I just got my T.H.E. SS guide rod and 22lb. recoil spring for my 21SF. I installed the washer behind the retaining bolt at the end of the guide rod with the spring installed. I am stuck at this point, as the rod with the spring attached will not go into place into the lug under the barrel. I know this is a stiffer spring than the stock spring, but I don't want to damage anything by trying to force it in place. I am guessing that this washer may not required on the ISMI recoil springs and was included for installing Wolff springs on this guide rod, as the stock guide rod with the polymer, attached retaining bolt does not have a washer behind it and it appears to be, of course, the same diameter as the T.H.E. retaining bolt. I didn't notice any instructions in the packaging, either I may have solved my situation here, but I just wanted to check with those who have installed this guide rod and spring on the 21 for any pointers. Thanks.