If it is the entire assembly you need just replace the assembly with your new one.
How to replace garage door opener sprocket.
Position a work table near the area under your motor unit to hold the garage door opener.
Remove the chain and cable assembly.
10 return to the floor and pull the manual release cord to refit the trolley to the chain arm putting your door opener back into automatic mode.
This is a video on how to change the gear sprockets in a craftsman garage door opener.
Use a step ladder to access the garage door opener s power cord and motor unit.
Fasten the sprocket cover back in place and plug in the power to the garage door opener.
This was a craftsman model 139 53976srt.
If it is just a gear you are replacing you should remove it after you have unbolted assembly and replace it with a new one by going in reverse order.
Simply remove those for replacement.
Now you should find you can raise your door using the opener without the chain jumping.