Undercarriage fastener grade is position- and model-specific: some applications use high-strength metric classes such as 10.9 or 12.9, but the applicable OEM part number, grade marking, and service specification control the selection. Use the grade from the service manual for the exact position, because the grade is what holds the clamping force under load. Read the head marking and confirm it before installation.
Related reading: for torque values, see our undercarriage torque guide, and for loosening causes, see our loosening guide.
What Bolt Grades Mean and How the Head Marking Reads
Bolt grades are standardized strength classes that set the tensile strength and yield point of the fastener. The grade is marked on the bolt head and must match the machine's specification for the position.
In Saskatchewan construction and ag work, the bolt grade is the difference between hardware that holds and hardware that fails. The marking is the quick check, and the service specification is the authority.
The grade standard is shared across brands, which is what makes the marking readable: an 8.8 bolt from any supplier carries the same strength class. The shared standard is why the grade check works at the counter and at the machine.
Tensile Strength and Yield Differences
Higher classes such as 10.9 and 12.9 carry higher tensile and yield strength than 8.8. The higher strength holds the clamping force that keeps undercarriage parts tight under vibration and load.
The difference matters most under repeated loading: a bolt at the edge of its yield point stretches, and a stretched bolt loses clamping force even when it feels tight. The grade is selected so the fastener stays below that point in service.
The yield point is the number that matters in service, not the tensile peak: a bolt loaded past yield is a bolt that will not come back. Reading the specification for the position tells you which margin the joint needs.
Which Grades Undercarriage Parts Need
The required grade is set by the position, the part, and the machine's service specification. A 10.9 or 12.9 requirement is model- and position-specific, so confirm the grade from the service manual rather than applying a general rule.
The same machine can carry different grades on shoes, rollers, and segments. The parts book for the serial number lists each position's hardware, and the grade check belongs on the same sheet.
Grade, Coating, and Plating: The Full Hardware Specification
The hardware specification is more than the grade: it includes the size, thread pitch, coating, and any locking feature the position requires. A bolt with the right grade and the wrong coating can corrode, seize, or change the joint's behaviour in service.
Confirm the full specification from the parts book and order hardware that matches all of it. The complete specification is what makes the fastener right for the position, and it is what a kit is built around.
Why the Wrong Bolt Grade Is a Risk
A bolt with a lower grade than specified can lose clamping force under load and loosen sooner. The saving on the fastener rarely covers the cost of a loosened or failed part, and the failure usually damages the part and the hole.
The risk is invisible until it fails: a weaker bolt looks identical until the head marking is read. That is why the marking check is part of the install, not a quality step for the lab.
How to Read Bolt Head Markings Before Installation Starts
The grade is marked on the bolt head with radial lines or a number. Read the marking before installation and confirm it matches the specification for the position.
The marking is the fastener's identity card. If the marking is unreadable or missing, the bolt should not be installed where a grade matters.
Head markings can be faint after plating or worn after service, so check a clean new bolt at delivery rather than a pulled one at install. The delivery check catches the mislabeled box before the bolt reaches the machine.
Grade Markings at a Glance: Reading the Head
Metric strength classes are marked on the head in a standardized way, and the pattern is quick to read on site. The table below is a general reference — the service specification for the position remains the authority.
| Grade class | Typical head marking | General use note |
|---|---|---|
| 8.8 | 8.8 stamped | General hardware where specified |
| 10.9 | 10.9 stamped | High-strength positions where specified |
| 12.9 | 12.9 stamped | Highest-strength positions where specified |
When Grades Are Mixed: The Weakest Bolt Sets the Joint
In a multi-bolt joint, the weakest bolt sets the load the joint can hold. One 8.8 bolt among 10.9 bolts becomes the failure point, which is why mixing grades in a repair is as risky as using the wrong grade throughout.
Order hardware by position and grade, and confirm the count matches the joint. A kit that arrives with the correct grades for every position removes the mixing risk before the job starts.
The mixing risk also applies across services: a bolt replaced from a bucket of shop hardware can silently change the joint's grade. Marking the kit by position and keeping the old bolts separate until the new ones are fitted is the habit that keeps the joint honest.
Buying the Right Bolts Online
Buy bolts by grade, size, and thread pitch from the machine specification, and confirm the quantity with the part count. A kit with the correct grades avoids mixing fasteners and the second order that piecemeal buying causes.
Check the delivery against the maintenance stop and keep the grade markings visible until installation. The bolt that arrives correctly marked is the bolt that can be installed with confidence.
Confirm the coating and any locking feature at the same time, because online listings can show the same size in several finishes. The position specification decides which finish is correct, and the confirmation travels with the order.
Fastener Verification Checklist
- Confirm the bolt grade for the position in the service specification.
- Read the head marking and match it to the grade before installation.
- Check size, thread pitch, coating, and any locking feature.
- Confirm the quantity and grade at delivery, before the job starts.
Conclusion
Undercarriage bolts should be the grade the machine specifies for each position, marked on the head and confirmed before installation. Weaker hardware is a false saving that fails under load.
Key Takeaways for Canadian Operators
- Use the bolt grade from the service manual for each position.
- Read the head marking to confirm the grade.
- Buy by grade, size, and thread pitch, not by price.
- Avoid mixing grades in a multi-bolt joint.
See the Bolt & Nut collection and the V110-32-11420 hardware item for your machine.
Frequently Asked Questions
What does 10.9 mean on a bolt?
10.9 is a strength grade — the bolt's tensile strength is approximately 1000 MPa with a 0.9 yield ratio. It is common for undercarriage hardware.
Can I use 8.8 bolts on track parts?
Only if the machine specifies 8.8 for that position. The applicable grade comes from the service specification, not from a general rule.
How do I read a bolt grade marking?
The grade is marked on the bolt head with radial lines or a number. Confirm it matches the specification before installation.