Left-hand threads: How to recognise them – and how to cut them correctly

Left-hand threads are encountered much less frequently than standard threads because they are not the standard, but the exception. For this very reason, they repeatedly cause uncertainty in workshops, maintenance and assembly.

The basic principle, however, is quickly explained: A left-hand thread is the mirror image of a right-hand thread. It is tightened counterclockwise and loosened clockwise.

It is used wherever a normal right-hand thread could loosen itself through rotation or where incorrect connections are to be prevented by design. Typical examples include the left bicycle pedal, rotating components and fittings on gas cylinders.

Difference between right-hand thread and left-hand thread

Right-hand and left-hand threads do not differ in their basic principle, but in the direction of rotation:

• With a right-hand thread, tightening is done clockwise.
• With a left-hand thread, tightening is done counterclockwise.

In practice, this difference is crucial because it determines whether a thread engages cleanly or is already damaged when being started.

How can you identify a left-hand thread?

In practice, you can identify a left-hand thread mainly by the following points:

• Check the marking: In drawings and designations, “LH” usually stands for Left Hand.
• Look at the flank direction: Viewed from the front, the thread turns rise from the bottom right to the top left.
• Consider the application: Left-hand threads are often used where a right-hand thread could loosen itself through movement.
• Do not use force: If a screw does not engage cleanly, the thread direction should always be checked first.

Where are left-hand threads typically used?

Left-hand threads are used wherever a normal right-hand thread could loosen itself through movement or direction of rotation. Typical examples include the left pedal on a bicycle, rotating components, certain clamping and adjustment elements as well as fittings on gas cylinders.

Especially in such applications, the different thread direction is not a special case without reason, but a design solution against malfunction and self-loosening.

Cutting internal threads: step by step

It always starts with the correct preparation of the core hole. For metric internal threads, the well-known rule of thumb applies:

Core hole diameter = nominal diameter – pitch

For an M8 x 1.25, this results in a calculated value of 6.75 mm; in the table, 6.8 mm is used in practice. The core hole is then drilled and the hole is lightly countersunk so that the tap starts cleanly. For blind holes, the required depth must of course also be taken into account.

For manual cutting of an internal thread, hand taps are used in several stages. Commonly used are taper taps, second taps and plug taps. The ring marking helps with the sequence: one ring for the first cut, two rings for the second, no ring for the plug tap.

This division reduces the chip load and leads more cleanly to the final dimension. Those working by machine use machine taps instead, which produce the thread in a single operation. The form is decisive here: Form B is intended for through holes, Form C or C/35° RSP for blind holes.

VÖLKEL recommends always aligning the choice of tap consistently with the application and type of hole. Especially with left-hand threads, a cleanly coordinated tool concept pays off, as errors occur more quickly due to the unfamiliar direction of rotation.

During the actual cutting process, control matters more than force. The tap must be aligned cleanly along the axis, the cutting medium should suit the material, and with a left-hand thread, cutting is carried out consistently counterclockwise.

The first start is particularly critical. If the tap is applied in the wrong direction or tilted, the first thread turns can already be damaged. Especially with left-hand threads, this can happen quickly if work is unconsciously carried out in the familiar direction of rotation.

From VÖLKEL’s point of view, straight guidance, the right coolant lubricant and stable process control are particularly decisive here in order to achieve a dimensionally accurate and clean thread.

Cutting external threads: how to do it cleanly with the die

For external threads, preparation is at least as important as for internal threads. First, the bolt is brought to the correct pre-machining diameter and lightly chamfered at the front end.

VÖLKEL provides its own tables with recommended bolt diameters for this; for an M8, the recommended pre-machining diameter is around 7.87 mm. Here, too, the following applies: A cleanly prepared start often determines whether the thread runs precisely or starts crooked.

The external thread is cut with a suitable left-hand die in the die holder, ideally supported by a guide.

From VÖLKEL’s point of view, clean tool guidance when cutting external threads is one of the decisive factors. A die guide ensures that the tool remains exactly aligned axially to the bolt and thus prevents typical starting errors.

With a left-hand thread, the die is turned counterclockwise onto the bolt. If the die is applied the wrong way round or turned in the wrong direction, it does not cut cleanly, but presses and damages the thread flanks. The result is a damaged thread that is no longer dimensionally accurate and can no longer be assembled.

Once the desired thread length has been reached, it is removed again clockwise. During the cutting process itself, cutting oil, a straight start and controlled working are decisive, not high force.

A practical example shows the difference between internal and external threads very clearly: For an M8 x 1.25 LH, the internal thread requires a core hole of 6.8 mm, while the bolt for the external thread is prepared to approximately 7.87 mm.

The thread direction therefore does not change the basic logic of preparation; it only reverses the cutting process.

The most common mistakes with left-hand threads

Most poor left-hand threads are not caused by the unfamiliar direction of rotation, but by classic preparation errors. The following points are particularly typical:

• incorrect core hole or bolt diameter
• missing chamfer
• crooked start
• too little lubrication
• wrong tap form for the respective type of hole
• confusing the direction of rotation

The last point in particular is especially critical in practice. If a left-hand thread is treated like a right-hand thread, this not only causes assembly problems, but often also permanent damage to the thread.

From VÖLKEL’s point of view, the cause of faulty threads is in most cases not the tool itself, but the preparation and application.

Those who prepare cleanly, start straight and choose the right tool for through holes or blind holes create the basis for a precise and load-bearing left-hand thread.

How to check the result

Anyone who wants to work precisely does not rely only on the feel when screwing in the mating part. Thread ring gauges are used for external threads, and thread plug gauges for internal threads.

A simple principle applies:

• The GO side must be possible to screw on or in completely and easily.
• The NO-GO side may only screw in by a maximum of about one to one and a half turns.

VÖLKEL recommends not judging threads only by feel, but consistently checking them with suitable gauges. This is the only way to ensure that dimensional accuracy and function are reliably guaranteed even under load.

If an existing left-hand thread is only slightly damaged, careful recutting with the appropriate tap or die can often already be sufficient.

In the case of heavier wear or clearly damaged thread turns, however, it should be checked whether reworking is still technically sensible or whether the component must be replaced. This is also part of practical work: Not every thread can be repaired economically or technically cleanly.

Conclusion

Left-hand threads are less common in practice, but with the right approach they can be produced just as safely as right-hand threads.

Anyone who identifies them before starting, observes the LH marking, prepares the workpiece correctly and consistently works counterclockwise when cutting will achieve results just as clean as with right-hand threads.

For practical use, VÖLKEL offers suitable left-hand hand taps, machine taps, dies and complete left-hand thread sets, in other words exactly the tools needed to cover workshop, maintenance and series applications cleanly.

From VÖLKEL’s point of view, left-hand threads differ not in the technology, but in the attention with which they are produced.