Removed the reversed tacho from bike but another twenty years and my decrepitude has meant I am unable to remove bezel without destroying it. Will have to rely on my memory??? Remove front, needle, face, and main casting. Remove main mech and odometer keeping odometer drive pinion and screw. Swivel main mech to one side ( keeping needle spindle central ) until there enough room to engage redundant odometer pinion between rocking pinion and drive pinion. Mark out the three new holes remembering that you need to tap the odometer screw hole. Lightly oil and assemble. Calibration is by adding/removing weight to the escapement wheel, more weight = higher reading, less weight = lower. Difficult to describe but hopefully you get the gist