Silv-O-Lite piston question - Hot Rod Forum : Hotrodders Bulletin Board
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Old 09-13-2002, 07:50 AM
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Question Silv-O-Lite piston question

This may be an amateur question, but here it goes. I trial fitted a new hypereutectic piston into my .060 350 this morning and noticed a huge difference between the bore and the piston. This skirt was snug but the top land was (maybe) 0.015 smaller than the bore. I measured the bore (4.059), than I measured the piston and notice a significant taper, top narrow, skirt wider. Is this normal for the eutectic pistons?

Thanks,

Paul

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Old 09-13-2002, 08:49 AM
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All pistons are smaller in diameter in the ring land area. Piston clearance should always be measured at the skirt. Actually you may notice that the skirt itself has a very slight difference in diameter from top to bottom. This why it is very important to have (and follow) the piston manufacturers instructions as to where to measure the piston diameter for their pistons. Most will say something like "measure 1.000" inches below bottom land of oil ring groove, 90 degrees from pin". As you can see, there can be a huge difference in the resultant piston to cylinder wall clearance if the directions are not followed explicitly. Hope this helps.

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[ September 13, 2002: Message edited by: engineguy ]</p>
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Old 09-13-2002, 09:00 AM
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Aren't pistons also an oval shape rather than perfectly round.
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Old 09-13-2002, 10:32 AM
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Thanks for easing my mind guys.

Paul
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Old 09-13-2002, 11:09 AM
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You are correct, MH. Pistons are what they call "cam ground", meaning that they are sorta' egg shaped. This is done because the piston does not expand evenly as it gets hot. The area around the pin has more material, therefore it absorbs the heat and doesn't expand as much as the rest of the piston does. This too illustrates why the piston must be measured exactly according to manufacturer's instructions.
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Old 09-13-2002, 05:01 PM
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A few tips on setting piston clearance. Always try to use the same micrometer to measure the piston and the bore. IE, use the same micrometer to set your dial bore guage (if you have one) or the telescopic "snap" guage. I learned this the hard way at a machine shop I used to work at. Also, all the pistons i've ever measured come with instructions telling where to measure skirt size. Hypereutectic pistons generally run much tighter clearance versus other "performance" pistons. Ol snake says, if it's a little loose, nobody knows, but it it's too tight, everybody knows. I like to add .001 inch clearance for this very reason.
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