Threads

There are a huge array of thread available. Depending on application and country of origin depends on which thread is chosen.

Over the years, many attempts have been made to unify all of these threads into one standard, but each attempt has simply added another thread to the list.

Many older English cars use BSF (British Standard Fine), BSW (British Standard Whitworth) or BA (British Association). When British engineering dominated the world, these threads were the norm. When American owned Ford and General Motors began to export to all corners of the world, the American unified UNF and UNC threads took over. Now that new car design has strong roots back in Europe the Metric threads have become virtually 100% dominant – in Europe at least (America will no doubt follow in due course). Some US cars have an array of thread types.

Certainly, as any car builder will tell you, many older nuts and bolts are only available from specialists. Metric threads are available from most DIY stores.

As a norm, threads are ‘right handed‘. This means they need to be turned in a clockwise direction in order to tighten them. ‘Righty tighty, Lefty Loosey’

How to identify a Thread

To accurately identify a thread several pieces of kit are and advantage:

Micrometer or Vernier Calipers
Thread Gauges

Thread Gauges Vernier Calipers

Step 1:

Measure the outside Diameter of the thread. Caution! some thread diameters are very close to others. This is not a diagnosis test on it’s own.

Step 2:

Measure the number of threads per inch (TPI). The easiest way of doing this is with a set of thread gauges.

NB. − For a metric threads, 12.7 per inch would be 2mm e.g. 25.4/12.7 = 2mm

With some threads it maybe better to count the number of threads over several inches, to get a more accurate TPI count.

38.1 turns over 3inches = (3 x 25.4) / 38.1 = 2mm
or 76.2 / 38.1= 2mm

Right and Left Handed Threads

On a car, the high speed rotation of a part and the associated momentum, can cause nuts and bolts to over-tighten or loosen. For this reason drive-train parts often use Left and Right Hand threads. A component on the near side of the car may have the opposite thread direction to a component on the off side.

Threads per inch (TPI)

This is the number of turns a thread makes in one inch. Metric threads are expressed ‘Thread Pitch’

Thread Pitch

This is the distance taken for a thread to complete one revolution. It is the distance from a point on one thread to the same point on the next thread. The measurement is taken parallel to the axial plane.

Thread Included Angle / Angle of Thread

The angle of thread is the angle of the ‘V’ shaped groove between two adjacent Flanks. It is measured at the cross section of the screw thread and includes the axis of the screw thread.

BA – 47½°
BSF – 55°
BSW – 55°
UNF – 60°
UNC – 60°
Metric Fine – 60°
Metric Course – 60°

Flank

This is the flat surface of the thread excluding the radius at top and bottom of grooves (Root & Crest)

Root and Crest

Some threads have a radius on the crest and roots of their profile. Others simply have a flat. This can be felt with many threads simply by running a finger over them. A sharp edge to the thread would be undesirable as would be easy to round-off, fracture or strip. The Metric thread standard allows for flats or radius.

Root

This is the bottom radius of the thread groove. It joins adjacent Flanks.

Crest

This is the top radius or tap surface of the thread groove. It joins the two side (Flanks) of a thread.

Flank Angle

This is the angle between an individual Flank and a line perpendicular to the thread axis. It is measured in the axial plane.

Lead

This is the axial distance a screw advances in one complete revolution.

Lead Angle

This applies to parallel threads and is the angle formed by the flank and a line perpendicular to the screw thread axis.

Major Diameter of an External Thread

This is the diameter of a virtual cylinder drawn along a External thread, touching the crests.

Minor Diameter of an External Thread

This is the diameter of a virtual cylinder drawn through an external thread, touching the roots of the external thread

Major Diameter of an Internal Thread

This is the diameter of a virtual cylinder drawn through an internal thread, touching the roots of the internal thread

Minor Diameter of an Internal Thread

This is the diameter of a virtual cylinder drawn through an internal thread, touching the crests of the internal thread

Pitch Diameter

This is the diameter of a virtual cylinder drawn through a thread, where the width of the thread and the width of the space threads are equal.

Simple Pitch Diameter

This is the diameter of a virtual cylinder drawn through a thread, where the thread groove width (determined by the axial direction of a thread) equals half of the reference pitch. The simple pitch diameter equals the pitch diameter for many standard taps.

Virtual Pitch Diameter

This is the diameter of a thread, that would fit without interference and play over a given thread engagement.

Thread Height

This is the distance between the:

Major Diameter Internal and the Minor Diameter Internal or
Major Diameter External and the Minor Diameter External

Thread Information

Close Up Thread Info