Auto lube drive cog brakeing

on saturday my brother was going down the road on his vespa with 210 kit he changed down from 3rd to second the back wheel locked up he pushed it home at first we thought it had seized took head and barrel off not seized as we had just put a t5 4th gear in on thursday thought we might have done some thing wrong when putting in 4th gear seamed strange as he had all ready done 12 miles with no problem the crank would only move about 20 mm back or forwards . so we split the caseing last night gears ok a piece of cog was laying in the engine turned out to be the auto lube drive cog that fits over the crank shaft has broken going to take out the clutch tomorrow night to see what else has broken has any one had this brake if yes do you know why thank Gary

Hi found out the problem was the barrel stud had wound into the autolube drive cog braking some teeth of it and the drive cog that goes onto the crank was broken into 7 pieces so i have cut 4mm of the stud and replaced the cogs scooter back on the road now. i have since read in scootering number 225 tech tips page 112 that this can happen on 200cc engines