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  1. #1
    apwachholz's Avatar
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    O.S. .25 SF (circa '92) Manual or Part # Needed

    @OS Guru
    I have an O.S. .25 SF that I purchased quite a while back and am looking to get a new piston sleeve (reason below). Clearly they don't manufacture the engine but, I'd like to find out if there is a comparable .25 that's built today that I can purchase parts from. If so, what engine? I'd really like to keep this engine going as it was my first r/c engine, and thus means a lot to me.

    Here's a link to another thread with pix of my OS .25.
    The Reason: I'm loosing power in-flight as the engine runs longer and I'm fairly certain the compression is becoming less and less as the engine heats up. I don't have any issues during idle (approx. 2500RPM on idle) but the longer it runs it starts to loose power. It's an open faced cowling (E-flite Texan conversion to glow) so getting air through the cooling fins doesn't seem to be an issue.
    Either way, I'd like to know if there is an existing engine out there I can swap parts with. Thank you.
    _aw

  2. #2
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    RE: O.S. .25 SF (circa '92) Manual or Part # Needed

    When an engine's compression starts to decline, the first symptoms are usually the idle and acceleration. They become bad and inconsistent. Top end performance usually doesn't suffer as much because of the high pressure of the combustion gasses. At low RPM though, the cylinder pressures are not as high, so performance will sag due to leakage of the compressed fuel/air mixture.

    If an engine tends to lose RPM during a flight, it's almost always due to overheating. The fact that an airplane has an open-front cowl doesn't mean that the air is passing through the fins of the engine and cooling it. You usually need a baffle to direct cooling air into the cylinder fins, and a good outlet that allows the air to leave. If it cannot leave the cowl, then it cannot cool the engine.

    As far as parts for your engine, you'll have to locate them on-line through various dealers, auction sites, or selling sites like the RCU Marketplace. Neither O.S. nor we have parts specific to that engine available, and there are no parts from other engines that would work, except for parts like screws, washer, and possibly bearings.
    Bill Baxter, Manager Hobby Services/Futaba Service/North America
    3002 N. Apollo Dr. Ste. 1 Champaign, IL 61822 USA
    Service Phone: 217 398-0007
    Email: hobbyservices@hobbico.com

  3. #3
    apwachholz's Avatar
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    RE: O.S. .25 SF (circa '92) Manual or Part # Needed

    @Bill
    Thanks for the insights on the O.S. .25 SF. I really appreciate the feedback, and in such a quick manner. As far as the engine is concerned I believe that my issues surround the overheating you spoke to. I've tuned the engine to the point that it idles at 2600RPM with no fluctuation in power at all and on the high end get a consistent 12,090RPM (+/- 100). And that's with it running somewhat rich. I have two additional questions/comments:
    1. Do you have any resources (i.e., URL links) that show good examples of building baffles for a T-6 style cowl / engine installation? Or perhaps do you have any personal recommendations? Note: The final installation does include an exhaust exit for a pitts muffler, however, it is cut close to the pipes and might need to be made larger, given the situation(see the attached image for my set up or here is the thread on the build)
    2. What physically happens to the engine that causes the RPMs to decrease when the engine heats up?
    Thank you again for all your help Bill. It really has shed excellent light on my situation.
    _andrew
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  4. #4
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    RE: O.S. .25 SF (circa '92) Manual or Part # Needed

    We don't have any drawings for cowl baffles, but they're easy to do. You just use thin plywood to force air through the cylinder fins. The plywood (or other stiff material) will have to be no more than about 1/16" away from the fins. All air entering the cowl should go through the engine's fins. If the air outlet for the cowl is only large enough for the muffler stacks, then you don't have enough air outlet, and you cannot get airflow through the cowl. Air exit should be at least 1.5 times larger in area than the air inlet to the engine.

    When an engine overheats, the piston expands more than the cylinder liner, due to the types of metal used. The oil starts to break down and stop lubricating. Both cause friction, and that will cause more heat, but also slow the engine. The only way to stop it is to cut the throttle and let the engine cool off. Adding throttle only makes things worse.
    Bill Baxter, Manager Hobby Services/Futaba Service/North America
    3002 N. Apollo Dr. Ste. 1 Champaign, IL 61822 USA
    Service Phone: 217 398-0007
    Email: hobbyservices@hobbico.com

  5. #5
    apwachholz's Avatar
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    RE: O.S. .25 SF (circa '92) Manual or Part # Needed

    @Bill
    Your knowledge is really is making a difference in how I look at installing my engines moving forward. Especially with my enjoyment of converting smaller .25 sized electrics to glow power. I think since my cowl is completely open on the front (per the previous post picture) I'm going to concentrate as much as possible on getting smooth airflow over the engine but, more importantly ensuring that it's exiting at a good rate as well.
    I'm testing my baffling concepts by creating them in cardboard, placing on the cowl, and then using a blow dryer to test and monitor the out flow of air. Seems to be working well so far.

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    RE: O.S. .25 SF (circa '92) Manual or Part # Needed

    Does anyone have a OS 25 SF engine manual? Please contach me at jgosch@ptd.net thanks John Gosch

  7. #7
    apwachholz's Avatar
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    RE: O.S. .25 SF (circa '92) Manual or Part # Needed

    Hey Gosh -
    I do have a PDF somewhere in my archives of the manual. I'll dig it up and post a link here on RCU. Also, I've kept an eye on these few pages as they are the only record I know of being able to I.D. any of the past engine parts (and #'s):

    http://www.osengines.com/parts/xr12650.html

    http://www.osengines.com/parts/xr12641.html

  8. #8
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    RE: O.S. .25 SF (circa '92) Manual or Part # Needed

    You can send an email to the address below, and we can arrange to send you the manual.
    Bill Baxter, Manager Hobby Services/Futaba Service/North America
    3002 N. Apollo Dr. Ste. 1 Champaign, IL 61822 USA
    Service Phone: 217 398-0007
    Email: hobbyservices@hobbico.com


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