Threads are classified by “grades.”
Simply put, these grades are standards used to control the amount of clearance and play between mating threads. In Japan, thread grades are generally classified as follows:
- Grade 1 → Minimal clearance and play
- Grade 2 → Standard general-purpose fit
- Grade 3 → Larger clearance between threads
It should be noted that Grade 2 threads are used in the vast majority of industrial products, including motorcycles and automobiles.
Did You Know? A Grade 1 Bolt Can Actually Fit a Grade 2 Threaded Hole!?
In general, bolts and threaded holes are designed to be used with the same thread grade. However, in practice, it is possible to install a Grade 1 bolt into a Grade 2 threaded hole.
In fact, we at moto rockman initially considered manufacturing our products with Grade 1 threads. The reasoning was simple: reducing thread clearance could help minimize positional variation and runout.
However, after evaluating and testing the idea in real-world applications, we found that the concept offered not only advantages but also several drawbacks.
Grade 1 Threads Do Not Necessarily Mean Higher Performance.
As thread clearance decreases with a Grade 1 thread, surface contact and friction increase. As a result, issues such as:
- Galling (thread seizure)
- Reduced ease of assembly
- Poorer serviceability
become more likely to occur. This is especially problematic when threading into aluminum components.
And perhaps the biggest issue of all is cost.
The manufacturing process itself is not significantly different between Grade 1 and Grade 2 threads. However, most thread-rolling equipment is set up and managed around Grade 2 specifications because they are the industry standard. Adjusting production specifically to achieve Grade 1 tolerances for a single customer inevitably leads to a substantial increase in manufacturing cost.
There is another important point as well. Even if thread clearance is reduced, clamping force does not increase. The clamping force generated by a threaded fastener is primarily determined by four factors:
- Friction
- Lubrication condition
- Seating surface condition
- Tightening torque
The slight increase in friction caused by a Grade 1 thread has very little effect on actual clamping force. For that reason, switching to Grade 1 threads does not change the specified tightening torque.
In fact, proper lubrication generally has a far greater influence on fastening performance than thread grade. We’ll cover that topic in more detail another time.
What Is the Role of a Bolt?
Reducing clearance can certainly improve precision.
But if that’s true, why aren’t Grade 1 threads commonly used in engines, where accuracy is critical?
The answer is simple: that job belongs to the dowel pins.
- Bolts → Provide clamping force
- Pins → Provide positioning accuracy
Each component has its own role.
If you try to achieve positioning accuracy through the threads alone, you end up introducing other problems, such as:
- Reduced assembly efficiency
- Increased risk of galling
- Higher manufacturing costs
That’s exactly why the vast majority of industrial products still use Grade 2 threads today.
Grade 2 isn’t the standard because it is merely “ordinary.”
It became the standard because, after balancing performance, manufacturability, serviceability, and cost, it proved to be the most practical solution.
Conclusion
When it comes to bolt customization, the following upgrades are generally more effective than converting to Grade 1 threads:
- Changing from 8.8-grade to 12.9-grade high-strength bolts
- Replacing steel bolts with titanium bolts
- Replacing corroded bolts with new ones
