Here is the translation of the quiz text:
Quiz: There are three types of M6×25 bolts with different shapes (A: Flanged hex bolt, B: Hex socket head cap screw, C: Hex bolt). Which of these has the largest tightening torque (the highest numerical value)?
① Option A: Because it has the largest contact surface.
② Option B: Because it requires a hex socket (allen wrench).
③ Options A, B, and C: Because all of them are the same M6 size.
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Answer
Here is the translation:
The correct answer is… A: Flanged hex bolt.
The specified torque for each is as follows:👇
| M6・Flanged hex bolt | 11.8 Nm |
| M6・Cap screw | 9.8 Nm |
| M6・Hex bolt | 9.8 Nm |
*Values may vary depending on the manufacturer.
Why is the torque different even though the size is the same?
When bolts have the same diameter and length, the clamping force (axial force) that secures the screw should inherently be the same. The reason the tightening torque differs is that a torque wrench cannot measure the axial force directly. It ends up measuring the friction (resistance) generated during tightening along with it.
motorockmanIf you want to measure the axial force accurately, you must use an “axial force gauge.” However, it is quite expensive. Furthermore, it is not a hand tool. It is also time-consuming, making it impractical for motorcycle maintenance.
Let me put it a bit strictly. Since A, B, and C all have the same size under the head, they reach the required axial force at the exact same amount of screwing in. Let’s assume that requires 10 full turns. For B and C, it takes 10 turns at 9.8 Nm of torque. However, for A, it has only turned 9 times at 9.8 Nm. It takes 11.8 Nm to finally reach 10 turns.
Recommended torque wrenches
Although it cannot measure axial force, a torque wrench is essential for measuring tightening torque. If you don’t have one yet, TOHNICHI’s MTQL70N is highly recommended.
Since it supports M6, M8, and M10 bolts, the manufacturer also markets it for motorcycle use. This should be your very first torque wrench.
motorockmanAt the 2017 Tokyo Motor Show, TOHNICHI introduced “JIXY,” a torque-wrench-style axial force gauge. In a few years, the inside of our toolboxes might shift from torque wrenches to axial force gauges.
