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  1. #1

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    Need help with OK Cub diesel


    I have a{seems like a nearly new) .075 OK cub diesel and need some help on a few questions.

    First,when it's fliped over it seems to have good compresion, but it won't hold it at TDC. After reading some of the past postings I wonder if it may need the sealing ring replaced.
    I can screw the contra piston down to where it stops but backing off the screw has no effect on the contra- pistons position. Looks like it's stuck.
    Second question: how do I get the head off to check the seal? I've tried moderate force to unscrew it but it wont budge.Will heating the cylinder with a heat gun help?
    If the contra- piston is stuck, how can it be removed?
    Sory about the long post, but I don't want to any thing wrong that would ruin the engine, this is my first OK cub.
    Been reading the diesel forum for a while and I know you guys have a lot of experience. thanks for your time!
    Ed Stout

  2. #2
    Mr Cox's Avatar
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    RE: Need help with OK Cub diesel

    I don't have any cub diesels, but the contrapiston should have a really tight fit and it will not ususally come back up untill you have fuel that ignites.

    If it's your first diesel it would help a lot if you join a club and can get help the first few times.

  3. #3

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    RE: Need help with OK Cub diesel

    The OK Cub contra should not stick in the traditional sense because it has an "O" ring seal. The head should screw off, but perhaps it has some castor varnish holding it...same with the contra. I THOUGHT the contra had a ball-socket connecting the contra to the adjustment screw but perhaps I am wrong. Only had the head off the one I had about 40 years ago. Long since gave it back to the person I got it from so he could recapture some of HIS childhood memories with that engine.

    If the compression is near the running setting, I would try to run off some primes and see if it budges loose. If it runs, all's well. If it doesn't, the fuel may set things free. It may also close up the seal and perhaps you will not need to change the "O" ring.

    Since you said it appears to be nearly new, I would give it a good break-in...just in case...once you get it running.

    I would suggest a 7x3 prop until you get it running. Small props would tend to "smite" my digits. [&o]

    George

  4. #4

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    RE: Need help with OK Cub diesel

    Mr. cox and gcb, thanks for your help! I've run a two diesels PAW .09, Supertigre .15 (still have them), a LONG while back, so I know a little about them.
    The Cub looks like a little differnt engine than those two. I'll do what you sugest and run it before I try any thing else.
    Thankyou! Ed Stout

  5. #5

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    RE: Need help with OK Cub diesel

    If I recall correctly, the Cub diesel had a spring between the compression screw and the contrapiston. the idea being to have a little "give" in the contrapiston to make the power impulse less sharp.

  6. #6

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    RE: Need help with OK Cub diesel

    Much information about the care and feeding of OK Cub diesels here:
    http://www.modelenginenews.org/ad/ok_diesel.html

  7. #7

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    RE: Need help with OK Cub diesel

    The O-Ring in the contra piston is a good place to start investigations on the leaky compression. You might try saturating the upper cylinder in ATF first to see if you can rejuvenate the O-Ring.

    However be warned about the difficulties of disassembling a +50 Y/O steel/aluminum motor. I absolutely ruined my very sweet running OK 075 diesel in trying to remove (unscrew) the soft unhardened cylinder from the crankcase using "the split hole in the piece of timber holding the cylinder" method.

    The head came off easily using a spanner. The contrapiston just popped out the top. It travelled a considerable distance , so be careful. Reassembled with a new O-Ring it wouldn't go back via the top of the cylinder. Hence the need to take the latter off to insert it from the bottom.

    In retrospect a heat gun on the aluminum crankcase might have been better, or maybe not.

    Good luck,

    Ray

    PS: I've just read the Adrian Duncan article cited above. Good stuff! Wish I'd have read it a few years ago []

  8. #8

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    RE: Need help with OK Cub diesel

    Thank's locktite for your advise on the "O" ring. Will try the AFT treatment first, before trying anything else.
    If It won't run I'll just keep it in my collection. I remember back in the early sixtys seeing the ads in the model mags for Americas Hobby Center. If I remember the OK cubs went for $6.95 or maby $7.95! I always wanted one , but never bought one then- wish I had.
    The article on the cub diesels was very good , a lot good info and history.

  9. #9

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    RE: Need help with OK Cub diesel

    Both the Mcoy and the Cub ere made before Viton and employ a silicone O-ring which takes heat but is not compatable with petrolium. Suggest you run the engine and you will find that it will improve with time untill the ring dies and then it can be replaced with a viton one.
    Bob Davis, Owner - Davis Diesel Development

  10. #10
    lildiesel's Avatar
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    RE: Need help with OK Cub diesel


    ORIGINAL: Jim Thomerson

    If I recall correctly, the Cub diesel had a spring between the compression screw and the contrapiston. the idea being to have a little ''give'' in the contrapiston to make the power impulse less sharp.
    The "bump spring" is a little Y shaped piece of spring steel measuring 3/64 thick. The spring was missing on both of the .049Ds I've acquired. Luckily I found them at a site specializing in off beat replacement parts for model engines. Without the bump spring in the top of the contrapiston it is almost impossible to adjust compression since the screw is too short. Since the OK instructions want the engine started at a lower compression setting than ideal for smooth running and then raised, not having the spring is a major drawback.

    The only other thing I want to add at this point is that the OK needle valve has to be out at least 6 to 8 turns to get fuel draw. None of the old needles I have are the same in respect to metering fuel and the NOS one I got from Herkimer is different from the other three. Needling both the D and the glow plug head B is very different from running a Cox.


  11. #11

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    RE: Need help with OK Cub diesel


    ORIGINAL: lildiesel
    The only other thing I want to add at this point is that the OK needle valve has to be out at least 6 to 8 turns to get fuel draw. None of the old needles I have are the same in respect to metering fuel and the NOS one I got from Herkimer is different from the other three. Needling both the D and the glow plug head B is very different from running a Cox.
    Some of the OK needles are blunt with almost no taper, that's why they need to be out so far. I add some taper to mine with a file and sandpaper. Without alteration, they can go from slobbering rich to peak in less than a turn.

    George


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