OK. Here is the tool I made for helping to refasten the strong chain brake spring that is used on almost all chainsaws that Stihl sells today and upwards of 20 years ago or more. Doing it with a screwdriver is almost impossible, although I managed to do it after severe frustration using two screwdrivers at once. Stihl offered a Pic of one in a couple of service manuals and perhaps even sold it at a few shops. My dealer, however, never had one in stock, so I decided to make an equivalent. This one works:
Another view:
I made the steel blade using a 5/16" ordinary bolt about 3" long. I cut off the head with a hacksaw and then milled a flat about 3/8" long, a little over halfway though. I milled a fillet inside the flat with a 1/4" dia. grinder stone to produce a cup shape on the inside where the flat was. That fillet eventually mates with the stud in the case that secures the strong spring.
Next I embedded the threads of the bolt inside a hardwood oak handle with a pre-drilled hole. Being a woodworker, it was easy for me to turn the handle on a small lathe. The threads on the bolt sink into a 9/32" blind hole that I drilled with a hand drill slightly over an inch deep, holding the handle horizontally and tight with a vise as I drilled. That handle will hold the machined blade forever and give you the leverage that you need.
Now I can use this tool very easily to set and once again attach the strong spring that controls the chain brake. That's a piece of cake with this tool but an absolute bear cat without it. The strong spring slips over the round shaft of the blade and slides down on the back side. You then remove the blade of this tool and the spring snaps into place onto the stud. A dab of grease or bar oil on the back side of the blade allows the end of the spring to slide down and off a bit easier.
Cheers!