Here is the photo of my sprocket. After adjusting the chain slack and axle, I can still hear a bit of grinding sound. Maybe because the chain is hitting the outer side of the sprocket (you can see the smooth sides of sprocket teeth). It appears to be a bit loose but I have no idea how to tighten those bolts. I mean do i tighten the 2 bolts on the side or do I need a special tool for the center thing. Please help.
You cannot actually tighten it to the shaft, those 2 bolts just hold the retainer plate, this is all that keeps the sprocket from coming off. Once the plate is removed you can slide the sprocket off of the output shaft.
Sprocket and output shaft are machined for a tight slip fit, so if there is play either the sprocket is worn, the output shaft is worn or both. Remove the Sprocket and inspect for wear, replace worn part(s).