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  1. #1

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    To install rings w/o removing motor 780

    I'm wanting to re-ring the slx780 with out taking the motor out.
    It looks like i have to move the exhaust out of the way to get to the manifold bolts......Is this correct.

    If you do then ,
    How do you remove the big bolts that hook up the large 180 degree pipe to the exhaust manifold. It looks to me that you have to re-position the motor to clear the hull. If that's the case then i might as well take out the motor.

    Another question. I have all the pieces for a 1993 750 motor except for one cylinder.can i use a 780 cylinder or do they all have to be the same.
    Also..will the 780 and 750 electric all interchange.

    thanks Rich


  2. #2
    She likes the bike. But the ski gets her wet!!!! xlint89's Avatar
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    Yes the exhaust pipe needs to be removed. Unbolt it and slide it forward. (the 4 large bolts clear on my 750, not sure on 780 hulls)

    780 cyls are different from 650/750. Bolt patterns are different on head and exhaust. Plus the displacement is larger, making the crank unbalanced.

  3. #3

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    So the way i see it the cranks are interchangable between the 780 and 750's. But cylinders and manifolds are specific. Are the cases that much different.?Can i just swap a 750 motor into a 780 machine?
    rich

  4. #4
    Moderator beerdart's Avatar
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    Quote Originally Posted by desertmariner View Post
    So the way i see it the cranks are interchangable between the 780 and 750's. But cylinders and manifolds are specific. Are the cases that much different.?Can i just swap a 750 motor into a 780 machine?
    rich
    Yes the cases are the same.

  5. #5
    She likes the bike. But the ski gets her wet!!!! xlint89's Avatar
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    Quote Originally Posted by desertmariner View Post
    So the way i see it the cranks are interchangable between the 780 and 750's. But cylinders and manifolds are specific. Are the cases that much different.?Can i just swap a 750 motor into a 780 machine?
    rich
    Yes, the crank is interchangeable from a 650, 750, or 780.

    A 650 cylinder cannot be bored to a 750. A 750 and 650 both can use the same exhaust manifold.

    A 780 uses an entirely different cylinder and exhaust manifold. The top half of a 780 case is slightly different internally to enhance performance. BUT, can be used on a 650, 750, or 780. Now the opposite is true as well. You just won't have the additional 780 channels. This may hinder performance ever so slightly. I doubt you'd even notice though.

    To answer your Q, yes you can swap a 750 engine into a 780, and vice versa. Only difference would be the CDI inside the elec box and impeller. 780 uses more advance timing than a 750, so fuel adjustments may be necessary. But you can always swap the CDI along with the engine and have no issues. Same goes with the impeller.

  6. #6

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    thanks for info.....
    The four large bolts on the top of the exhaust......On the 780 i think i have to either get the motor tipped over or get out the sawsall. Anybody on this?
    BTW I just rechecked the compressioon on these 3 cylinders and they are 90,90,95. Am i right in thinking this is too low to run. Or can they get stuck. Smokes like a SOB. So if i can remove and rebuild these pistons and cylinders i should be able to re-ring and get better compression.?
    Imahopeing.
    Rich

  7. #7
    She likes the bike. But the ski gets her wet!!!! xlint89's Avatar
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    Quote Originally Posted by desertmariner View Post
    thanks for info.....
    The four large bolts on the top of the exhaust......On the 780 i think i have to either get the motor tipped over or get out the sawsall. Anybody on this?
    BTW I just rechecked the compressioon on these 3 cylinders and they are 90,90,95. Am i right in thinking this is too low to run. Or can they get stuck. Smokes like a SOB. So if i can remove and rebuild these pistons and cylinders i should be able to re-ring and get better compression.?
    Imahopeing.
    Rich
    Instead of a swazall, how about drilling a hole for the bolt or a socket extension to go through?

    90's are low, but readings are supposed to be taken on a warm (expanded) motor.

    Smoke is prob from fogging the engine. That's a good thing.

    As long as cyls and pistons are not damaged, re-ring will bring up compression. Definately do a hone on the cyls whenever re-ringing.

  8. #8

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    So I've been working on the slx 780 with low compression. Turns out the ring gap to cylinder wall was way too much. Supposed to be .012-018. and it was about.250. Also it looks like the heads were reworked ( i think SBT) to enlarge the dome portion. I was told that SBT machines the heads to be able to reuse them and also to detune the motor. By lowering compression they have less failures.
    If you were going to re-use a piston/Cylinder combo you removed do you have to do anything to it or can you just reinstall it and move on. Two of the cylinders and pistons had 160psi and look great.
    BTW , I met BArt from Stuart fl. Great guy and he's been servicing Polaris and other brands for a long time now..... Seems to know his stuff and has lots of parts.

  9. #9
    She likes the bike. But the ski gets her wet!!!! xlint89's Avatar
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    As long as they are a matched set, I'd say run with it. You will need new gaskets and circlips of course. While you're that far in, I'd inspect the wrist pins, upper bearing, and re-ring the piston.

    How are the ring gaps on the other cyls? 160 psi sounds pretty high, esp. for an SBT.

    FYI: when re-assembling the cyls, you need to mount the exhaust manifold to the cyls before torquing the cyls down. This will help properly align the cyls and prevent gasket damage.

  10. #10

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    The two cylinder/piston sets were from the other machine. Not the SBT. They had wsm rings and pistons and it looked like the heads may have been cut.
    The SBT's had 85-90 psi.
    I'll be sure to install ex manifold for alignment. Thanks for the tip.
    rich

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