Prime-Line Products H 3630 Wood Window Sash Balance, 12-Pound, Steel Construction Used in single and double hung WOOD windows with both single and dual installation of sash balances Requires 3-1/4 in. x 6-3/8 in. opening Replaces balances manufactured by Duplex, Superior, Preset and Fabco Constructed of steel housing and aircraft cable lifters with coil spring retraction