You might find on a high mileage bike that whatever means used to line up the split gear in the drive unit, the crank may not stay stationary due to old age, so preventing it rotating by locking it in gear as in my method may need to be done. My old White Pan had 200,000 miles plus when I repaired my alternator, which had siezed and cracked the case due to water ingress, common on Pans used daily in all weathers. Mine had two careful owners before me, the Police and a Courier 

