The tine unit on this 1986 Troy-Bilt PTO Horse rototiller wouldn't engage due to a seized dog clutch/PTO shaft. This video shows disassembly of the dog clutch, freeing the seized mechanism/PTO shaft, reassembling the dog clutch/PTO and packing with grease. Freeing the seized mechanism required penetrating oil, vibration (light tapping), and heat. If you encounter this situation, one (or all three) of those techniqes may be necessary.