I am struggling to get my head round the spatial operation of this lot in my head without seeing it working in the flesh. Perhaps you can help?

I know that the freewheel gear 'locks' when trying to turn it anti clockwise when viewed from the mag end and 'slips', or turns freely when turned in a clockwise direction due to the cams in the sprag clutch. What I can't seem to get my head around is if this is the case how is it possible that should the engine back fire on starting (ie. Not enough inertia provided to get the piston over top dead centre) that the freewheel gear is able to turn the intermediate starter gear and therefore the starter motor in the wrong direction? Shouldn't the freewheel gear slip to prevent this? I never checked this out for myself when I had the opportunity of having an engine on the bench with the ignition housing off and now I just can't get my head round it.

I see that with the starter motor removed from the engine the intermediate starter gear does indeed rotate in both directions when the prop is turned by hand both in the correct direction and the wrong direction (not good of course for your starter motor).

I am obviously referencing the wrong components with respect to what grips/ slips relative to what. Too many cogs! 

  • Re: Sprag Clutch (Too many cogs!)

    by » 8 months ago

    The Freewheel-Gear, Idle-Gear and the Starter always turn in unison with each other.

    The Sprague Clutch Housing always turns in unison with the CrankShaft.

    The Sprague Clutch alternately connects and releases the Freewheel Gear from the Sprag Clutch Housing.

    The Freewheel gear does NOT turn unless the starter is engaged OR the CrankShaft is rotated in reverse.


    The intermediate Gear does not freely turn in unison with the Crankshaft when the Starter is installed.

    The forward rotation you saw was just an artifact of the Freewheel-Gear turning from the minimal friction of the surface tension of the Oil.

    You could have stopped it from rotating with your little finger. 


    Bill Hertzel
    Rotax 912is
    North Ridgeville, OH, USA
    Clicking the "Thank You" is Always Appreciated.

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