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Old 07-14-2011, 11:05 AM   #1 (permalink)
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Default Which is the real NPR P29?

So I screwed up one of my pistons when installing it (thread in engine building section) so I ordered another set of pistons to replace the one I screwed up. The new set comes in and they are quite different. Both pistons came from the same place on ebay and both claimed to be NPR p29's. So which is the real deal NPR?








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Old 07-14-2011, 01:03 PM   #2 (permalink)
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Measure the compression heights if the are the same, rock on. If not, go with the new set of pistons.
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Old 07-19-2011, 05:38 PM   #3 (permalink)
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they both look about the same, beaver is right if the compression height(from the centerline of the piston pin to the deck of the piston) is the same 29.0mm and the dome cc's (7.2) look identical or the description of the dome cc's are the same on the boxes I would say your fine, also when your installing the pistons make sure the deck and top corners are deburred. get some emry cloth and polish em up. any burr left on a piston deck or edge will create a hot spot and a magnet for predetonation(pinging)

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Old 07-20-2011, 07:51 PM   #4 (permalink)
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So using my cheap harbor freight calipers I couldn't figure out a reliable way to measure compression height from the center of the pin so I went from the top of the hole drilled for the pint to the top of the dome and mesaured 23.1mm on the old ones and 23.4mm on the new ones. Is that a significant enough distance to matter?
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Old 07-21-2011, 08:12 AM   #5 (permalink)
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Get a more accurate measuring tool or take them to a machine shop, they can measure it for free if you ask nicely. The NPR P29's are 30mm, regular are 29mm. I know from experience that the harbor freight digital calipers can be off by as much as 1mm and aren't the best tool to measure compression height with. Also, why do you have a picture of the NPR P29's and then some PM7's? Is that what you're comparing it to?
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Old 07-21-2011, 08:45 AM   #6 (permalink)
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I know from experience that the harbor freight digital calipers can be off by as much as 1mm and aren't the best tool to measure compression height with.
WOW really, the $20 sets of digital calipers around here have a guaranteed accuracy of +/- .02mm (the 6" set ones)

edit also the bottom 2 pics are pm7's not p29's they have it stamped on them. ZCHONDACRX91
already said this tho

i also believe the p29's have bigger valve relefs in them then the pm7's
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Old 07-21-2011, 10:33 AM   #7 (permalink)
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Yeah, I was sent both of those sets of pistons and was told they were both NPR P29's by the seller. That's why I am here asking the question. I was under the impression that PM7's and P29's were the same pistons. Is that not true? I'm comparing the first two pictures to the second two pictures. I have 3 of the PM7's on the rods and one of the P29. Do I need to have the machine shop switch out the other 3 or can I run as is? That's what I'm trying to figure out.
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Old 07-21-2011, 10:50 AM   #8 (permalink)
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Woodi-

To properly measure the compression height, you take the measurement you got from measuring from the top of the hole to the deck of the piston and then add that to HALF the dimension of the wrist pin (which should be 19mm).

Don't measure to the dome. Just the flat part of the side of the piston.
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Old 07-21-2011, 10:52 AM   #9 (permalink)
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Also, looking at the pictures again and more closely, the top ones are the NPRs. The valve reliefs are larger and slightly deeper, and there are many more oil drainback holes. (I totally just noticed that.)

If the piston in the top pictures are the new ones you got, those are the NPR P29 pistons with the 30mm compression height.
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Old 07-21-2011, 11:29 AM   #10 (permalink)
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the ones in the top picture are the new ones I got. And I was thinking they were the real deal for all the reasons you just mentioned. I'll need to more accuartely check compression height but it sounds like the other ones are most definitely not going to be the same. I'm stopping by the machine shop tonight I think I might try and get a better answer out of the machinist tonight. I want this engine to run when I put it back together not blow up in 2 weeks because it has mismatched pistons. Seems like for peace of mind it might be worth paying the little bit of extra money to get the other three swapped since I already have them. I'm pretty sure the machinist i've been working with is going to look at me like I'm crazy though.
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Old 07-22-2011, 08:54 AM   #11 (permalink)
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Yes, he might look at you like he's crazy, but, run the NPRs. Your engine will like them.
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Old 07-23-2011, 10:02 AM   #12 (permalink)
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the top pistons are p29 pistons out of a 88-89 teggy and the bottom ones are jdm pm7 out of a 88-91 jdm dohc zc engine, the reliefs are slightly different but the compression numbers should be identical, I have not seen them in person but according to other users they should be interchangable, but to keep things sane here just get a complete set of the same.
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Old 07-23-2011, 11:38 PM   #13 (permalink)
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Nope, Arny, you're wrong and I'll tell you why.

