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Thread: How big a bore is too big?

  1. #1
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    Default How big a bore is too big?

    1975 Merc triple (similar to in-line sixes). What's the maximum acceptable (ie: without problems) bore allowed on these motors?

    Thanks,

    Jeff
    "We live at the bottom of an ocean of air." - General Marvage Slatington

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    A clarification: How big a bore out is acceptable without incurring liner warpage and other problems?

    Trying to save an old Merc.

    Jeff
    "We live at the bottom of an ocean of air." - General Marvage Slatington

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    Team Member zul8tr's Avatar
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    What is the largest oversize "Mercury" piston that was available back in 1975 for a over bore job? IMO that would define the max bore diameter with proper skirt clearance included.
    " Three may keep a secret if two of them are dead" Ben Franklin

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    The largest overbore sold by Wiseco is .040. I have heard of .060 over slugs being used, however.

    Jeff
    "We live at the bottom of an ocean of air." - General Marvage Slatington

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    Team Member zul8tr's Avatar
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    Jeff
    Just because Wiseco has 40 over does not mean that Mercury was willing to go there with there available oversize pistons. As I asked what was the max oversize that Mercury offered back then?
    Have you measured the bore accurately and get max worn values to determine how far worn from stock?
    Any deep scratches if so where are they?
    Pete
    " Three may keep a secret if two of them are dead" Ben Franklin

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    My boring service guy cleaned up the bore at .030 on two holes and went to .040 (without my knowledge or permission) on the third hole. All three bores look perfect, so idle and shifting into gear should not be a problem. Mu only concern is that the liner might warp on me.

    Time will tell.

    Thanks for the responses.

    Jeff
    "We live at the bottom of an ocean of air." - General Marvage Slatington

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    Team Member zul8tr's Avatar
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    Run lots of oil like 25:1 and make sure the cooling system is in great shape and passages and all areas are free of scale.
    Not good your machinist went 40 without your permission. Does he have lots of experience with blind bores?
    " Three may keep a secret if two of them are dead" Ben Franklin

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    ..."Not good your machinist went 40 without your permission."

    You said it! I was pissed.

    ..."Does he have lots of experience with blind bores? Does he have lots of experience with blind bores?"

    Yes, and he's pretty good.

    Oh well.... Time will tell.

    Jeff
    "We live at the bottom of an ocean of air." - General Marvage Slatington

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    I wouldn't sweat it. Yes it's more than you probably should, but really, it's only .005 on a side and that likely isn't going to be much different than a .030 overbore. As Z noted, keep it from overheating and you should be fine. Which hole is it that they overbored? If it's the top, then it likely got out of shape when it starved for water in the first place.

    Also if the motor got overheated once it's less likely to distort from now on.

    Also, nothing is forever, it'll be fine for a while..

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    .030 is as big as I have ever went on an inline. I went .044 on a v6

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