 18 mm.
The correct diameter must match the mating nut or threaded component exactly.
Installing a different diameter simply because it appears stronger is not appropriate.
How to Measure Hub Bolt Diameter
Use a vernier caliper to measure across the outside edges of the threaded section.
The measurement should be taken across the widest points of the threads.
If the diameter measures close to 18 mm, for example, the nominal thread may be M18.
However, diameter alone is not enough.
The thread pitch must also be identified.
2. Hub Bolt Thread Pitch
Thread pitch refers to the distance between adjacent threads.
A metric hub bolt might be specified as:
M18 × 1.5
Here:
M18 means the nominal thread diameter.
1.5 represents the thread pitch in millimetres.
Another hub bolt may use:
M18 × 2.0
Although both have the same nominal diameter, the thread pitches differ.
These components should not be assumed to be interchangeable.
How to Measure Thread Pitch
The best practical tool is a thread pitch gauge.
Place the gauge against the bolt threads until the profile aligns correctly.
For OEM and bulk requirements, the technical drawing should always be treated as the primary reference.
Using the wrong thread pitch can cause:
- Cross-threading
- Nut incompatibility
- Damaged threads
- Poor engagement
- Installation problems
Never force a nut onto a bolt where the thread does not engage smoothly.
3. Hub Bolt Overall Length
Overall length is another important specification.
Depending on the bolt design, the measurement may run from the underside of the head to the end of the bolt.
However, different component designs can use different measurement conventions.
For this reason, a technical drawing should clearly show the reference points.
Why length matters
A hub bolt that is too short may not provide enough usable thread.
A bolt that is excessively long may:
- Extend beyond the required position
- Interfere with surrounding components
- Create installation difficulties
Always use the specified length rather than estimating from a photograph.
4. Thread Length
Overall bolt length and thread length are not necessarily the same.
Some hub bolts may include a smooth shoulder or knurled section before the thread begins.
The threaded length should therefore be measured separately.
For example, a hub bolt could have:
Overall length: 90 mm
Thread length: 45 mm
The remaining length may consist of:
- Shoulder
- Knurled section
- Head
Thread length is particularly important because sufficient engagement is required with the mating component.
5. What Is the Knurl on a Hub Bolt?
Some hub bolts or wheel studs include a knurled section near the head.
Knurling consists of small ridges formed around the circumference of the bolt.
This section may be designed to create an interference or press fit within the hub or related mounting component.
The knurl can help prevent the fastener from rotating during installation or removal of the mating nut.
Because of this, knurl diameter is a critical dimension.
6. Measure the Knurl Diameter
Knurl diameter is measured across the outside of the raised knurled section.
Use a vernier caliper and measure carefully across the widest points.
A small difference in knurl diameter may significantly affect fitment.
If the knurl is too small
The bolt may not fit securely in the mounting hole.
If the knurl is too large
Installation may become difficult or may damage the hub.
For OEM and bulk requirements, knurl dimensions should always follow the approved drawing.
7. Measure the Knurl Length
Knurl length is the axial length of the ridged section.
This dimension determines how much of the hub mounting area engages with the knurl.
When measuring, record:
- Knurl diameter
- Knurl length
- Position from the bolt head
These dimensions are particularly useful when replacing an existing hub bolt.
8. Shoulder Diameter
Some hub bolts contain a smooth shoulder section.
The shoulder may sit between:
- Head and knurl
- Knurl and thread
- Other application-specific sections
Measure the shoulder diameter using a vernier caliper.
Do not assume it is identical to the thread diameter.
Depending on the design, the shoulder may be larger.
9. Shoulder Length
The length of the smooth shoulder can also affect fitment.
Measure from the point where the shoulder begins to where it transitions into another section, such as:
- Knurl
- Thread
- Head
For customised hub bolts, include shoulder dimensions in the technical drawing.
10. Head Diameter and Head Height
Hub bolts can use different head profiles depending on the application.
