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Thread: OS FS-70 repair


  1. #1

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    OS FS-70 repair

    One of my two FS-70's has MANY of hours on it and I've noticed that it's lost a lot of compression over the past flying season. They've been great engines, so I decided to break this one down and see what I could do in the way of a minor overhaul to possibly rejuvenate it.

    The head and piston had some carbon buildup but it was easy to remove. The valves look fine, as do the piston and cylinder walls, i.e. no scuffing, etc. I decided to order a new piston ring, liner, and gasket set, all of which should arrive in a few days.

    In the meantime, when I removed the old ring from the piston, I didn't see any indication that there was an "top" or "bottom" side to the ring. Is this correct? In other words, when I install the new ring, is there a top and bottom, or does it matter?

    Also, what about the position of the ring gap, relative to the front of the crankshaft? Is there a proper orientation for the gap, or do I just leave it where ever it happens to wind up?

    Thanks for any help or info you can provide.

    Rod Robbins
    Beaverton, OR
    Rod Robbins
    Beaverton, OR U.S.A.

  2. #2
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    OS FS-70 repair

    Rod,

    On the O.S. Max four-stroke engines, there is no 'top' or 'bottom' to a brand-new piston ring. If you're installing a used one, then it does make a difference because of the wear it would have received during use. Used parts must always be installed the same way they came out.

    As far as the ring gap is concerned, it makes no difference in the four-stroke engines where you put it. As the engine runs, the ring will tend to rotate around the piston. Because there are no ports in a four-stroke engine's sleeve, there's nothing for the ring's ends to get caught on.

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  3. #3

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    OS FS-70 repair

    Bill,

    That's exactly the info I was looking for.

    Thanks very much!

    Regards,
    -- Rod
    Rod Robbins
    Beaverton, OR U.S.A.


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