Single Hung Window balance springs

Question

window frames are 31.75" (H) x 51.5 (W)
Stamp on balance spring 29 5 3 SI RM
Length of aluminum without tips: 30"

Bob · Accepted Answer

Thanks for posting. This looks like an S380-30 or S385-30 Channel Balance. The only difference between these two balances is the 'depth' of the bottom nylon shoe, or 'heel-to-toe' measurement. See the diagram below for reference. Measure your old balance's shoe to see what matches best. For a stamp option choose 2950. Let me know if you have other questions.

Comparison of the S378, S380 and S385 balance shoes.