Overbore size...what is the real size? - Team Camaro Tech
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post #1 of 5 (permalink) Old Aug 16th, 03, 01:05 PM Thread Starter
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Join Date: Jan 2001
Location: skiatook, ok
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I was just about to stuff the pistons into my in progress 388, when I thought I would attempt to measure the actual bore diameter and piston diameter to get my piston to wall clearance. This is a .060 over 350 4 bolt block, BTW. With my chinese calipers, I am getting bore measurements of 4.0575-4.058. The wiseco pistons are supposed to have a minimum .004 p/w clearance, but when I measure them I am getting 4.048-4.050...meaning huge p/w clearance no matter which piston goes into which hole. Am I putting too much faith into a set of calipers (I know it isn't the right tool for this anyway), or have I been sweltering in the heat too long today and suffering dain brammage? The shop that machined my block is very reputable 3 man operation, FWIW.

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post #2 of 5 (permalink) Old Aug 16th, 03, 01:22 PM
Join Date: May 2003
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The clearence should be cut on the piston,and I think you will need the proper outside mic. to get an accurate measurment on the piston skirt where the measurment should be taken. I think a set of calipers will only give you a " close" measurment of the bore, like if you were buying a used block and wanted to see if it was over bored or not. Find a machine shop that has a Sunnen dial bore guage and have it properly checked. I guess there's other maufacturers, but that's the one I'm familiar with. The right tool, VERY VERY accurate. Reads out 1/10 of 1/1000"

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post #3 of 5 (permalink) Old Aug 16th, 03, 02:50 PM
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Where are you measuring the piston? It must be measured on the skirt not at the top.
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post #4 of 5 (permalink) Old Aug 16th, 03, 04:53 PM
Join Date: Aug 1998
Location: Waldorf Md. USA
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You really cannot measure accurately enough with a caliper. You need a micrometer and a dial bore gauge, or at least an inside micrometer. Your bore size really should be at least 4.060 unless the pistons were manufactured wrong.
Bit of info for everyone...each piston manufacturer has a different place they have you measure the piston. Some do it at the bottom of the skirt, Wiseco typically says about 1/2" above the skirt, but they vary. Others measure at 90 degrees from the wrist pin bore. The point is...you cannot guess at where to measure...you have to read the instructions that come with the piston. Sealed Power / Speed Pro actually prints a minimum bore size on the lable of the box. On thier skirt coated pistons you cannot measure the piston, you have to finish the bore to the size on the box, or larger, depending on the intended use of the engine.
Once again...you cannot guess, or go by what another piston manufacturer does...you hve to follow the instructions for YOUR pistons. Period !
Hope this helps,

Bill Koustenis
Advanced Automotive Machine
Waldorf Md
1971 Chevelle "Heavy Chevy" original owner

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post #5 of 5 (permalink) Old Aug 16th, 03, 09:22 PM Thread Starter
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Oh come on guys, give me some credit here! One these particular pistons you measure the diameter 1.130" below the bottom ring groove, or even with the flat area just below the pin bores. These are a very short skirted piston...the actual "skirting" only extends maybe another 3/8" below the pin bosses. I went ahead and assembled everything and it all felt fine...not too loose or too tight. Everything turns over like it should. I know I really should have measured everything but I have done a great many engines without measuring (trusting the machine shop, in other words) and have never had any problems.

Interesting too is that my deck height is .000-.001...I expected it to come out around .010 or so with considerably more variance between cylinders.
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