Balance spring replacements

Question

Hi a I need to replace 2 balance springs and not sure which to order.
Shows 26 4 and BSI
1/2 inch wide x 5/8 inch high
Thank you Penny

Bob · Accepted Answer

Hey Penny. This looks like either an S380-27 or S385-27 Channel Balance. The only difference between these two balances is the depth of the bottom nylon shoe, or the measurement from 'heel' to 'toe'. See the illustration below for reference. Compare both balances with your old one to see what matches best. In either case, for a stamp option choose 2640. Let me know if you have further questions.

Comparison of the S378, S380 and S385 balance shoes.