Conference vmszoo::rc

    Re -.1
    
    You are correct... the gray/black color is aluminum from the engine
    wearing it self out.
    
    I believe that the condition is due to not enough lubrication.  Try
    adding a couple oz of Castrol bean oil, etc... to a gallon of fuel or
    switch brands.
    
    Hope this helps,
    
    Chris 

Hi,

	When you mention the piston arm moving, are you
implying that it is not correctly attached to the 
crankshaft? If so, a possibility could be big end shells
having worn away, or wearing. These are normally light
aluminium and could give rise to the grey fluid! I am not
that familiar with this particular engine however, so
don't treat my suggestion as a highly likely cause! I'm
not sure if this engine has shells in the big end at all,
but on larger engines(like cars) they do.

-Trev

    Check for a bent con rod or wrist pin.  That can cause rubbing on the
    backplate.  It is normal to have minor rubbing but any signs of
    aluminum in the exaust are not normal.
    
    charlie
    

    
    I have a engine manual that covers OS. I can look up some specs for you
    that might be of help. Just a point, if you can't see or feel any play
    in the rod, pin or any other internal parts that make it rub againist
    the back plate, the problem may only be present at high RPM and high
    temperature. If the engine has been broken in, then running it with this
    problem will only wear the piston and cyl until you have loss of power.
    Once you go that far its cheaper to buy a new engine than fix it. If
    you can live with the low power than run it rich, VERY rich.
    

    Hmm, some good points guys.
    
    Fuel: Most of us here run "straight" fuel that we mix ourselves. 
    	  20% Castor oil and 80% Methanol, well mixed. This seems fine for
    	  all of the two-strokes at our club.
    
    Shells: The OS46SF has a plain bush bearing between the con rod and the
    	  crankshaft pin. 
    
    Bent Bits: Charlie, my thoughts exactly. I pulled apart my 40FSR and
    	  noticed some very slight rubbing on the backplate. I also pulled
    	  apart my very old 25FP, this had quite a bit of rubbing, but has
    	  never emitted black gunge. I have checked the con rod and
    	  crankshaft pin for straightness, I guess the wrist pin is up in
    	  the piston? I haven't checked it yet but it looks fairly easy to
    	  get out, so that will be my next step. I was also considering
    	  facing off the backplate so it is nice and flat, the con rod
    	  could be digging in instead of floating. Of course the tolerances
    	  required are very difficult for me to measure, and at 14k RPM it
    	  wont take much to cause a problem.
    
    Manual: Actually, it may be too difficult to properly measure the
    	  tolerances, but I will keep the offer in mind. As you say, the
    	  aluminium gunge will be acting like a cutting compound wearing
    	  the motor out - not desirable.
    
    Keep those ideas coming in guys, of course if I can't fix it I will
    give it back to the original owner but these little beasts cost over
    $200 here so it would be nice to be able to get it going!
    
    Thanks all, Rob.

Chances are most of the backplate wear is due to starting. If you use an
electric starter you're pushing into the engine pretty good when starting.
This puts the crank at the back of it's "slop" and might put it in contact
with the backplate. When the engine is running the thrust of the propeller 
is pulling the crank to the front of it's "slop". You might find some wear
on the front housing from this during running. Easiest thing to do is turn
the engine over with the backplate off and watch the sides to see how close
the connecting rod is coming to the sides of the crankcase. An easier 
method than "facing" the backplate would be to put an extra or thicker
gasket on the back to move it away and see it that cures the problem. If it
does, you now have a lead on what to do (or just leave in the gasket). The
increased crankcase volume shouldn't effect the engine.

    It is easy to bend a rod if you use an electric starter.  All you have
    to do is overprime the engine and get enough fuel in there to
    hydro-lock it.  I have straightened several rods for people that did
    just that.  If you run with a bent rod, you will wear the bushings and
    get aluminum comming out the exhaust.  Also, crashes can push the crank
    back in the bearings and cause the crankpin to wear on the back cover.
    
    
    Actually, bad ball bearings are a major cause of aluminum wear and
    black gook.  The fit of the crank to the case between the front and
    rear bearing is pretty tight because it has to form a gas tight seal. 
    If one of the bearings gets play in it, the crank starts wearing
    against the case and ruins the case.  It is important to replace bad
    bearings as soon as they are noticably sloppy or noisy.
    
    Hope this helps,
    
    Charlie