The NPR P29s are NOT stock pistons. They have a 30mm compression height, more oil return holes, different alloy, larger valve reliefs, and a lower dome (relative to the "deck" of the piston). I am guessing that the total dome on the NPR P29s is about 5.2cc, but I sure won't know until someone actually CCs them.

The stock PM7 and P29 pistons are the SAME casting. (I have had factory pistons in my hands.) They have a 29mm compression height and 7.2cc total dome.

Overall, the NPR piston will give a slightly higher static compression ratio and also yeild better quench and combustion dynamics, which being slightly more detonation resistant, though all P29s are pretty weak in the upper ring land area.
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Old 07-26-2011, 03:47 PM   #14 (permalink)
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like I said before beaver I have never seen them in person, but good spot Matt, I had no Idea that NPR would rob the p29 badge and change the dynamic of the piston, those bastards*fist shaking* It does make me wonder why they did that and still sell them for d16a1s/zcs as replacement pistons??? I guess until we get an exact #on the dome cc's we will never know...haha for curiosity what static compression #s were you getting with both different sets? were they close, I was tempted back in the day to install a set, and turbo them but gave up on the idea for forged sets..lol

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Old 07-26-2011, 03:59 PM   #15 (permalink)
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like I said before beaver I have never seen them in person, but good spot acid beaver, I had no Idea that NPR would rob the p29 badge and change the dynamic of the piston, those bastards*hand shaking* It does make me wonder why they did that and still sell them for d16a1s/zcs as replacement pistons??? I guess until we get an exact #on the dome cc's we will never know...haha
The NPR's have 30mm compression height compared to the stock 29mm compression height, 30mm is better. They didn't rob the P29 badge, they made the pistons better and they are a great stock replacment or for a nice build. The dome is the same volume on the NPR's and stock P29's. You need to learn the facts before you talk.
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Old 07-26-2011, 05:59 PM   #16 (permalink)
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The dome is the same volume on the NPR's and stock P29's. You need to learn the facts before you talk.
Are you sure on those dome cc's zchonda? I don't think anyone knows the volume of them,have you clayed any motor with em yet?plz fill me in!!! by my calculations and what I've seen, and heard I would put them somewhere in the +5.2cc area, they do have the 30mm compression height, and they sit a smidge taller in the dome, but have larger reliefs,?? its a crap shoot, we need some solid info!!! help!!

woodi if you go with the npr(30mm) setup on your zc/d16a1 (88-89) with a stock.048 head gasket and .010 shaved off the head(cleanup) you will be in the 10.2-10.7 comp. range(depending on the dome volume you decide to go with 5.2cc or 7.2cc)its pretty much 6's at that point, but getting close to out of pump gas range, do you have cams too? Is your computer tunable? I've heard good and bad things about the npr pistons, I personally think you will get better quench and thermal efficiency(shortened combustion chamber) with the npr's but I've heard ycp make a good p29 piston as well, they may be 29.0mm compression heights though. Either way with a good tune those npr's should be fine!

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Old 07-26-2011, 06:07 PM   #17 (permalink)
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Are you sure on those dome cc's zchonda? I don't think anyone knows the volume of them,have you clayed any motor with em yet? by my calculations and what I've seen, and heard I would put them somewhere in the +5.2cc area, they do have the 30mm compression height, and they sit a smidge taller in the dome, but have larger reliefs,?? its a crap shoot, we need some solid info!!! help!!
Oh, yeah you're right, the dome of the NPR's are about 5cc, but the compression ratio would be about the same. The NPR's are a much better choice than stock P29's.
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Old 07-26-2011, 06:26 PM   #18 (permalink)
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yea buddy! with a mls head gasket and .020 shaved head you'ld be flirtin with a high compression race beast!!!!lol!!
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Old 07-28-2011, 10:55 AM   #19 (permalink)
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They are swapping over the rest of the pistons to the real NPR's. I'm going to run them with a .051 cometic hg a cleaned up head and block and pretty much all else stock. Just want to get the car back on the road at this point so nothing exotic.
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Old 07-28-2011, 12:02 PM   #20 (permalink)
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sounds good woodi good luck homie.
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