Important dimensions may include:
- Head diameter
- Head height
- Head shape
- Flats, if applicable
- Flange dimensions
These dimensions can affect how the fastener seats within the hub assembly.
Commercial buyers should therefore avoid specifying only the thread dimensions when ordering.
Example of a Hub Bolt Specification
Instead of requesting:
M20 truck hub bolt
a more useful specification could look like:
Application: Commercial truck hub assembly
Thread diameter: M20
Thread pitch: As per drawing
Overall length: 95 mm
Thread length: 45 mm
Knurl diameter: As specified
Knurl length: As specified
Shoulder: As per approved sample
Head dimensions: As per drawing
Property class: Required grade
Surface finish: Specified finish
Quantity: 5,000 pieces
This gives the manufacturer significantly more information.
Hub Bolt vs Wheel Bolt Dimensions
Hub bolts and wheel bolts can be closely related, but their specifications should not automatically be assumed to be the same.
Wheel bolts often require close attention to:
- Thread
- Bolt length
- Wheel seat profile
- Head design
Hub bolts may additionally require careful measurement of:
- Knurl diameter
- Knurl length
- Shoulder diameter
- Shoulder length
- Press-fit section
For a detailed comparison, read Wheel Bolt vs Hub Bolt: What Is the Difference?.
You can also explore the Wheel Bolts manufactured by Benz Auto Corporation for related commercial vehicle applications.
Why Knurl Size Is Important for Press-Fit Hub Bolts
Where a hub bolt uses a press-fit design, the relationship between the knurl and mounting hole is important.
The fastener must fit according to the engineering specification.
An incorrectly sized knurl can create problems such as:
- Bolt movement
- Rotation during nut installation
- Difficulty pressing the bolt into place
- Damage to the hub
- Poor fitment
Because press-fit requirements depend on actual tolerances, buyers should never estimate the knurl specification from appearance alone.
Use the OEM drawing or approved sample.
Hub Bolt Strength Grade
Dimensions are only one part of hub bolt selection.
Commercial vehicle hub bolts may also require a specified property class depending on the application.
Common metric fastener property classes can include:
- 8.8
- 10.9
- Other specified classes
Higher strength does not automatically mean better.
The correct grade must match the vehicle and engineering specification.
A dedicated guide on Hub Bolt Grades 8.8 vs 10.9 can further explain this topic.
Material Requirements
Commercial vehicle hub bolts may be manufactured from suitable carbon or alloy steels depending on the required properties.
OEM and bulk buyers should clearly define:
- Material specification
- Mechanical-property requirements
- Property class
- Heat treatment
- Hardness
- Surface finish
Generic terms such as “heavy-duty hub bolt” are not enough for precise manufacturing.
Surface Finish
Hub bolts used on commercial vehicles may operate in environments involving:
- Rain
- Road water
- Humidity
- Dust
- Mud
- Road contaminants
- Temperature variation
Where corrosion protection is required, the approved surface treatment should be specified.
Surface finish should be treated separately from strength grade.
How to Identify an Unknown Hub Bolt
If you have an existing hub bolt but no drawing, record as much information as possible.
Measure:
- Overall length
- Thread diameter
- Thread pitch
- Thread length
- Knurl diameter
- Knurl length
- Shoulder diameter
- Shoulder length
- Head diameter
- Head height
Also note:
- Strength marking
- Surface finish
- Vehicle application
- OEM or reference number
Providing the original physical sample can be extremely useful.
Should You Measure a Used Hub Bolt?
A used bolt can provide valuable information, but it should not automatically be treated as a perfect master sample.
Used components may have:
- Worn knurls
- Damaged threads
- Corrosion
- Deformation
- Surface wear
Where possible, compare the measurements against:
- OEM drawing
- New approved sample
- Hub dimensions
This reduces the risk of copying wear into a replacement specification.
Common Hub Bolt Measurement Mistakes
Measuring Only Thread Diameter
Knowing a bolt is M20 does not tell you its pitch, length or knurl dimensions.
