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850 cylinders on a 750 motor

Discussion in 'Norton Commando Motorcycles (Classic)' started by hsi88, Dec 28, 2005.

  1. hsi88


    Sep 15, 2005
    Hi, I have a 1970 Model S (that's the date on the matching title, although the frame has 1969). Its a basket case for the most part and I am rebuilding it to utilize a Drouin Supercharger. I realize that a late 850 motor is probably a better starting point strength wise, but I don't happen to have one (outside of my fairly stock roadsters). But I do have a set of 850 cylinders and pistons. I was wondering if the 850 cylinders could be used in place of the original 750 cylinders. And yes, I do plan on replacing the main bearings. Any advice would be welcome! Thanks!
  2. Black/gold750Comando


    Dec 27, 2005
    i have seen this done using the 850 pots but then sleeving it to fit 750cc pistons as the 850 pots are alot stronger than the 750 ones.
  3. Ron L


    Feb 27, 2004
    Often racers will use an 850 bottom end and cylinders sleeved down to 750, but I don't know of any who grafted the 850 jugs on a 750 bottom end. The later cases are stronger and early cases must be machined out to accept the larger 850 cylinder, making them weaker.

    Adding a supercharger to an already marginal set of cases is probably asking for trouble. I'd stick to the 750 jugs if you are intent on using the weaker early 750 cases. Otherwise, find a set of 850 cases to use your 850 cylinders. If you are using the 750 cases, have them reinforced with weld and inspect them closely for cracks

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