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  1. #1
    Canadian Beaver Inspector jkindt's Avatar
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    Boring...... pt. 2

    I can see two reasons why one would bore out their jugs.

    1. To repair damaged cylinders by creating a new surface for larger pistons and rings.

    2. To put bigger pistons in to create more power!!!

    The question is...how far is too far? Is there any disadvantages to boring the jugs out to the max to gain as much power as possible through increased displacement? In an air cooled engine such as a motorcycle, boring the jugs out can cause overheating issues, I don't see this being as much of a concern due to the constant supply of cooling water. Is the power increase between 72.0mm pistons and 71.5mm or smaller pistons worth the fact that if you blow a ring, you may make the cylinder unusable without a resleeve? Of course I am talking about a 750 or 780 cylinder, either can be bored out to 72.0mm max (wiseco's). Does reliablity become a concern? What about tunability? I just need to make some decisions instead of just wandering off without direction on my Sleeper project. (I have decided on 750 ported jugs).


  2. #2
    bowsniper's Avatar
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    Maybe use some kawi 800 pistons! I heard of people using snowmobile pistons in place of stock or store bought stuff to increase displacemnt, but i dont have any experience in this area. Maybe someone on here can enlighten us!

    And dont worry about jugs, just buy another one off fleabay. there a dime a dozen. I have 3 here myself! I hear also that flat tops are better to use. Just need to check squish and pressure
    by the way, you ever thought of doing your own porting? I hear the only way to port is thru a laser and a cad program. and of course that 1 in a million guy that ports heads by hand!

  3. #3
    She likes the bike. But the ski gets her wet!!!! xlint89's Avatar
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    .5mm isn't going to make much of a felt power difference.

    I doubt 1mm would either.

    BUT, 2.25mm (72mm) in a 750 cyl would make a difference. You're adding almost another 50cc to the displacement.

    Obviously the compression is also going to be raised when boring, unless you have the heads machined for the different bore size.

    More power = more heat
    More compression = more heat

    So cooling will need to be looked at. The triple pipe instructions have you drill additional 1/4" holes in the head gaskets to aid in cooling.

    So any engine mod will affect tuning.

    Once you're MAX bored, you'll need to resleeve, or have the cyls plated to restore the lost material in the event of excessive wear or damage.

  4. #4
    Canadian Beaver Inspector jkindt's Avatar
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    Yeah, I was thinking that I would bore the cylinder 1 size smaller (71.5mm) than the maximum (72mm). Then if something bad happens, I just bore to 72mm and rebuild again!

  5. #5

    Smile

    I really like forged pistons secret is to setting up a race engine slightly looser,if it calls for .004 piston to cyl clearance go .005 and watch your ring end gap,also heat cycle the rings a few times before you put it on water,in other words start engine with no cooling water bring it up to temp about 10 to 20 seconds of running at fast idle, let cool completely then do it twice more,I repeat do not break in engine on flush kit or water hose unless you tie into the true water inlet line and let water preheat in exhaust manifolds,I have seen more than a few well built engines attempted to break in on backflush kit.as the cold water hits head and cylinders causing a seize.Hey just my opinion but over a 1,000+ engines I built cant be wrong!!>Marvin

  6. #6
    Canadian Beaver Inspector jkindt's Avatar
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    Thanks for the advice. I think I will end up with forged Wiseco's. I will be using high compression, hopefully I can get away with 93 octane. I will have to compression test it once it is together.

  7. #7
    She likes the bike. But the ski gets her wet!!!! xlint89's Avatar
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    Quote Originally Posted by jkindt View Post
    Thanks for the advice. I think I will end up with forged Wiseco's. I will be using high compression, hopefully I can get away with 93 octane. I will have to compression test it once it is together.
    If you're going 71.5mm (1.75mm OS) bore with the 750 OP head, you'll need to machine the head for the bigger bore.

    So, at that point, you can make the head any compression ratio you want.

  8. #8
    Canadian Beaver Inspector jkindt's Avatar
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    This is true! I don't want to have to run VP all the time! Too expensive for a rec ski!!! 93 octane is reasonable, and I can get 94 if I have to.

  9. #9
    Canadian Beaver Inspector jkindt's Avatar
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    Anyone know what the deck height is supposed to be on a 750?

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