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Its the width of the bore. 20 over is .020" oversized from the stock. When you rebuild an engine you need to keep getting bigger to be resurface everything.
 

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Discussion Starter · #5 ·
93smgturbo said:
.020 is pronounced "twenty thousandths of an inch". 20mm's would be about 3/4 of an inch. That would run the cylinders into eachother.
thanx for the help guys, i was just wondering about that, if i put new sleeves in, should i run stock size pistons.... no (10 over, or 20 over)? Im goign to go JE, any contest to that? :) Oh and if im planning on running 20+psi and around 400rwhp on this build should i go with 8.5:1 rather than 9:1?
 

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Discussion Starter · #6 ·
Also, what is the 10 over and 20 over when you are talking about other things like bearings, what is the difference there? sorry, im kinda new at this whole building from the block up, but you have to learn for the first time at some point right? thnx again
 

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Its the same measurement. When machinests talk, they talk in thousands. Here's an example:

Average person says .020 = Two hundreths
Machinest says .020 = Twenty thousandths

Average person says .0005 = Five Ten Thousandths
Machinest says .0005 = Half a thousandths

So when a machinest wants to recondition a part of an engine, usually about .020 is a good amount of metal to come off to resurface the object whether its a cylinder wall or bearing. If damage is very deep, then 40 over or .040 can be used. Lets say you ruin a bearing on the crank. A machinest will take twenty thousandths off of the crank to resurface and make it perfectly round again. Since .020 is gone, you need to get .020 bigger bearings for it to fill the difference.
 

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93smgturbo said:
.020 is pronounced "twenty thousandths of an inch". 20mm's would be about 3/4 of an inch. That would run the cylinders into eachother.
even though im wrong i did not say 1 mm.. i said 0.10 for 10 and 0.20 for 20

but i live in a country that uses mm instead of inch so its not that strange that i think the way i think :D
 

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.020 over is .5mm and .040 is 1mm larger. ex. my arias pistons are 87mm. the stock bore is 86mm. so the block was machined .040 to allow the 87mm pistons to fit.
 
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