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Piston Ring Clearances

tegratt

By tegratt,
in Engines & Forced Induction

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tegratt Newbie

tegratt

Posted
Posted

I have new CP pistons with new piston rings but I do not know how much clearance there should be the piston rings. The engine develops 580bhp and I have heard that the clearance of piston rings must be greater when the effect is so great. If the clearance is small, it will destroy the cylinder. The engine is standard bore and compression.

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GTRNUR Grand Master

GTRNUR

Posted
Posted

There is a data sheet that came with the pistons that tells you the correct ring gapping based on application. It is a calculation based on cylinder bore diameter.

Yes it does need to be correct, as too much gap means excessive blow by/engine pressurisation. Not enough and you gall the cylinders and damage the pistons.

djr81 Grand Master

djr81

Posted
Posted

Usually it is a number in thousanths of an inch multiplied by the bore.

An RB26 has an 86mm bore which is 3.386 old speak.

Now the number of thou depends on your piston manufacturer, your application and experience. Mostly the recommendations are conservative. This is done because a too large a gap is a small, manageable problem. Too small a gap on the other hand can lead to the ends of the ring butting together and they stop working at all well when that happens.

Now if I remember correctly CP recommend bore * 0.0055 which gives you 18.6 thou. How conservative or otherwise this number is I dont know but I have seen larger numbers used. Conversely I have seen reference to this being too large.

Out of interest do you have the 1.0mm thick top rings in it?

GTScotT Grand Master

GTScotT

Posted
Posted

I posted a detailed response to this thread which somehow got deleted...


The CP specs would be 18 thou top ring and 22-24 for the second. They say the second ring should be 4-6 thou larger than the top.

Oil rings must be 15 thou MINIMUM and they say DO NOT file these. Also, remember that you need to see both the green and red tabs on the oil expander, do not let the expander overlap. Also the oil rings sit in a groove on the expander, they DO NOT sit above/below it they sit ON it.


I did my own motor (86.5mm bore) at 18 and 22 and it has turned out awesome. Seeing as you are aiming for a little more power I would increase the second ring to 5 thou larger, so 18/23 for a street motor or up to 24 on the second ring if you want it to be a little looser. I would not be scared to go 19/26 on it.

At 18/22 my oil control in the SR is perfect. I went 18/25 in my brothers motor and that is a VERY free motor. I did that because it is an older mitsubishi motor which gets a lot of blow-by in factory guise and a few very experienced builders advised it would help in oil control, plus will make the motor a little happier in terms of power making.

Feel free to PM if you need help.

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