Hi fella's, appreciate your reply.
(dadof2) All of the gears/crank are original, I have recently replaced the crank seals, clutch plates and various ignition parts but other than that all of the engine is original, I haven't split the engine at all.
(feetupfun) Both sprockets I have used are Talon 10T. I have had the same thought about faulty sprockets but Talon products are usually top quality.
No lub was used during assembly.
I'm now thinking that it maybe the 'locking tab washer' thats at fault.. It is the original and appears to be opening at the point where it contacts the sprocket (not where it contacts the nut) I have torqued the nut to 50 ft-lbs both times and I'm sure the torque wrench is correct.
At the moment, I only have pictures of the 1st sprocket that split but it definitely has witness marks on the inside edge where it has been rubbing against the engine case.. It must have been under a massive amount of force to move it that far up the taper shaft!!
I will post pics of the 2nd sprocket when I get time to have a look at it.
Any thoughts???