The length of full-form thread actually in contact between a bolt or screw and its nut or tapped hole. A common rule is at least one bolt diameter of engagement in steel, about one and a half diameters in cast iron and brass, and two in aluminum and other soft materials, so the screw breaks in tension before the internal thread strips. Chamfers and incomplete threads at the hole entry do not count.
Only five-eighths of thread engagement with a half-inch screw in that aluminum boss, it'll strip, go longer or put an insert in.
When a thread strips instead of the screw breaking, check engagement and nut grade before blaming the torque wrench.
Also heard as
- length of engagement
- LE
Related terms
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.
Percent thread
How deep an internal thread is compared with full theoretical depth, set mainly by the tap drill size.
Nut
An internally threaded part that mates with a bolt or stud, graded so the bolt breaks before the nut strips.