I have the SP-101 probably the 2.5" bbl (never measured). It's a brick but I sold my 686 15 yrs ago and now the price went up $300, 7 round cylinder, and I said the heck with that!
I also bought the Wold spring kit and after joining a certain forum, got the pdf download that has detailed instructions with pics of taking the SP-101 down, what to file, sand, buff etc. replaced the factory springs with 9 or 10# mainspring, cleaned the firing pin spring chamber and left that spring alone.
When re-assembled the revolver fires double action way smoother than my stock 686 was and I can shoot soda cans all day long - double action. Buttery smooth!