Broken Block and Tackle

Thanks for posting. I am afraid the cord or spring in these channel balances are not meant to be sold separately; when they break, the entire balance must be replaced. Start by looking at either the S380-18 or S385-18 Channel Balance. These look like yours. The only difference between the two of these is the depth of the bottom nylon shoe. See below. Measure your old balance's shoe depth to see what matches best. For a stamp option, choose 1730.

Comparison of the S378, S380 and S385 balance shoes.

For your S385-18, choose 1730, which matches your old balance's 17-3 stamp. This indicates your balance can support a sash in the weight range of 12 through 14 pounds.