So you're trying to use the kind that hooks two sides of the spring. That works on an M103, but not an M116/7 with the cam in place. Just about any auto parts store has the kind that's a 15" bar with a couple of hooks on the end that catch the cam and a couple of bars midspan than press down on the spring retainer.

Sixto
91 300SE
87 300SDL
81 300SD