[time-nuts] Standards for units

Chuck Harris cfharris at erols.com
Tue Apr 3 23:57:30 EDT 2007

Hi Brooke,

Metric screws are spec'd by major diameter, and the number of mm between
the peaks of the threads.  English are spec'd by diameter, and the
number of threads per inch.

A 40TPI screw corresponds to a (1/40) * 25.4 = 0.635 mm pitch.  There
are standard metric pitches of 0.4, 0.45, 0.5, 0.6, 0.7, 0.8, and 1.0 mm.

What you are seeing is probably an M3-0.5, M3-0.6, or an M4-0.7 screw.

-Chuck Harris

Brooke Clarke wrote:
> Hi:
> There was a recent incident when a passenger aircraft (maybe Canada) had 
> to make an emergency landing because of a wrong metric - English 
> conversion resulted in not enough fuel to get to the destination. 
> Are there metric equivalents to different series of English threads.  
> For example I recently purchased a tap for 8-40 threads after mistakenly 
> purchasing a 6-40 tap.  Most good hardware stores in the US have 4-40 
> taps, dies, various lengths screws and nuts.  Although 6-40 and 8-40 are 
> standard sizes they are not commonly stocked.  Are there a a number of 
> alternative combinations of root diameter and pitch in the metric system?
> Have Fun,
> Brooke Clarke

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