Happens when a fastener enters at an angle and its thread climbs over the mating thread, so every turn damages both. It shows up as sudden tightness after only a turn or two, or a nut sitting cocked on the stud. The cure is prevention: start every fastener by hand and turn it backward until the threads drop into step with a small click before turning it forward.
Somebody cross-threaded the spark plug, it went tight after two turns and they just kept cranking with the ratchet.
Also heard as
- cross-threaded
- cross-threading
Related terms
Chase
To run a tap, die or thread chaser through an existing thread to clean and reform it without cutting it oversize.
UNF
Unified National Fine: the inch thread series with more threads per inch, such as 1/4-28, 3/8-24 and 1/2-20.
Thread stripping
Failure where the threads of a nut, tapped hole or bolt shear off instead of the bolt breaking.
Helical insert
A coil of diamond-section wire screwed into an oversize tapped hole to repair a stripped thread or armor a soft one.