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  1. #1

    Running different bore sizes?

    I have a GP 1200r with 2 nik sleeves and one steel sleeve. The steel is scored and needs to be rebored, so I was wondering if I can run this one cylinder oversize, and the others stock?


  2. #2
    SLOWBOY31's Avatar
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    no they all have to be the same size

  3. #3

    bores

    ok, thanks for the info, I wasn't sure on these motors because on some I have done in the past, ( outboards) they allow different bore sizes up to a certain size

  4. #4
    Moderator DrewNJ's Avatar
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    Not "recommended"....However, I've seen a few "junkyard" motors with some pretty odd setups that ran fantastic... More "do-able" on a carbed ski, not so much on an EFI ski.

    Again, not the best of ideas, but.........

  5. #5
    Haven't fallen yet. MJE's Avatar
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    you'd have too much strain on one part of your crank...don't recommend that at all sir.

  6. #6
    PolarisNut's Avatar
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    No offense to anyone, but have any of you guys actually done this, or are you going by hearsay? You do realize that the crankshafts are NOT "balanced", right? Stock piston weights vary a decent amount as well. This isn't a high dollar 4 stroke V8 we're talking about here. If the pistons are close in size, it's perfectly fine. I've done it many times in motors that used steel sleeves...I'm not replacing all of the pistons for a .25-.5mm overbore on one cylinder. It wasn't uncommon for snowmobile racers to run piston sizes staggered by many millimeters, since "tech" only checked the one "small" cylinder...old school cheater mod. Motors never grenaded. If you want to go the extra mile, carefully grind some material away from the underside of the piston just above the wrist pin to get the weight close. Then you have nothing to worry about. This is all in carb'd applications, btw...not recommended with EFI, since you can't control the fuel flow separately to each cylinder.

  7. #7
    Eastside Powersports Slow 91's Avatar
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    Quote Originally Posted by PolarisNut View Post
    No offense to anyone, but have any of you guys actually done this, or are you going by hearsay? You do realize that the crankshafts are NOT "balanced", right? Stock piston weights vary a decent amount as well. This isn't a high dollar 4 stroke V8 we're talking about here. If the pistons are close in size, it's perfectly fine. I've done it many times in motors that used steel sleeves...I'm not replacing all of the pistons for a .25-.5mm overbore on one cylinder. It wasn't uncommon for snowmobile racers to run piston sizes staggered by many millimeters, since "tech" only checked the one "small" cylinder...old school cheater mod. Motors never grenaded. If you want to go the extra mile, carefully grind some material away from the underside of the piston just above the wrist pin to get the weight close. Then you have nothing to worry about. This is all in carb'd applications, btw...not recommended with EFI, since you can't control the fuel flow separately to each cylinder.
    +1. When we build circle track cars,guys want one bank bored .060" over while the other bank is stock incase they have to pull a head and measure the bore size. I also heard Seadoo says its perfectly fine to bore only one cylinder if it has a seizure while the other cylinder is left stock.(2stroke)

  8. #8
    PolarisNut's Avatar
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    Quote Originally Posted by no_more_rice View Post
    Hmmmm....400+ cc pistons revving at about 7400 rpm, and wide open throttle operation most of the time. With all the crank failures reported with these skis, I'd want it damn close to perfect, but that's just me.
    The thing is, it's NOT perfect..even when all pistons are the same size. The crank isn't even balanced...who knows where it ends up...for all we know, one heavier piston could put it closer to balance than it was stock...not saying it does, but you just don't know.

    Quote Originally Posted by Slow 91 View Post
    +1. When we build circle track cars,guys want one bank bored .060" over while the other bank is stock incase they have to pull a head and measure the bore size. I also heard Seadoo says its perfectly fine to bore only one cylinder if it has a seizure while the other cylinder is left stock.(2stroke)
    Haha, nice to hear people are still at it

  9. #9
    All hail the Chief! fullboogie's Avatar
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    Polaris is right on. As long as we're talking about the same general bore size, i.e. 80mm, it should be ok. If it were me, I'd buy a digital scale and get the pistons within .5 or 1 gram of each other which is very easy to do with a die grinder and the right bit.

  10. #10
    Thanks again for the info, but I just realized I cant buy an over size piston for the gpr, or can I ?

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