Thats a good question. Today I was just thinking of the same thing, I wanted to know what was the difference between standard pistons and oversized .25 pistons. I’m thinking of getting JDM CTR for my setup and I needed someone to answer me those questions.
Originally posted by 92GS-R
[B]and for those of you who know the part #'s (ie: leftcorner ) , and parts gurus, what are the different part #'s.
ITR=P73
CTR=PCT
A000 (USDM?)
J000 (JDM?)
is there a different part # for .25 over vs standard bore?[/B]
Yes, the standard and oversized pistons have different P/N’s (as do the rings). I have all the numbers in my garage, but I’m about 850 miles away from them right now, which does neither of us any good. Email me this weekend if you need them.
I want to know more about the difference between standard bore and the .25 over sized. I know the standard bore works with regular bottom ends but why has Honda made .25 oversized pistons? I think that they shouldn’t fit in any unmodified Honda block because they’re too big BUT since they are OEM parts, there has to be a Honda block that has them installed since they come out of the factory.
Oh wise and and all knowing G2 tech specialists, please be so kind to enlighten us with your knowledge and vanish the ignorance that all us newbies are suffering from!!
Originally posted by 92GS-R the oversized pistons increase your compression ratio even higher. however, you have to bore out your block in order to use them.
they don’t necessarily increase compression, but it will increase displacement (by an extemely negligible amount). the reason for going, say, 0.25 over is due to the fact that the cylinders need to be rehoned (if applicable) to accept the larger pistons for a clean piston to wall mating and back to acceptable compression/leak down numbers. basically renewing a used/tired block.
Tony, what is your goal for comp. ratio? (I know you said under 11:1) Have you thought about P30s? Those will put you in the low 10s. They are also relatively cheap compared with the P73s (A0 or 00).
Elmer is currently running this setup in his 11:1 CR B17A with P30s. He took me for a spin at WCM 2002 - it felt very strong and pulled hard.
The only problem I can foresee running at this compression is the level of octane you are able to use locally and any detonation issues associated with that.
However, I am not advising against the P30 route - in fact I think that CR or anything higher than the stock CR will be noticeable, Tony.
I don’t know my CR, but what I Can tell you is:
50 thou milled off my head
10 thou decked off my block (since I had a gouge in it)
B16 pistons (how much do these raise ur compression by vs the stock LS pistons, anyone know?)
Probably one of the mose underrated pistons out there are the PR3 pistons. Very similar dome size to the JDM ITR, readily available in the US standard or oversized, no machining of the rods needed, and around $40 a pop retail. Not too bad if you ask me. ~10.7:1 with a B17 mill which is pretty moderate. An outstanding alternative to the P30 and JDM ITR pistons. If you want that extra couple of points of compression, mill the head or do it with a head gasket.
My detonation issues are really minimal, and 11:1 on the street isn’t too bad. I just gap the plugs a little tighter and back the timing off during these hot summer days. ~18-19 deg. BTDC.
Originally posted by JETSPD1477
[B]Probably one of the mose underrated pistons out there are the PR3 pistons. Very similar dome size to the JDM ITR, readily available in the US standard or oversized, no machining of the rods needed, and around $40 a pop retail. Not too bad if you ask me. ~10.7:1 with a B17 mill which is pretty moderate. An outstanding alternative to the P30 and JDM ITR pistons. If you want that extra couple of points of compression, mill the head or do it with a head gasket.
My detonation issues are really minimal, and 11:1 on the street isn’t too bad. I just gap the plugs a little tighter and back the timing off during these hot summer days. ~18-19 deg. BTDC. [/B]
hey elmer (yes i’m still up too… hehe) thanks for the tip bro. I just checked the compression calculator, and I never tried the PR3 pistons, and hey that’s pretty much what I’m looking for in terms of compression numbers. Like you said, just slap on a thinner head gasket or mill the head and i’m right at the same #'s of the p30 or JDM ITR pistons. Looks, like so far that might be the route I end up going (not anytime real soon, cause I need to find some time when I can get my teg into a shop)…
PCT = CTR (no distinction between JDM or USDM, as there were no US CTR’s)
All pistons will have stamp on the dome indicating whether it’s oversized or not.
I had the JDM ITR overbores in my b18c block, and with the B16 head, it gave me roughly ~11.1-11.2 CR. I used the Spoon 2-ply headgasket and gave me around 11.5CR, runs fine with 91 octane. With that compression, pinging is minimal, only at low low rpm, just backed off the timing slightly.
Originally posted by 92GS-R thanks, what’s the mark on the dome for “overbore” say? is it obvious?
Yes, it is obvious Tony. If you have the full OEM over bore pistons, it will say “25” on the top of the piston. I have the P73-00s (JDM ITRs), but like Jason, I left the part numbers at home. I’ll try to remember to write them down and either email you back or tell you tomorrow when I see you.
has anyone calculated the displacement difference these overbore pistons make? I dont know the equation, but if anyone knows it or has a link to it please post- thanks guys!!
) , and parts gurus, what are the different part #'s.
)
) , and parts gurus, what are the different part #'s.
Oh wise and and all knowing G2 tech specialists, please be so kind to enlighten us with your knowledge and vanish the ignorance that all us newbies are suffering from!! 
