Big bore kits

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    Has anyone fitted such and what, if any benefits are to be had – drawbacks too!

    Have any other modifications been required – carbs etc.



    Hi Dibble,

    there are no big bore kits ready available. All big bores are made individually.

    There is an ongoing discussion what is the max. displacement possible.

    Some say it is limited to approx 860 ccm, some others did up to 970 ccm (Wrenchmonkees).

    The crankcase needs to be modified as well. The larger cylinder liner needs space in there !

    Maybe a double spark plug per cylinder is a good option when displacement is increased.

    One of the german guys did a big bore to approx 860 ccm with dual spark plugs. Power increases from nominal 50 HP to measured 55 HP.
    Measurements taken according to german DIN-HP. Overall power increase is just 10%.

    The power increase is low, the torque increases in relation to the displacement increase.

    There is a bottleneck in the Twin motors. It is the inlet camshaft.
    No chance to get sufficient mixture in. There are some Twin motors out there, that use a modified exhaust camshaft at the inlet side. Will try this when I have time and money to rebuilt a motor. No warranty that this will work without any issues, but it should 🙂

    Cheers, Michael


    That is bang on Michael. As far as the inlet cam is concerned I am led to believe the exhaust cam has a bigger lift than the inlet cam and is quite easily machined to be accepted as an inlet cam therefore allowing a bigger dose of fuel mixture to be delivered. I too am looking at this option but first will be shaft drive after Belgium.


    Hi KK,

    lift of inlet camshaft: 8 mm
    lift of outlet camshaft: 10 mm

    Cheers, Michael


    Thanks guys – much appreciated.

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