The thread tension dial holds the cotton at a specific tension to allow for an even stitch when the thread passes through the fabric. The bobbin tension is set to hold the bobbin thread at the proper tension. If the tension dial on your machine is no longer working, you can replace it. The first step in replacing the dial is to make sure it is bad. Clean the discs between the dial and retry the stitching. Hopefully, the dial will just be dirty. This is a much easier fix than replacing the dial.
Turn the tension dial to the left to unscrew, on machines with a tension dial on the outside of the machine. Continue turning the dial until it comes off the pin. Remove the springs and the discs. Place the items in the order they are removed from the machine for easy reassembly. Look at the pieces and make sure the new dial has the same parts.
Open the new tension dial. Insert the new dial on the machine in the proper order. Note: The common placement of the parts begins with the spring, two discs, spring and dial. Check your order to make sure.
Tighten the dial halfway. Thread the machine. Sew a sample seam. Adjust the tension until it is correct.
Open the top or front plate on your sewing machine. Locate the tension dial mounting.
Remove the mounting with a small screwdriver. Remove the tension dial from the housing. Make sure to place the items removed in a logical order for reassembly.
Insert the new tension dial in the proper location. Reassemble the housing and attach the new tension dial in the machine.
Thread the sewing machine. Sew a sample seam. Adjust the tension accordingly.