CP Piston Ring gap question

[CosmoE36]

i am currently assembling the motor and was told that when you buy CP pistons that come wiht cp rings that they come pre gapped but if i wanted to check the ring gap i should compress the ring in the bore and measure the gap and it should be between 12 and 15 thousandths. what i didnt ask was whicn ring should that measurement be of... there are 2 compression rings and which one should have the gap of 12-15 thousandths? or should bothl the rings have that same gap?

any guys in here that have CP pistons have any input? for what its worth i have CP 86.5 mm bore pistons.

[mazur]

Both compression rings should be in spec.

[CosmoE36]

Both compression rings should be in spec.

i understand. now hopefully someone here with cp pistons can confirm what ring gap they used or had with their setup.

[CosmoE36]

Both compression rings should be in spec.

actually just checked my documentation and CP website and they state that te top ring has one gap and the 2nd ring has a 2nd gap size so they are both not the same gap size.

CP has a generic ring gap chart to follow





well according to their equation and chart. i follow this equation to convert my boresize from mm to inches

86.5/25.4= 3.4055 inhes

and since im classified as nitrous/turbo/supercharger Top ring- 3.4055 x .0065 = .022 or 3.405 x .007 = .0238

and 2nd ring - 3.4055 x .007 = .0238 or 3.4055 x .008 = .027

just using their generic ring gap conversion table for my application states that my minimum ring gap should be between .022 and .0238 for the top ring.

and between .0238 and .027 for the 2nd ring.


both of these specs are well beyond the 15-20 thousandths of an inch i was told to use for my ring gap.


getting kinda worried here lol.

any other thoughts or advice?

[mike radowski]

I have CP pistons also. With an 84.5mm bore I set my top ring to .021 and my second to .023. I also made sure all the oil expanders had at least .015 clearance. I would listen to the piston manufacturer.

[CosmoE36]

I have CP pistons also. With an 84.5mm bore I set my top ring to .021 and my second to .023. I also made sure all the oil expanders had at least .015 clearance. I would listen to the piston manufacturer.

ok this makes more sense. when we measured with the ring compressed in the bore it came back with about .0020 on the top ring. so this sounds about right that CP pregaps these rings for the pistons when they are ordered.

Mike did you buy rings separately? or did your pistons come with rings? ust trying to figure out if you still regapped your rings or left them alone at what CP put them at

thanks for your help

[mike radowski]

I got pistons and rings from CP.

Mine were way too tight to drop in as-is. I file fit every ring.

[gboezio]

At least you checked them out, some people just don't care about important details like this and end up fracturing rings. In doubt I'll go slightly larger.