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Hey guys, quick question:

I am putting together an rb26 using CP pistons and an 87mm bore and .010" shaved off the block. Everything else is std sizes. I am looking for help getting the right head gasket thickness - I would like to end up with around 8.5:1. However, I can not find out what cc the dome of the pison is so I can not calculate it myself.

I tried calling CP to get help on this but have a several attempts I couldn't wait any longer.

Thanks

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https://www.sau.com.au/forums/topic/367818-rb26-compression-ratio-cp-pistons/
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Bore (in) 3.425 in.

Bore (mm) 87.000mm

Piston Style Dome, with no valve reliefs

Piston Material Forged aluminum

Compression Distance (in) 1.193 in.

Piston Head Volume (cc) -15.50cc

Wrist Pin Style Floating

Pin Diameter (in) 0.827 in.

Gapless No

File Fit No

Top Ring Thickness 1.0mm

Top Ring Material Steel

Second Ring Thickness 1.2mm

Second Ring Material Cast iron

Oil Ring Thickness 2.8mm

Thats from CP for part number SC7311

You guys are missing the point.

Combustion chambers have NOTHING to do with piston crown volume

Compression Distance (in) 1.193 in - So from the centreline of the gudgeon pin to the edge of the piston crown is 1.193". If the piston was dead level along the crown aka a flat top piston the crown volume would be classed as 0cc, if it is raised aka dome top than it would be + 15.5cc. A dished piston (or inverted dome as some call it) will have a negative volume.

As an example the first one in this list (SRP 138093) has a -5cc volume due to the valve reliefs

The one underneath (SRP 140674) has a +11cc dome due to the raised crown.

http://www.jegs.com/webapp/wcs/stores/servlet/KeywordSearchCmd?Ne=1%2b2%2b3%2b13%2b1147708%2b1147708&Ntt=350&langId=-1&Ntk=all&storeId=10001&catalogId=10002&N=4294964093&Nty=0

So really I was just having a dig at the CP website.

For eg, im building a V8 up.

Now I have heads with a 58cc chamber volume and I need pistons with a -6cc volume at 0 deck height to get the comp I want (11.5:1). If I was building it to take a turdbro or a supercharger I would need something like a -15cc dish to have a lowish comp ratio.

Now if CP listed there Ford Cleveland pistons like that, I would see the -15.5cc volume and order those because they will give me low comp....So imagine my suprise when I open the box and...."Oh look they are a dome top, I wonder how a blown v8 will like 16:1 comp?"

See my point?

oh and to the bloke that asked yes they can and do sit slightly proud. up to you to set the right deck height and chose the right thickness gasket to get the right CR and also to ensure they interact properly with the squish pads but don't hit valves or anything else. not always just bolt them in and whack it together. you do need to do some measuring.

I dunno? it made perfect sense to me. -15cc dome top piston means take 15cc off your chamber volume...

Ye. It works that way as well. I'm not denying that. Just simpler to do work it how all the other piston maker dudes do it.

RBs are easy because they are application specific. But when you start doing funky stuff like 6" chev rods and pistons designed for a 347 stroker windsor v8 in a Cleveland v8 it becomes dicky when some people say -ve and others say +ve lol

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