4 Pack threaded star type tubing connector 1/4-20 threads. Fits 1-1/2" outside diameter round or square tubing with 16/18 gauge wall thickness. Our metal tube connectors are designed to grip the inside of tubing and prevents the insert from slipping out of square or round tubing. Heavy duty and tube connectors are utilized to secure tubing members to other pieces of tubing or to plates and walls. These versatile products are commonly used in ductwork supports, rack units, RV ladders, and cross member attachments.