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How to measure POPUP Piston no specs available May 17 2003

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How to measure POPUP Piston, no specs available May 17 2003 at 5:39 PM    73sport  (no login)
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I have a compression ratio calculator on my website and a fello wrote asking how to determine the dome volume of a popup piston (chevy). Here is my best guess. Will this work?


If the piston was say 4.251 + .060 = 4.311 inches round by 0.125 inches tall that would be 29.89 cc's added.
Formula is: Bore Bore Stroke * 0.7854 = C.I. To convert to CC take C.I. * 16.3870 = CC
Bore Bore Stroke * Constant= C.I. 
4.311 * 4.311 * 0.125 * 0.7854 = 1.82455 c.i. convert to cc's 1.824555 * 16.3870 = 29.89 cc's

In order to measure, we need square corners as in the example above. Or we need a means of physically getting the data.
Picture this: if the piston was in the bore and you placed a straight edge across the deck where the top of the piston just made contact with the dome. No part of the piston is above the deck. Now measure the depth from the deck down to the top of the lowest part of the piston near the cylinder wall. 
Now place a piece of plexiglass over the bore with a small hole in it. Fill the volume up with alcohol or some other liquid (Top ring is required w/grease) measuring the exact contents of the liquid as you go.

Now we have the volume above the dome.
I think we can subtract the 0.125 bore volume of the popup (in cc's) "29.89 cc's" from the volume we added (in cc's) the difference should be the dome volume! 

Thanks for your input, I think we could all learn something here.

Randy A

(Login machoneman)
Here's an easy way....    May 18 2003, 5:16 AM 

to measure just the pop-up's cc displacement. Take some stiff window caulking compound. Form a disc 1" thick and 4"+ in diameter. Press same into oiled-up dome and remove. Place impression face-up, level it, add plexi plate and measure cc's added from graduated cylinder. Can also use plaster of paris (art supply stores, but is messy) or Play-Dough (easy to distort when removing, fix is to bake it at 350F for 1/2 hour before removing it). 

Method is 100% accurate and even better, can use mold to 'cast' a parrafin wax impression later. Hold wax model up to the head's chamber and easily see where tight spots may interfere with flame front. Wax can be cut with an Exacto knife to form a better looking dome (used when one is ordering custom pistons). 

Worked for me! Your method works, but only when domes are flat-milled. When domes match heavily-tuliped valves, data becomes skewed.

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