I’ve noticed that the front sprocket on my DRZ400 seems to be too large. It has cut a groove into the swingarm. I’m obviously quite shocked to see this and want to fix it so there’s no risk of more damage.
The front sprocket has 15 teeth, and the rear 48. The chain adjuster is on the tightest/minimal setting.
Consider this. When the sprocket went on, it can’t have fouled the swingarm, otherwise the bike wouldn’t have moved.
Hence I suspect the sprocket fitted back then, and slowly as the bearings wore out, the chain pulled it forward and the sprocket ate into it a little at a time.
Pop the chain off the sprocket, stick a car jack under the sump (get Claire to hold the bars) and you’ll be able to wiggle the swingarm if I’m right.
As it is there is no way the sprocket would have fitted on to the shaft. You’ll need to remove the chain and relax the tension on the swing arm to remove the sprocket. New swing arm bearings, chain & sprockets kit for sure and possible weld up the cut on the swing arm. i’m surprised you didn’t notice a tendency to fish tail or that it passed muster on the MOT, I’m assuming it has a ticket.
edit: Check the gearbox output shaft bearing while your there too.