Wish I could help. I ran into the same problem a few years ago with a ASM 51 Navy. I tried three different springs--standard Colt, Wolfe and I forget the other--nothing was long enough. I just put the ASM spring back in and hoped for the best. No longer have the gun. I did try something--I elongated the screw hole on a Wolfe spring so it would ride forward. It actually worked but you had to fuss around with the spring when tightening the mainspring screw. I believe I even used a lock washer. Not an elegant solution, but sometimes you get desperate.