Sprockets could need replacement in several ways, a tooth can break off (rare), the teeth can wear out too much (common), or the sprocket can shear off. Shearing is caused when the track starts to slip off the sprocket and the torque causes the retaining bolts to break. Big pain in the butt to replace since the broken off bolts have to be tapped out of the holes.
Teeth wear is when the gap along the edge of the sprocket teeth start to wear away, The teeth have little wear marks to indicate how much wear is left in the teeth (much like a tire tread wear indicator). Nice thing about sprockets is that you can change the left one with the right one and then the wear starts on the other side of the teeth.
That mechanic is a stud if he can loosen up those sprocket bolts with one hand
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usually takes 2 guys standing on a cheater pipe. Very good looking dio, I really like the welding equipment.