What bolt length should I use for this motor?

I should use a M8 x 1.25 bolt that is longer than 20.3 mm right?

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  • Anonymous
    1 month ago

    Probably, but that's not what the 20.3 mm means!

    What are you mounting this device onto?  How thick is the mounting bracket or mounting surface? 

    There is a rough formula for bolt thread engagement and strength. 

    1.5 × the diameter of the thread. 

    8 mm × 1.5 = 12 mm 

    So add 12 to the thickness of your mounting surface.  If you are using washers or lockwashers, add their thickness also. 

    The threaded holes in your device are likely deeper than 12 mm.  You should use bolts that engage at least 12 mm of threads for maximum security of the bolted connection. 

    You could use a longer bolt that goes deeper, but you must make sure that your bolt is not too long which would cause the bolt to bottom in the threaded hole before the bolt attaches your device securely to its intended mounting surface. 

    My logic tells me that the threaded hole in your device is at least 20.3 mm deep, but you don't need to engage an 8 mm bolt more than 12 mm for maximum strength. 

    Metric bolts are commonly available in steps of 10 mm length.  They may be available in steps of 5 mm in length, but they are not as easily available by every source. 

    If you can get bolts that allow 12 to 15 mm of engagement, that would be ideal. 

    So how thick is your mounting surface?? 

    And don't use low grade bolts from a hardware store or home improvement store.  Use minimum grade 8.8 hex head bolts.  The bolt will have 8.8 stamped in its head.  If no number is visible, then the bolt is of inferior quality and should not be used. 

    Some applications for fasteners don't require high strength, like household furniture, etc.  But you have a geared power device that needs quality fasteners. 

    I might have more to say if you can give the mounting surface thickness or if your device is made of aluminum.

    Source(s): I work with metric fasteners daily
  • Anonymous
    1 month ago

    Calls out   an 8mm x 1.25mm pitch x   20.3 mm. Use it. No longer. If you haveta grind on a hardware store 1" long screw, so be it. 

        Any longer, if threaded into aluminum,  will strip threads.

      Ref: Common Sense 

  • 1 month ago

    Depends on the material thickness you are mounting it too. You should not have to go any deeper than 2 x 8mm = 16mm deep into the motor housing if it's aluminum or 1.5 x 8mm if it's steel/iron. Add the material thickness to that. You do not want to calculate it to reach the bottom of those threads, leave some clearance.

    -old fart mechanical designer

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