These brass inserts used mainly in the injection moulding industry enable m3 screws to be retained in plastic parts. They are 4mm OD and 3mm long. Take up much less space than m3 nuts commonly uses in 3d printing. Insert them either during printing, or by screwing them onto the end of a long m3 screw, then heating them with a micro blowtorch for 5 seconds then pulling them into the plastic, allow 3-5 minutes to cool. Take care with high temperatures to avoid burns. Pk of 10 inserts.