Unless you've rebuilt these before, take lots of pictures while testing it down. These are like watches in some respects as they have a very fine and easily damaged "Hair Spring" a device that ads resistance to the magnet and creates the speed indicator for the needle. Use a good synthetic click oil to lubricate the gears ..to get it to work properly it will need to be torn down completely so that all the gear pivots and the holes they sit in can be cleaned before reassembly.