Replacement spring balance

Question

Milgrad single hung window.  Weight 40lbs length of old spring balance 33".  No stamps or markings on old spring balance.  Windows originally purchased in 1996.

Bob · Accepted Answer

Hello there. Is the metal channel, not including the nylon attachments at both ends, 33" long? If so, then this looks like either an S380-33 or S385-33. The only difference between these two balances is the depth of the bottom nylon shoe, or the 'heel-to-toe' measurement. See the diagram below for reference. Compare both with your old hardware to see what matches best.

Comparison of the S378, S380 and S385 balance shoes.

In either case, for a window sash that weighs exactly 40 pounds, choose a stamp option of 3270, which will support a window sash in the weight range of 39 through 42 pounds.