A helix wound opposite to the standard right-hand thread, so the fastener tightens when turned counterclockwise as seen from the head. It is used where rotation or vibration would undo a right-hand thread, such as left bicycle pedals, the left wheel nut on many bench grinders, one end of a turnbuckle and fuel-gas cylinder fittings. Left-hand nuts are often marked with a groove or notches across the corners, and callouts add LH.
That's the acetylene fitting, left-hand thread, see the notch on the nut corners, turn it the other way.
Also heard as
- LH thread
- reverse thread
Related terms
Righty-tighty, lefty-loosey
Rhyme reminding that standard threads tighten when turned clockwise and loosen counterclockwise.
Lead
The distance a thread advances along its axis in one full turn, equal to pitch times the number of starts.
Pitch
The distance from one thread crest to the next, measured parallel to the screw axis.