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  1. #1
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    Question What bore size for oversized pistons?

    I have a 1997, 2 stroke Mercury Sport Jet (2.5L XR2 175HP).

    Stock bore is 3.501" and I have 0.030 pistons. Would the oversized bore be 3.531? That seems a bit tight?

    Here are the pistons: Only 2 pictured, but I am doing all 6..

    Click image for larger version. 

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    No specs came with the pistons.

  2. #2
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    most all pistons i have bought for rebuilds will tell you in the box what it should bore to, promarine and vertex say .3.530

  3. #3
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    Quote Originally Posted by TEXAS20225 View Post
    most all pistons i have bought for rebuilds will tell you in the box what it should bore to, promarine and vertex say .3.530
    That is the size of the piston.

  4. #4
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    I would call that the advertised size of the piston. If you measure it you will probably see it is actually about .006/7 smaller than 3.530.

    Rock
    Team Junk

    No sparkling wiggles in here, only dump truck grinches.

    "Screamin Heathen"

  5. #5
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    Rock is correct a .030 piston is not 3.530
    Last edited by TEXAS20225; 07-20-2014 at 05:42 PM.

  6. #6
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    So what are you guys machining bore sizes to?

    My stock finish bore in my Mercury service manual is 3.501. 0.15 over is 3.516 finish bore. The manual says the tolerance is 0.006" from standard finish bore. My service manual does not mention specs for 0.030" over.

    Standard piston measurements are 3.494 +/- 0.001"
    0.015 over piston size are 3.509 +/- 0.001"

    So from what I can see, specs seem to be 0.007" over piston size? with an additional 0.006" tolerance?

    What would you guys think about going to 0.008" over piston size? I feel this would keep me a bit "safer" from pistons seizing in the bores when hey warm up?

    Thanks for the input guys! I'm nervous, I only get one shot at this.

  7. #7
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    So no luck getting in touch with anyone at GLM Marine that was able to help. I did speak to a helpful tech at Mercury tech support. He recommends a bore of 3.531". Stock finish bore was 3.501".

    I sent the block off to the machine shop and I should get it back in a couple of days. *fingers crossed*

    Also, when I asked the machine shop about the port chamfer, he told me he never chamfers those blocks that he simply de-burs them. I'm under the impression that de-burring is actually a light chamfer no? How can I make sure the chamfer is "good enough" ?

    Thanks!

  8. #8
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    You need to know if the pistons are cast or forged. .006 is fine for cast but forged should be .008. Deburring ports will be fine. All you are trying to do is make sure the edge of a port don't catch a ring. Good luck.

    Rock
    Team Junk

    No sparkling wiggles in here, only dump truck grinches.

    "Screamin Heathen"

  9. #9
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    Quote Originally Posted by rock View Post
    You need to know if the pistons are cast or forged. .006 is fine for cast but forged should be .008. Deburring ports will be fine. All you are trying to do is make sure the edge of a port don't catch a ring. Good luck.

    Rock
    Thank you for the advice! Yep, my pistons are cast and not forged.

    Cheers!
    Eric

  10. #10
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    well. $hit... When it rains it pours...

    The machine shop just called me and their boring machine is not capable of going deep enough into the block... The crankcase plate is in the way... anyone know of a machine shop near Ottawa Ontario (Canada) or as far out as Montreal that can do this? I need to bore the holes 0.030" over.

    Thanks!

  11. #11
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    send it to Rock he does that stuff and he is a good mechanic to boot just ship it at nite across the boarder then you wont get robbed by the establishment

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