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Please forgive me if this has been answered before, I'm at level dummy when it comes to proper searching, but know that I tried.

I just dropped my block and pistons off at the machine shop and they want to know where on the piston skirt the manufacturer specifies to measure the diameter. In my Honda service manual it says to measure 16mm up from the bottom of the skirt but these are YCP Vitara pistons and they didn't come with any info regarding how to properly measure them. And they do vary about a thousandth, plus or minus a little depending on where you take the measurement on the skirt. I have found plenty of P2W advice out there but nothing on this. I'm waiting for an email reply from YCP but figured I would post this in the mean time to see if anyone knows, thanks.
 

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usually when i bore em out at work i measure 90° of the pin. Tell em to measure it and use the biggest spot in the skirt.
 

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This is a really good question actually, I've actually been checking in to see if there is a new answer.

The answer is normally "wherever the piston manufacturer tells you to measure," but with all of these RS/YCP/Nippon pistons out there that come with no documentation, it's kind of hard to come up with something on your own.

The answer is always 90° to the pin axis, since you're most interested in the width on the thrust side of the piston. I bounced around on Google/Google Images for a bit to see what I could find, and it seemed that a lot of what is documented suggests measuring at a line that is tangent to the bottom of the pin (think of the level of a table top of the pin was sitting on it).

The answer still comes back to "whatever the manufacturer says" because different manufacturers have different design concepts and standards. In the absence of direction, I would think the best you could do would be to measure the diameter in a few places between the bottom ring land and the bottom of the skirt, and then take the largest number.

In for an expert answer though :)
 

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Meat Popsicle
91 CRX Si
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Frankly, I think that this hasn’t been answered because there’s no certain answer and nobody wants to contribute misleading or incorrect information.

Without supporting documentation, I would personally measure in a couple of places, slightly below the centerline of the wrist pin, and use the largest value. But keep in mind, that’s just my gut, based off no factual information, feeling.

And, like other said (and just to be clear), you’re measuring perpendicular to the wrist pin.
 

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Meat Popsicle
91 CRX Si
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Just a thought; since these are an OEM replacement piston style for the vitara engine, would a Suzuki FSM have the correct info regarding where to measure?
 
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