Ignoring the Knurl
For press-fit applications, knurl size may be critical.
Guessing Thread Pitch
Always verify pitch using a gauge or drawing.
Ignoring Shoulder Dimensions
A shoulder can affect both positioning and fitment.
Measuring a Worn Knurl
A removed hub bolt may no longer represent its original knurl diameter accurately.
Ordering From a Photograph
A photograph is helpful for identifying overall shape but is not sufficient for precise manufacturing.
Hub Bolt Sizes for Trucks and Trailers
There is no single universal hub bolt size for all trucks.
Specifications can vary according to:
- Vehicle manufacturer
- Axle
- Hub assembly
- Vehicle model
- Load rating
- Wheel system
This is why searches for “truck hub bolt size” should always be followed by verification against the actual vehicle specification.
Custom Hub Bolt Manufacturing
Commercial vehicle OEMs and spare-parts distributors may require hub bolts in specific or non-standard dimensions.
Custom hub bolts can be manufactured according to:
- Technical drawings
- Approved samples
- Thread specifications
- Knurl dimensions
- Shoulder dimensions
- Head profile
- Material
- Strength class
- Surface finish
Benz Auto Corporation manufactures Hub Bolts for trucks, trailers and automotive applications according to customer specifications.
What Should You Send to a Hub Bolt Manufacturer?
For accurate quotation and manufacturing, provide:
Application
Truck, trailer, bus or other commercial vehicle.
Thread diameter
For example, M20.
Thread pitch
Exact pitch required.
Overall length
Clearly marked.
Thread length
Usable threaded portion.
Knurl diameter
For press-fit designs.
Knurl length
Where applicable.
Shoulder dimensions
Diameter and length.
Head dimensions
According to drawing.
Property class
Required mechanical grade.
Surface finish
Specified coating or treatment.
Quantity
Expected order volume.
Drawing or sample
Strongly recommended.
Related Automotive Fasteners
Commercial vehicle buyers may also require other related components.
Wheel Bolts
Wheel Bolts for wheel fastening applications.
U-Bolts
U-Bolts for truck, trailer and leaf spring suspension systems.
Centre Bolts
Centre Bolts for commercial vehicle and leaf spring assemblies.
These related product categories help OEMs, distributors and automotive parts buyers source multiple commercial vehicle fasteners.
Frequently Asked Questions
How do I measure a hub bolt?
Measure the thread diameter, thread pitch, overall length, thread length, shoulder, knurl and head dimensions according to the bolt design.
What is the knurl diameter on a hub bolt?
It is the outside diameter across the raised ridges of the knurled section used in applicable press-fit designs.
Is hub bolt size the same as thread size?
No. Thread size is only one part of the full hub bolt specification.
Can two M20 hub bolts be different?
Yes. They can have different pitches, lengths, knurl sizes, shoulders, heads and strength grades.
Are all truck hub bolts the same size?
No. Specifications vary according to vehicle, axle and hub design.
Can hub bolts be manufactured to a sample?
Yes. An approved physical sample can be useful for identifying dimensions, particularly for specialised aftermarket requirements.
Can custom hub bolts be manufactured?
Yes. Hub bolts can be manufactured according to application-specific drawings, dimensions, materials and strength requirements.
Final Thoughts
Correct hub bolt identification requires much more than knowing the thread diameter.
A complete specification may include:
- Thread diameter
- Thread pitch
- Overall length
- Thread length
- Knurl diameter
- Knurl length
- Shoulder dimensions
- Head dimensions
- Strength grade
- Surface finish
For commercial vehicle OEMs, distributors and bulk buyers, a detailed drawing or approved sample is one of the most reliable ways to avoid fitment and manufacturing errors.
Benz Auto Corporation, Ludhiana, manufactures Hub Bolts for trucks, trailers and commercial vehicles, along with wheel bolts, U-bolts, centre bolts and other automotive fasteners. Buyers can share their drawings, samples, dimensions and quantity requirements for standard or customised hub bolt manufacturing.
