Its called a"disc key" have seen some sold at one time or another ,but best is to make one up if you can .Grind one out of a wide blade screw driver with a big grip or if it can be used in a old style carpenters brace so you can put pressure on it to lessen the chances of it sliping out and damaging the disc . When disc's are in tight they can take a lot of getting out and it is easy ,as they say in the trade ,to "totaly xxxxxx it up."