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Are they in anyway different? I thought I saw PM7 stamped on the block somewhere, does this mean it is different? I was looking on the compression calculator page, and while A6 pistons are slightly dished (3.4cc) the "ZC" option (I dont know if they are referring to SOHC or DOHC ZCs) had 7.2 cc of dome. Hmmm...
 

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They can also be stamped p29, which I think are the standard OEM replacements for the PM7s. I've also heard that the rods on the PM7/P29 are the best D series rods, though I don't know if that's true.

And Dren, I'm not running them yet ;)
I don't know what's up with these pistons, but they seem to make the car sound like a monster.
 

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Looking at that picture, I almost want to say the pm7 is a bit higher CR. At the top of the picture you can see that more material for the p29 is cut out at the valve reliefs. Once thought to be exactly identical, now they are not, at least not by appearance alone.
 

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89civicdx said:
I may be wrong
BUT the P29 piston pictured looks like it is .050 oversize as the stock bore size
...then

Sonny said:
That is a stock bore P29. Maybe it looks a little bigger because it's cleaner than the rest?
I'm talking about something totally different. 89civicdx is refering to oversized pistons meaning the diameter is bigger. I'm talking about that little area right in between the two valve reliefs. Look at how the ZC has more material right there. Given that with everything else being equal, the ZC would yield a bit higher CR than the A1 because there's more material there to take up space. That leaves less space for the air fuel mixture during the compression stroke resulting in a tighter "squish" meaning higher CR. Hope I didn't lose anybody there.
 
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