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Metric and imperial bolts

  1. Jan 13, 2010 #1
    Hi guys, i want to ask about bolts, the metric and imperial bolts, are they the same but just one of them is in inches while the other one is in mm? What would a 1/4 inch bolt be equivalent to, a M6?

  2. jcsd
  3. Jan 13, 2010 #2


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    The thread profiles and pitches are different, so as a general rule one metric bolt can not be used as a replacement for an imperial bolt.

    For example, a 6-32 bolt has a thread pitch of about 0.79 mm/thd, and a body diameter of 3.5mm. These dimensions are not equivalent to any standard metric fasteners.
  4. Jan 14, 2010 #3
    A whole book could be written on that question. I regularly see equipment where someone has tried to screw a 1/4" npt male into a 1/4 bsp hole which ruins the hole. I think that even among similar size and pitch, flank angle is also different.
    With metric, SAE, Imperial, Wentworth, and then the "poor versions" of these same threads that are made in other countries for cheap products, proper fastener selection, and proper use can be somewhat "problematic at best"

  5. Jan 15, 2010 #4
    it is just a bit weired that you have all these different size systems for bolts, so i would believe that ASME will use the imperial bolts, then which standards uses metric bolts? DIN, AD maybe?
  6. Jan 15, 2010 #5
    all the styles and standards came from the evolution of technology
    actually interesting history

  7. Jan 15, 2010 #6


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    Everybody except the US (and owners of classic British motorbikes) uses metric today.

    An English guy called Whitworth invented the first standardized thread in about 1850, it's actually a lot more complicated than just picking a width, he designed the mathematically best angle and shape for the groove.
    The US standardized on roughly the same size but simplified the angle so it was easier to make.
    In about 1900, everybody in europe (except the Brits) standardized on metric.

    There are few hangovers from the very early designs, because Britain was the biggest market for scientific instruments things like microscope objective threads, camera lenses and still today tripod screws are based on whitworth, even if they were made in metric countries.
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