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Thread: Cylinder Boring

  1. #1

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    Cylinder Boring

    When you bore a cylinders as big as they can go how far off do you throw off the port timing .


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    She likes the bike. But the ski gets her wet!!!! xlint89's Avatar
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    Never really thought about that. It would decrease the port timing wouldn't it? (lower the opening)

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    I was just thinking they don't make oversize pistons for the 785's but the 750 pistons will work in the 785's so why not get the biggest oversize 750 pistons and have the 785 cylinders bored and renicasiled . I remember mentioning it to Randy once but i think he said it will throw off the port timing to much . Just wondering then would you need to report the cylinders also ???

  4. #4
    What got u thinking? the 14 cores u have?
    I can't picture the bore size changing the port orientation allot?
    what is the std bore on the pro? vs the biggest piston os for a 750?
    I know I'm missing something? I would have thought a 780 oversize piston would have been a better candidate? please enlighten me

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    The 750 and 785 pistons are almost identical the 780 is bigger for some reason . Beerdart had all the specs somewhere . Since you can't get 785 pistons oversized i would like to try some oversize 750 pistons . when you install the stock 750 pistons in a 785 they rev up quicker because the 750 pistons are lighter

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    Moderator beerdart's Avatar
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    The 785 pistons use keystone rings..

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    Moderator beerdart's Avatar
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    .020 is the largest oversize from Polaris for the 750.

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    Connecticut CrazyA's Avatar
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    Quote Originally Posted by xlint89 View Post
    Never really thought about that. It would decrease the port timing wouldn't it? (lower the opening)
    Correct.

    Because ports enter the cylinder at an angle, when you overbore, the transfer ports drop in height, retarding the timing and duration.

    The exhaust port gets narrower. Most tuners will make porting adjustments for bigger bore motors.

    If not, all things being equal, one would have a helluva low-end ripper!!!



    c

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    Quote Originally Posted by beerdart View Post
    .020 is the largest oversize from Polaris for the 750.
    Thats it how about from WSM

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    Connecticut CrazyA's Avatar
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    Also, you gotta really know what you're doing with squishband area too. Just cutting the heads to proper diameter isn't the only thing to worry about when overboring.

    Just cutting them bigger without altering the area can lead to squishband turbulence resulting in a motor that runs "flat" on the top end, along with other nightmares. (midrange deto)


    b

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