Hello GRB,
As I have not had any experience with a Model 40 I've to date not chimed in.
I must admit and no disrespect to CyberJack whatsoever as we all have different methods, but the SOP for this type of situation is to put the bed knife into a vice and clamp it hard right in the location of the set screw and with the screw facing upwards, then using a drift of a slightly smaller size as the screw head and then give the screw a solid sharp blow. Don't forget to add some Penetrene or similar. From there you should be able to use the correct size slotted screwdriver and undo it. If all else fails then I would go down the road Cyberjack has suggested. I have watched quite a few fitters use the above method and it works just about everytime.
As far as impact drivers are concerned I've found them to be nothing more than screw head destroyers and they generally maggot the screwdriver receptacle area, thus making the job even harder.
Regarding the sprocket removal, I've learnt something new from Cyberjack.
