thanks for the info .i pulled the cowl off and the two flywheels are the same dia but the 12hp one is taller even without the ring gear.i spose for the charging system.i havent pulled the existing flywheel off as yet as looks different for the coil setup i couldnt see the magnets on the outside of the 12 hp flywheel. on the brighter side i was wrong about shaft and it is a 25mm output shaft the same as the 8hp so i can rebuild and fit to the old girl it looks to be a standard bore which is clean and the piston and rod look good.as i said it is in pieces will need a valve grind and decoke but apart from that a freshen up kit and should be a good engine.
