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  1. #1

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    O.S. Gemini 120 twin

    I have a 120 twin first gen i had a bad crash and bent the crank pretty good is it possible to send in for repair i can't seem to find a replacement crank for it i work in a machine shop and i was even thinking about straightening the crank but i would like to get a new one just sounds like the better idea

  2. #2
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    RE: O.S. Gemini 120 twin

    Parts for the first-series FT-120 Gemini Twin are not available. They have not been available for a long, long time. We'd have no idea where you might be able to locate any, other than an "items wanted" ad on the various RC websites.

    Parts for the FT-120 II engines are no longer available, either.
    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

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    RE: O.S. Gemini 120 twin

    well thats not what i was hoping to hear ok next question i wanna pull the crank out so i can try to straighten the crank and i'm kinda worried about the bearing caps on the con rods i was wondering is there any important info about them or anything else i should keep in mind while tearing this motor down i've had the back off the motor before and the cam out but i never messed with the crank or bearings i had to heat the back cover to get the cam bearing out of the back plate i'm thinking i'm gonna have to do the same thing to rear bearing on the crank and are the bearings still available or will i have to take a trip to the bearing shop

  4. #4
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    RE: O.S. Gemini 120 twin

    We likely have bearings that will fit. OS uses the same bearings in a lot of engines. It's normal to heat the case to remove bearings.

    When you take the conrod ends off, make sure you note which way they were on. The rods should be replaced in the same direction they were when you took them apart. The rod ends and rods should be marked with a small dot to show which sides line up together.
    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

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    RE: O.S. Gemini 120 twin

    cool thanks for the info i got another question not about 120 twin i ended up with a 91 DF motor i don't have a DF jet of any sort but i do have a SIG King kobra i'm building and i wanna stuff it in the front of this plane it's the model with the large heatsink head i'm thinking i'm gonna need the smaller head but thats just a guess i already took the spacer off the end and replaced it with the prop drive plate but what i don't know is what size prop and fuel i need my cousin gave me the motor i remember him saying something about 5% nitro only also i can't find the manual for the motor on the O.S. site

  6. #6
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    RE: O.S. Gemini 120 twin

    The O.S. Max .91VR-DF engine had large and small heads available for it. These parts are available. If it's the older VF engine, no parts whatsoever.

    If you have the large head, it will tend to overcool if you use a prop. Make sure your propeller will allow the engine to turn at 13,000 RPM or more at full throttle. More RPM is better. The engine is ported for high RPM and will be a turkey if you try to prop it like a regular .91-size engine.
    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

  7. #7

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    RE: O.S. Gemini 120 twin

    I seen on the spec page they rate the engine at 4.8hp @ 22000 rpm is that what i should shoot for or am gonna have to just buy like a half dozen props and do some static thrust test and Is there any way to tell the new and old 91 df motors apart

  8. #8
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    RE: O.S. Gemini 120 twin

    22,000 RPM is what you would see when using a ducted fan system. To get that RPM with a propeller would require an astonishingly-small one. Really, you should shoot for 15K or so, but you will likely never be satisfied with the performance, because if you get a prop that will give you a reasonable top end, you'll have a lousy idle. Wind up with a good idle, and you'll likely have a lousy throttle transition and maybe a lousy top end. With a ducted fan, the idle was 5,000 RPM or so...not something you want with a propeller. You can play with it and see if you can get it to work, but we'd not have confidence that it will suit.
    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

  9. #9

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    RE: O.S. Gemini 120 twin

    so here is an odd question for you can i buy the crank for the newer 120/160 and modify it to fit the older engine i have access to grinder at my work so if its just a matter of grinding a couple diameters for the bearings i can do that but i don't know how much the same the cranks are

  10. #10
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    RE: O.S. Gemini 120 twin

    The cranks are very different. Different lengths, diameters, and so on. You will have much difficulty trying to adapt a modern crank to the old FT-120.
    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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