bushing that protects the mower engine if you hit a rock?

on my craftsman 917.377390, it seems to easily engage a safety feature to protect the engine, my father (previous owner) told me that he had to get it fixed a few times, but sears charges 90 bucks a pop to replace the "bushing" if that is really what it is. I guess when the feature engages the busing sheers and then throws off the timing and shuts the mower off. He was also told by someone that it's really not very difficult to do yourself. The problem with my mower is that it happens whether a rock is hit or not, but just over time it decides to give way. Well I'm not paying 90 bucks every 2 years when this happens. I'd prefer to learn how to do it myself. Does anyone have any suggestions on how to get the "bushing" replacement and realign the timing on this mower?