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  1. #1
    Ghostrider55's Avatar
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    97 Polaris Slt 780 Cylinder size?

    Hello, I measured my cylinders and the inside diameter is 71.30 mm. Is this standard size?


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    MSX 150 guy lives on Mr. GP1800's Avatar
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    According to the manual 71.30 is standard size

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    Ghostrider55's Avatar
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    Well, I put new standard Pistons in and to me it seems like the middle piston has a small amount of play at TDC. But the measurement is 71.30 mm. Does this seem normal? Piston measures just under 71.3 mm

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    MSX 150 guy lives on Mr. GP1800's Avatar
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    Quote Originally Posted by Ghostrider55 View Post
    Well, I put new standard Pistons in and to me it seems like the middle piston has a small amount of play at TDC. But the measurement is 71.30 mm. Does this seem normal? Piston measures just under 71.3 mm

    I always measure the piston with a micrometer and the cylinder with a bore gauge, and figure clearance from there. Were you measuring the cylinder with a caliper? If so, you may have been only catching the very top of the cylinder where the rings don't reach and there is virtually no wear. The bore farther down may be worn more than that causing the loose feeling piston.

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    Ghostrider55's Avatar
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    Yes I was using a caliper, this is the center cylinder that seems to have play, but this is also the same cylinder that had 150 PSI compression before removing old piston. The other 2 were the ones with lost compression, but those seem to be fitting a lil more snug. Would you go ahead and install heads and check compression on the cylinders?

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    MSX 150 guy lives on Mr. GP1800's Avatar
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    I would at least check the ring gap. If it is too wide then you will most likely know that your cylinder bore is too big. I bought a dial bore gauge. That can show you a lot of things, cylinder taper, out of round, etc. Things you will never see with a caliper.

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    Ghostrider55's Avatar
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    What is the maximum ring gap?

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    MSX 150 guy lives on Mr. GP1800's Avatar
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    Ring gap is .012-.020 on the 1997 SLT 780

  9. #9
    Ghostrider55's Avatar
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    ok I used feeler gauge and .018 is very snug between piston and cylinder. Is this allowable?

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    Ghostrider55's Avatar
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    Also question about high speed adjustment on carbs. The manual for this year and model recommends it to be adjusted 1/8 turn out from seated position on all three carbs. That seems very lean to me. Is this safe? Lol

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