I took the advice of member Rob Lloyd while rebuildiing my 71' 500, and stuck a 75' 550 motor in it. It still says 500four, the only give away is the clutch assembly and hangar, which are WORLDS better on this motor. I got the bike for $200, the 550 motor for $105 off eBAY, it was execptionally clean. The best of both worlds... I have 2 750s as well, and the 500 does get the attention. (However it is lowered and has open headers)