Note May 14, 2010: The newest batch of these spring that I received are too weak for most clocks. When released from their clamps, the springs open out only to about 5 inches (as compared to over 11 inches on the original batch). I cannot recommend these springs any more. I just tried one on the time side of a Seth Thomas No. 89 movement, and it was too weak to be satisfactory.
Merritt’s Antiques has a new 3/4 inch by 0.0165 inch by 96 inch mainspring for American clocks. It is part no. P-1956, currently selling for $7.00 each individually (quantity discounts are available). It is a higher quality, American made version of their P-1496 that I have blogged about earlier.
I bought one of these springs to test. The quality appears to be excellent. The thickness measured as specified, 0.0165 inches. It is tempered to a nice uniform blue, and is well finished with a nicely made, neatly attached loop end.
It seems to have plenty of power, as the coils open way up when the spring is first uncoiled. It may be too strong for some clocks.
Tomorrow I am going to install it in an Ansonia long drop octagon clock with early type movement.