The "D" shaped end is actually a "nut" threaded onto the recoil rod. The only way I could get it unscrewed without damaging the rod was to push the spring down the rod at the other end, wrap several layers of duct tape around the rod (to hold the spring/guide back and to protect the rod) and hold the rod where you taped it with a pair of locking pliers. I then used an adjustable wrench to turn the "D" shaped end off. Be careful of the last few threads, the spring is under a lot of tension and can send parts flying.
After removing the old spring, install the new one, compress it and thread the "D" shaped end back on enough to hold it together. Then, use your wrench to turn it on the rest of the way.
BTW, I didn't tighten it all the way, I backed it back out about half a turn. It makes the recoil assembly fit just a hair tighter in the gun. I shot it with the new spring and the assembly tighted all the way, same groups as before. With the "D" shaped nut half a turn out, groups were half of the previous ones.
Once you get it back together, carefully remove the tape and use solvent to clean the tape glue.
There's probably a better way, but this was the only way I could do it given what I had on hand at the time.
Chris
