how do i replace the upper gear on the verticle shaft that drives the bobbi?

Hello Brenda,
That was the very first model I ever sold to my first customer in 1968, if you e-mail me drivewithken@blueyonder.co.uk I will type up full instructions for you, do you have the new gear? if not tell me if you are in the UK or US and I will tell you where to get one, they are quite cheap, I won't be able to type up instructions until later today because I have to go to work just now.
Ken

drivewithken@blueyonder.co.uk
January 2008