1. This site uses cookies. By continuing to use this site, you are agreeing to our use of cookies. Learn More.

750 head on 850 barrel

Discussion in 'Norton Commando Motorcycles (Classic)' started by marinatlas, May 15, 2019.

  1. marinatlas

    marinatlas

    Joined:
    Nov 4, 2007
    What is the problem when fitting a 750 head (after changing the outer head bolt spacing) on a 850 barrel , without re -hemispher the head ......? will increase the CR for sure , will it work ........? more turbulences ........? just thinking o_O
     
  2. 84ok

    84ok

    Joined:
    Aug 10, 2014
  3. comnoz

    comnoz VIP MEMBER

    Joined:
    Dec 10, 2008
    By the time you open up the squish bore to 78.5mm the compression will be about the same.

    The smaller ports will be a plus on a street engine. Jim
     
  4. marinatlas

    marinatlas

    Joined:
    Nov 4, 2007
    I was just thinking of not opening the squish bore ........? and fitting 850 flat top pistons which do not protrude as the 750 doe, when we had done it for the 750 Maney jugs it was only because it had a 850 bolt pattern , so it was easy just plug two outer head bolts holes and redone the correct spacing ......
     
    84ok likes this.
  5. Time Warp

    Time Warp .......back to the 70's. VIP MEMBER

    Joined:
    Dec 3, 2012
    On that note was there ever a 73 mm bore sleeved ? 850 type cylinder and head to suit.
    Folk seem to like bigger capacity engines but that would make for a tough 750 in stock form. (compared to the early engine case and cylinder)
     
  6. lcrken

    lcrken VIP MEMBER

    Joined:
    Mar 15, 2009
    Steve Maney used to supply his alloy cylinders with 73 mm bore sleeves and 850 head bolt patterns on request. I used to get similar alloy cylinders from Drew Robertson many years ago, but I don't know if anyone is still making them. I believe AN can supply 73 mm bore iron cylinders with through bolts, like the 850, but don't know if they can do them with 850 head bolt pattern. Might be worth asking them.

    Ken
     
    Time Warp likes this.
  7. comnoz

    comnoz VIP MEMBER

    Joined:
    Dec 10, 2008
    I have dropped sleeves in an 850 engine to make them race legal. It works great. Jim
     
    Time Warp likes this.
  8. jseng1

    jseng1

    Joined:
    Nov 26, 2009
    Here's a 750 head on 850 cylinders (my old monoshock racer). If you carefully elongate the four outer bolt holes near the head and check that there is some area left to clamp the head gasket - you're fine (it worked for me). I think this is also what Steve Maney used to do. Or you can plug the holes like Ken Canaga did and re-machine.

    [​IMG]

    The head originally had 28mm ports and that gave me plenty of design room to give it raised ports which ended up as wide and high as I could get them without welding without going too far beyond Axtell's port specs.

    Before it went on the 850, the same head was raced in my Featherbed 750 (stock valves with raised 32mm ports) and managed a 2nd place at Laguna Seca in 1984 (below).

    [​IMG]
     
  9. marinatlas

    marinatlas

    Joined:
    Nov 4, 2007
    Hi Jim (s), both of you ! just asking about the non -matching 750 head hemisphere/850 jug bore , just to know if it is a non sens ??
     
  10. comnoz

    comnoz VIP MEMBER

    Joined:
    Dec 10, 2008
    The hemisphere is the same between a 750 and 850.
     
  11. marinatlas

    marinatlas

    Joined:
    Nov 4, 2007
    Thanks !
     

Share This Page