{{Techniques infobox
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|parts           = [[Bolts]], [[Nuts]]
|techniques      = 
|tools           = [[Wrenches]], [[Screw drivers]]
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|reversible      = true
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[[Category:Techniques]]

=Introduction=
By common convention, right-handedness is the default handedness for screw threads. Therefore, most threaded parts and fasteners have right-handed threads.

=Challenges=
Left-handed thread applications include:
* Where the rotation of a shaft would cause a conventional right-handed nut to loosen rather than to tighten due to applied torque or to [[Precession (mechanical)|fretting induced precession]]. Examples include:
** The [[bicycle pedal|left hand pedal]] on [[bicycles]].
** The left-hand grinding wheel on [[bench grinders]].
** The axle nuts, or less commonly, [[lug nut]]s on the left side of some automobiles.
** The securing nut on some circular saw blades – the large [[torque]] at startup should tend to tighten the nut.
** The spindle on [[brushcutter (garden tool)|brushcutter]] and [[line trimmer]] heads, so that the torque tends to tighten rather than loosen the connection
* In combination with right-hand threads in [[turnbuckle]]s and clamping studs.
* In some gas supply connections to prevent dangerous misconnections, for example:
** In gas welding the flammable gas supply uses left-handed threads, while the oxygen supply if there is one has a conventional thread
** The [[POL valve]] for [[LPG cylinder]]s
* In a situation where neither threaded pipe end can be rotated to tighten or loosen the joint (e.g. in traditional heating pipes running through several rooms in a building). In such a case, the [[Coupling (piping)|coupling]] will have one right-handed and one left-handed thread.
* In some instances, for example early [[ballpoint pen]]s, to provide a "secret" method of disassembly.
* In artillery projectiles, anything that screws into the projectile must be given consideration as to what will happen when the projectile is fired, e.g., anything that screws into the base from the bottom of the projectile must be left hand threaded.
* In mechanisms to give a more intuitive action as:
** The leadscrew of the cross slide of a [[Lathe (metal)|lathe]] to cause the cross slide to move away from the operator when the leadscrew is turned clockwise.
** The depth of cut screw of a "Bailey" (or "Stanley-Bailey") type metal [[plane (tool)]] for the blade to move in the direction of a regulating right hand finger.
* Some [[Edison base]] lamps and fittings (such as those formerly used on the [[New York City Subway]] or the pre-[[World War I]] [[Sprague-Thomson]] rolling stock of the [[Paris Metro]]) have a left-hand thread to deter theft, since they cannot be used in other light fixtures.

=Approaches=
Used to recall the direction a standard screw, bolt, or nut must turn to either tighten or loosen, right meaning clockwise and left counterclockwise.

=References=
* [https://en.wikipedia.org/wiki/Screw_thread Wikipedia: Screw thread]
* [https://en.wiktionary.org/wiki/lefty_loosey,_righty_tighty Wiktionary: lefty loosey, righty tighty]