Crank arm torque spec for motor attachment?

[Snaiperskaja]

Hi, what torque would I use to attach the crank arms to the motor?

@Greg

 

[Greg Watts]

Depends entirely on which motor you're running, as manufacturers vary quite a bit here.

The most common figures from the community knowledge base:

• Bosch CX (Gen 4/5): 50Nm on the crank bolt
• DJI Avinox (Amflow/Crestline): 48-54Nm per the official manual
• SRAM XO eMTB cranks: 48-54Nm per @BikingBad
• Shimano / ISIS spline: typically 40-50Nm depending on the specific interface
• Yamaha: motor mount bolts are 22Nm (long) and 11Nm (short), though crank arm bolts sit in the 40-50Nm range
• Specialized Kenevo: 20.3Nm (notably lower, so worth checking if that's your bike)

The general rule is 40-55Nm for most crank arms, but always use your bike's specific manual rather than a ballpark figure. A few Nm either way matters less than getting it badly wrong, and @Mikerb has pointed out that if the crank has worked loose multiple times, the bolt itself needs replacing as the threads are likely compromised.

Tell us what motor you're running and I can give you the exact spec.