Usage:
Threads in bolt holes can become damaged from cross threading, loose parts and using damaged bolts. Retapping the hole will not give the quality of thread that is needed to meet torque specifications in most applications.
Heli-Coil inserts and installation tools have been released for making repairs to damaged threads. Heli-Coil inserts are available in most fine and coarse pitch, unified and metric size threads. A wall chart, Form SMHS8438, gives Heli-Coil sizes and tools needed to install Heli-Coil inserts.
The lengths of all Heli-Coil inserts, except the 6V9074 Insert and 6V9076 Insert, are one and one-half times their diameters. The lengths of the 6V9074 Insert and 6V9076 Insert are two times their diameters. The inserts may be cut to a shorter length or multiple inserts may be stacked if required lengths are not standard.
Hole Preparation
1. Use the wall chart to find the correct drill size. Drill the hole that has the damaged threads. The drill depths suggested allow for tap end clearance, countersink, and chamfer of the top. These depths are minimum and should be increased when possible. On thread relieved holes, such as head bolt threads in the cylinder block, depths should be held from the bottom of the counterbore.
NOTE: It is recommended that a magnetic base drill, drill press or radial drill be used when possible, especially with larger diameter holes.
2. Countersink the drilled hole using a countersink with an included angle of 120°. Countersinking is not required on thread relieved holes.
3. Tap the drilled hole using the tap and depth shown in the chart.
Installation Of Coarse Thread Inserts
1. Screw the insert on the installation tool with the tang toward the front. Install the insert on the tool until the tang engages the driving contour of the tool.
2. Position the tool and insert squarely over the tapped hole.
3. Rotate the tool, exerting slight pressure against the work, until the insert is engaged in the tapped hole. Continue to rotate the tool until the insert is one-fourth to one-half thread below the top surface. On thread relieved holes, install the insert to the depth of the original thread.
Installation Of Fine Thread Inserts
1. The tool to install fine thread inserts has two parts, the body and mandrel.
2. Remove the mandrel from the body. Place the insert on the mandrel and engage the tang on the driving contour of the mandrel.
3. Install the insert and mandrel into the tool body. Rotate the mandrel until two threads of the insert are engaged in the tool body.
4. Place the tool squarely against the tapped hole. Rotate the mandrel, while holding the body of the tool with one hand, until the insert is one-fourth to one-half thread below the top surface. On thread relieved holes, install the insert to the original thread depth.
NOTE: Do not hold the tool body tightly against the work surface. Allow the tool to float.
Tang Removal
The tang must be removed after the insert is installed to allow the bolt to be engaged to the correct depth.
On .5 inch, M12 and smaller inserts; the tang must be broken off at the notch in the bottom coil. Use a punch or rod that just fits into the assembled insert. Make sure the punch or rod has a square end. Place the punch or rod in the assembled insert until it rests on the tang. Holding the tool squarely, strike the tool sharply with a hammer.
The tang on inserts .562 inch, M14 or larger, the tang must be broken off at the notch in the bottom coil by using a long nose pliers. Bend the tang up and down to break it off at the notch.