Crossthreads

Crossthreads is a term used in mechanical engineering to describe the misalignment of threaded components, such as a bolt entering a tapped hole, where the thread profiles do not engage along the correct helix. The plural form is used to refer to multiple instances of such misthreading within an assembly or batch of parts.

Causes of crossthreads include misalignment during initial engagement, use of incompatible thread standards (for example metric versus inch), damaged or dirty threads, and applying force with the part at an angle. Loose tolerances, worn screw holes, and cross-threading when starting threading too aggressively can also contribute.

Crossthreading can damage the threads, reduce clamping force, increase wear, and lead to fastener failure or leakage in pressurized or load-bearing joints. In severe cases, the hole or the fastener may become stripped, requiring repair or replacement of components or threads.

Detection methods involve visual inspection and feeling resistance during threading, as well as the use of thread gauges or go/no-go tools to verify pitch and engagement. Torque testing can reveal abnormal binding or reduced torque capacity that indicates misthreading.

Prevention focuses on proper alignment, cleaning and lubrication of threads, and ensuring the correct fastener size and thread pitch are used. When assembling, start threads by hand to maintain alignment and avoid overt tightening. If crossthreading occurs, stop and realign; damaged threads may be repaired with inserts such as Helicoil or Time-Sert, or the component may need retapping and replacement.