Hi, just hoping that someone might be able to confirm which CP piston this is?

is it a 11.5:1 SC7118 or 12.5:1 SC7121 ?





thanks!

btw - i have emailed CP and got dome volumes (1.1cc & 5.3cc respectively), but the trail went cold once i asked them to identify second hand pistons...

 

those are 5.3

 

EDIT: thanks Chris! :D

also - do you know what the 1.1's look like?

do they look like the pic on page 6 of the CP catalogue?

http://www.cppistons.com/LinkClick.a...nY%3D&tabid=64

if so - could i safely reduce the CR by milling the top of the piston?

 

u can always reduce it like that... not recommended if u got no experience doing it. Why would u want to reduce it btw?

 

i would have a machinist do it

well, i bought them as 7118's - so expecting 1.1cc dome and aiming at high 12's for CR...

Parts:
B16A head = ~0.04' milled (at wear marker? not sure what this equals?)
chambers = 41cc (welded + flat face valves)
B20B block = decked 0.002', bored to 84.5mm

so according to zeal i am at around 14.4:1 CR with std head gasket
and according to c-speed i am at around 13.6:1 CR with std head gasket

i wasnt planning on having this high compression, as it has 'stock sleeves'

but simple solution is thicker head gasket, a 0.06' would yeild aroudn 12.9:1 with the 5.3cc domes (zeal) which is where i was initially aiming...

is a thicker head gasket a better option? it will also help my crank/cam timing to come back to better alignment i guess?

 

that head should be in the high 38 - 39cc's btw... u should cc'd it for accuracy.

thicker head gasket is an option or the mill of the pistons, whatever rocks you re boat... i would do the pistons. personal pref specially with heavy std cp slugs.

 

what makes you say the head would be 39cc? without even seeing it?

 

Well if you remove 0.04" from the head the volume of the combustion chamber would drop. Add to that your flat faced valves and "welds" and you have to be under 41cc if stock CC is 42.7-43cc's.

So you have a combustion chamber with a radius of 40.5mm then you remove 0.1016mm from the heads surface. That right there alone will remove 5.3cc's per combustion chamber. Therefore, mathematically, your already down to 38cc without subtracting the volume for your flat faced valves and "welds".

I would suggest that you CC the combustion chambers just to make sure.

 

thanks, yes, i understand that, but the head shop who cc'd the chambers for me after welding said they were all exactly 41cc.

but i will re-cc them myself to be sure.

thanks for your help.

 

I would just to be sure. I am pretty much in the same position you are with having to use a thicker HG. I actually planned my build to use a .060 HG. Where my bore was being taken out 4mm I had to get rid of the 85mm combustion chamber lip. Milling was a much easier option than having it welded and worked. Then it would have to be milled in the end anyway...lol

If those pistons aren't the right CC for your setup. I would suggest selling them and getting the proper compression height and dome piston. Unless you have an in at a machine shop and your getting the work done for a good price/free...lol

 

OK, i cc'd the chambers using a 50ml syringe and a 3ml syringe (0.1cc graduations) and came out with an estimate of: over 40cc but under 41cc

however, my high CR "problem" has gone away since i realised i was using the online-calculators wrong... i.e. if you *know* the combustion chamber volume, you don't have to insert the details of the amount the head has been milled...

 

Yeah, I wouldnt rely on those compression calculators for their determined volumes. Zeal says a stock h22 cylinder head has a volume of 53.8ccs. My h22 head with welded chambers, and flat face valves on the exhaust has a volume of 54ccs that I have measured myself.

 

Hey im in the same situation as you. I got a set of CP pistons on eagle rods and the guy told be they were 12:1 but i realized that they dont make a 12:1. Just not to sure what i have they are 84.5 11.5:1 or 12.5:1. What numbers did yours have on the bottom of the piston? Mine had P/N209848 and 563269.

 

mine have a "83-2A" below the CP on the under crown.

also - the dome is about 4.2mm high and around 16mm wide on the flat surface.

best way to tell is from pictures i have found, i have confirmed from CP that mine are SC7121's...

got pics?

 

Only pics of have of them which probably wont help much and they are at my machinists shop. Next time I am there though I will definitely check the measurements out.

 

look like 11.5:1 to me...

EDIT: looks like two different sets of pistons to me?

 

Good to hear because they measure there compression ratios with b16 heads and im using a GSR so it would be higher and I want to spray this motor.

 

Where are the measurements taken from?

 

dome height is taken from piston surface. (i.e. what you see as the dome height on picture two in post #1)

length of dome is of the flat surface between the centres of the two round edges of the dome.