The axial length over which external and internal threads are in contact.
Engagement length determines whether the bolt or the nut's threads fail first. The traditional rule is one times the nominal diameter for same-material joints, or 1.5 times when the nut is softer than the bolt.
Percent engagement is a different quantity and describes how much of the full thread depth the tap drill leaves — about 75% is the usual target when selecting a tap drill for steel.
Confusing engagement length with percent engagement. One is a length in millimetres, the other a percentage of thread height.
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