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I have this data sheet for a stepper motor:

stepper motor data sheet

The pitch diameter of the pinion attached to it is listed as "Ø6". What unit is this? Looking at the wikipedia page for Ø it seems like this is just a nonce symbol indicating that the measurement in question is a diameter, but then the question still remains, what unit is it? Does it mean 6 millimeters? Is there some convention about default units in data sheets I'm missing?

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In the lower right corner of the document/drawing, you can see four (or five) lines of text upward: UNIT: mm

Another piece of useful information in the data panel is the scale, shown there as 3:1. You would be able to measure the components of the drawing and divide by 3 to determine real dimensions, but only if the drawing is printed to 1:1 and not "fit to page" as if sometimes done.

Your other research result is correct.

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Engineering drawings come in two flavours metric and north american (mainly US only). Metric drawings are using mm as a unit, even in architecture where a mm is a slightly small unit. North american drawings are using inches as their base unit. If they would use anything else then the unit would read next in the dimension (this is incredibly rare).

Now, the sheet states that the unit is mm in the title block. The title block applies to the whole drawing. Other much weaker hints are the A4 size paper (ISO size indicates mm), projection rotation rule (image is drawn in first angle, US almost exclusively use third) and scale info. Although you really shouldn't measure distances on drawings since the numbers are what counts and there can be reproduction errors and/or dimensions not in scale.

Other possible hints would be invoking any ISO standards vs ASME standards which here are not present (for example a general tolerance). Also simply assuming mm* would work for the bulk of drawings made for any other purpose than US consumption. Besides in this case inhes would sound incredibly big.

* Although, inches are much more common in electronics than mechanical drawings, so best not guess anything.

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Yes this drawing is all in metric. Therefore, "Ø6" = 6mm diameter

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    $\begingroup$ If the asker knew the drawing was in metric, they probably wouldn't have needed to ask the question. So it would be helpful if your answer included information on how the asker could have known that the drawing was in metric. (Probably not much point editing that in now, since two other answers now explain it. But, in general, we're looking for detailed answers that teach people something and explain why they're correct, not just one-liners answering the literal question.) $\endgroup$
    – David Richerby
    Jan 29, 2018 at 9:43

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