I have one, picked it up cheap. the owner had taken some it apart breaking every bolt on the top cover. Don't know what he did once he opened it up but when I got it running it sounded like a lifter needed to be bled and refilled . I took it for a few rides, they are much quicker than our sohc and the same engine with different bores and strokes were used in other nighthawks so there are parts out there.
A few years before picking up this 81' 550 ,I picked up an 83' 650 , both engines are the same size and configuration so the 650 is a direct fit into the 550 frame except for the air box, so when I swap motors into the lighter ,smaller 550 frame , the airbox to carb boots will be swapped from the 650 to the 550 airbox completing the direct swap using the larger carbs that came with the 650 engine. Both frames are not interchangeable ,only the engine cradle is , the 650 is a heavier/stronger frame and the drivetrain and brakes are also larger. The 650 it's been said has different ( better ) gear ratios with a real overdrive . the 55o overdrive wasn't much overdrive and felt like you were actually loosing speed after shifting to overdrive. Can't tell you if the 650 engine will chew up the drive on the 550 ,but when it's finished I should be able to report on that.....whenever I get it finished. Oh , I put 3 inch rise bars on the 550 for the test drives, so much better, threw out those stock 550 bars.
