SMOOTH OPERATOR

Frankw is undoubtedly the top guru when it comes to AMC twins. Does the central main bearing make the engine smoother or not? I’ve read so many totally conflicting opinions on the subject that I don’t know what to believe. Is this a heresy, or does it perhaps indicate that quality control at AMC left something to be desired, so that some engines were properly balanced and others not ? I’ve always liked the look of the design but have never owned one. Bob Torrens, member It’s highly likely that the engineering and theory suggest that supporting the centre of the crank should both strengthen the bottom end, ease the stresses on the outer main bearings and reduce crank flexing, which should – should! – reduce vibrations at a given engine load and speed. However, many, many years and untold thousands of miles aboard all manner of Brit twins reveal that a well set up 650 AMC twin vibrates about as much as a well set up pre-unit BSA or Triumph twin.

Also that everything is tremendously variable, especially now that the engines are all ancient and never ridden very hard.

Although 500 and 600 AMC twins have no history of crank problems, very early 650s did. The problem was solved by changing the material the crankshaft is forged from. So not so much a question of quality control more of material specification. Probably. Ride a few, and see whether you like one today. What testers in 1960 thought doesn’t really apply now. Top Guru!