ISO Metric System

ISO Metric  System

Although there are two systems in use (Unified Thread Standard and ISO Metric) the thread itself is basically the same (60°). Difference is in the major and minor diameters and pitch.

The organizations concerned with these standards are for UTS ASME/ANSI and for the ISO Metric system the International Organization for Standardization.

Identifying

UTS type thread hardware is identified by its nominal diameter and by the pitch in tpi (threads per inch) and sometimes followed by the letter combination UNC, UNF or UNEF for the coarse, fine and extra fine pitch series.

ISO Metric hardware is coded the same way (diameter x pitch) but leaded by the letter M. For example: M5x10 which has by default a coarse pitch.

Tolerance Class

The UTS system uses a classification for interchangeability of fabricated nuts and bolts. Class 1 is loose fit and 3 is close tolerance. Class 2 is most commonly used. The class number is followed by an A for external or a B for internal threads.

Under load

When a bolt and nut are put under load they stretch a little and the distribution of the applied load will not be equal on allthreads. In fact, for a infinitely long bolt the first thread takes about 30%, the first three threads 75% and the first sixthreads take all the load.

So its not necessary to have more than six threads on a nut, which saves weight and money.

Preloading

Preloading or torquing a bolt and nut makes sure that the bolt does not fail from fatigue. Preloading is normally done by a torque wrench calibrated in inch/lbs or newton/meter but an experienced A&P or Lame can torque a bolt by feel. More info about torquing bolts and nuts.

NPT

For pipes and fittings mainly used in fuel and oil lines the National Pipe Thread, NPT, is used. With NPT, the thread (also 60°) is tapered under an angle of about 1.7899°.

By using a tapered thread a good seal is obtained where a straight thread is only able to hold the parts together. The taper makes sure that the flanks are also compressed together. For a leak free seal a teflon thread seal tape or sealant compound (Loctite®) is used, this also prevents corrosion and facilitates future removal should that be required.

Whit worth

British Standard Whitworth (BSW) has a thread angle of 55°, so these won't fit ISO metric or UTS ASME/ANSI hardware. Used mainly in the United Kingdom and in Australia, but ISO Metric is becoming more popular than the old Imperial units. ISO 7 also still defines BSW for British Standard Pipe Thread.