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  1. #1
    Sly Fox DKFox's Avatar
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    Question Rotary valve shaft question

    Tore into my 1998 SPX 787 engine last night after it has failed to start after troubleshooting all possible electrical causes. Found that the rotary valve shaft is not turning at all! From just peering into the cylinder ports, I can see that my top end rebuild still looks perfect. (Each cylinder still had 160 psi prior to engine removal.) Pistons, rods, and crank all appear to be turning like they are supposed to do. Had to stop there last night because it was getting very late.

    Isn't there a brass gear on the rotary shaft that sometimes strips? I don't think the shaft itself is sheared or broken. I don't hear any clanking of metal when the crank turns. I've got the service manual... but I've never torn into the lower end. I've only messed with top ends on these engines. I know how to set the timing of the rotary valve, but I've yet to ever have to REMOVE the rotary shaft. Just wondering what the most common cause might be?


  2. #2

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    There is a brass gear that strips out, I dont remember if it is on the cranks or shaft or both. I believe that you need a special puller for the rotary shaft.

  3. #3
    Sly Fox DKFox's Avatar
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    Dang.... got into the bottom end tonight. It was definitely the brass gear on the rotary shaft that was causing the problem. Notice all the brass shavings in the attached photo! I was expecting it to be worn down, not demolished!! Would a bent crank do that? Or just any small object that may have gotten between the gears torn it up like that?? The crank and all the bearing appear to be fine.
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  4. #4
    4.0l sahara's Avatar
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    you can make a puller with a long bolt and a big socket,

  5. #5
    Sly Fox DKFox's Avatar
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    Can I get away with just replacing the brass gear? What might have caused it to do this??

    (Just want to make sure I fix the cause and not just the consequence after going to all this trouble.)

  6. #6
    mibunkerking's Avatar
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    yes you can replace just the gear. what probably happened is that the RV seized/got stuck/something jammed in it at some point that striped the gear enough to throw the timing off. the starter turning the motor over and over filed it down which is why there are so may shavings. but that is what it is designed to do to prevent major damage. you need to make sure the shaft is not bent though or it will strip again. safest bet is to replace the whole thing. you cant get a new one from....*cough*sbt*cough*

  7. #7
    If i knew,...I wouldn't ask sea-one's Avatar
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    I've heard of this happn'n but never run across one in my years of repair.how does this happen when the gear is bathed in injection oil?

  8. #8

    seen many times

    Quote Originally Posted by sea-one View Post
    I've heard of this happn'n but never run across one in my years of repair.how does this happen when the gear is bathed in injection oil?

    I have seen this problem many times you can replace brass gear after you do leave out spark plugs and slowly turn crank without rotary plate and cover on make sure it turns freely and with no hangups if you feel any remove and replace the shaft itself. I have replaced many of these shafts do not know how they get bent but I usually keep one in stock just in case this happens!!

  9. #9

    Help

    Would you happen to know how i take the rotary valve shaft out?I can not figure it out i have the c clips out the nut off the back is off and now im stuck can u help me?
    Quote Originally Posted by 4.0l sahara View Post
    you can make a puller with a long bolt and a big socket,

  10. #10
    mibunkerking's Avatar
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    Quote Originally Posted by AdamHenry View Post
    Would you happen to know how i take the rotary valve shaft out?I can not figure it out i have the c clips out the nut off the back is off and now im stuck can u help me?
    you need a puller. you can either buy one or make one with a big socket, a bolt and nut. the hole i the center of the gear is threaded. this will allow you to pull the shaft out.

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