Which replacement do I need? 20-4 BD5 CMC 87

Thanks for posting. I'm pretty confident we can replace this balance for you, but we need more information to be able to help. The stamp code only tells ut the strength of the spring inside the balance. We need to see photos fo the nylon fittings on either end of the balance; this is what determines the Series or type of balance you need. From there, we'll do our best to narrow down precisely what you require. Thanks. 

These photos are perfect, thank you. Have a look at our S385-21 channel balance. This comes assembled with a 19-005 balance top and 15-004 bottom balance shoe. You will need to compare your nylon fittings with ours to confirm this is the right balance. If everything looks correct, then you can go with the 2040 option that holds a sash weight of 15-23 pounds.