Thread inserts
An M8 wire thread insert 1.5D long needs an 8.3 or 8.4 mm drill, an EG M8 STI tap, 13.25 mm of full thread and a 23.5 mm deep drilled hole when the hole is blind and tapped with a plug tap. Type or pick the thread below: the Make tab’s Tap drill view, on Insert, gives each maker’s drill for the STI tap, the tapped hole’s limits and its gauge. Beside the drawing, pick the parent material or the insert length, the hole class (6H mod or the closer 4H5H for metric, 2B or 3B for inch), a free-running or screw-locking insert, a blind or through hole and a plug or bottoming STI tap: it works out how deep to drill and tap, or how thick a plate a through hole needs.
The insert is a coil of diamond-section wire. Its outside screws into an oversize thread, the STI thread, cut at the screw’s pitch; its inside is the screw’s own thread. The hole is the same for a free-running insert and a screw-locking one.
How deep to drill and tap for a thread insert
Tap full thread for the insert’s length plus one pitch, so the coil sits wholly in full thread below a 120° countersink. Drill deeper by the tap’s lead and a pitch of clearance: six pitches beyond the insert for a plug tap and four for a bottoming tap. Small plug taps, 5/16 in and M8 and under, have a pointed male centre, so add half the diameter for it. These are the minimum depths in NASM33537 and the makers’ tables.
A through hole needs a plate at least as thick as the full thread: the insert’s length plus a pitch when countersunk, the insert’s length when not.
Free-running or screw-locking?
A free-running insert lets the screw turn freely, as a tapped thread does. A screw-locking insert has a coil wound out of round that grips the screw’s flanks, so the screw holds by friction without a lock nut or thread locker and can be taken out and put back. Both go in the same STI tapped hole.