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Atlantic Green website: about 810 Dunstall head

Discussion in 'Norton Commando Motorcycles (Classic)' started by pierodn, Jan 10, 2018.

  1. pierodn

    pierodn

    Joined:
    Nov 10, 2012
    Hi.
    The Atlantic Green/Dyno Dave web site says:
    “ 71 head with a "rotated away from exhaust" big intake valve Dunstall conversion. It has 32 mm intake ports. The squish band being completely machined away. It was for use with a Dunstall 810 barrel kit and the R3 cam which was very similar to the norton combat cam. I suspect this combination was the claimed 75 hp engine. “
    Please, what means “rotate away from exhaust”?.
    Big valve: could you explain the differences between the stock one?
    Thanks.
    Piero
     
    Last edited: Jan 10, 2018
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  2. lcrken

    lcrken VIP MEMBER

    Joined:
    Mar 15, 2009
    The Dunstall big valve conversion involved boring out the intake valve guide holes at a steeper angle (26.5° instead of the stock 28°) to fit a larger valve. That also required fitting a larger diameter valve guide and a larger valve seat. In Dunstall's case, he was doing the conversions on 750 heads, and the new guide could be the same size as the 850 guide. After Dunstall quit doing the conversion, other tuners (Mick Hemmings, Steve Maney, etc.) continued. Just recently, Jim Schmidt posted a video here showing one way to do the conversion. The stock Commando intake valve is 1.490"/37.85 mm in diameter, and the Dunstall conversion used a 1 5/8"/41.3 mm diameter. Norton did pretty much the same thing for their AMA Homologated Combat 750 head. The AMA Racer head they produced for their factory race conversion kits (Production Racer and F750 Formula Racer) was similar, but with slightly smaller intake valve and slightly larger exhaust valve. The factory used the same geometry when they produced the 850 pattern big valve heads for the short stroke 750 engines.

    This is one of the original Dunstall catalog descriptions of the head:
    Dunstall Head 100 GS.jpg
     
  3. Bernhard

    Bernhard

    Joined:
    Apr 20, 2011
    Picture of Dunstall is self-explanatory –larger 1 5/8in place of one and half inch.
     
  4. pierodn

    pierodn

    Joined:
    Nov 10, 2012
    Hi.
    What it needs to reangle the intake guide/valve from 28 to 26,5 degree?.
    Thanks
    Piero
     
  5. Bernhard

    Bernhard

    Joined:
    Apr 20, 2011

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