XT600 Front Sprocket Disaster.

[DRPMC]

I've got a 1985 XT600. Up until a couple of days ago it's been a super bike and competely reliable. Its done about 35,000 KM (spent the first part of its life in Italy so has a KM speedo). Then when I was pulling away from the last roundabout on the way to work I suddenly lost all drive. Engine running, gear box still OK but open the clutch and nothing happened. I thought at first that something in the clutch had gone but was horrified to find that the front sprocket has completely worn away the splines on the gearbox output shaft. Before I took the sprocket off I had thought it was the other way round so felt sick to see the shaft worn round.

So I'm going to have to drop the engine out and split the cases but would like to establish what has caused this to happen. Wonder if anyone else has had this happen? The chain was a new heavy duty one and kept well oiled and I fitted a new retainer plate with the sprocket, which was a pattern part rather than OE. The chain was also kept tensioned but was by no means tight.

Any thoughts and suggestion gratefully recieved.