Ring Gaps For Engine Builders.

[MJTru]

Hi guys i came across something wierd today building a rb25. I might be wrong here and wondering what you guys think. I have a block that was bored to 86.5 and i just bought a set of new cp pistons over sized for the bore. The machine shop had a piston so he could get the right clearance. The problem is the ring gaps look big like really big in the bore. The photos will show you guys what i mean. The gaps are like .064 in inch. Sorry my gauge isnt in mm. Damn yanks products. Lol

Im figuring that cp would have made it tight with room to adjust the gap.

[tridentt150v]

Wow that is big, I don't reckon I'd use them. What is the recommended gap?? its usually more like 0.006"- 0.008" or 6 to 8 thou - yours is 10 times that.

On the bright side rings won't bind, no matter what the heat!!!

[MJTru]

Ok so i have figured out its to big of a gap. How do i find from cp pistons a set of rings for my 86.5 over size pistons that have a smaller gap.

[MJTru]

I have done the math and figure ill need 1.75 mm bigger ring with a 86.5mm bore. Wow wonder if thats avalible. I notice that the part nber from cp is cnp 3.406 thats the actual sizing of the rings. So if i was to add 1.75mm which is .060 inch the size i would get would be 3.466

Wonder how that will work. At that size ill need to gap them myself. Im planing on going with an end gap of .010 or so for occasionally hitting some NOS. max boost of 25psi.

My big problem is i cant find a chart for cp pistons and the ring gapping or size option. I feel stuck.

Does anyone one of a way to address this??

[MJTru]

In retrospect i have set my rb26 way smaller than the my rb25 build right now. See attached of rb26 ring gap.

[KiwiRS4T]

Have you tried to contact CP direct?

http://www.cp-carrillo.com/LinkClick.aspx?fileticket=Q7LAfTKUjgE%3D&tabid=83

[MJTru]

I will try to contact them directly in the morning. Thanks for the chart. (Document) mate

[CEF33Y]

Have you measured the bore yourself? not a machine shop issue?

[MJTru]

Yes. Right now i cant remember the actual size i got. I will update tomorrow. Work was super busy and will contact cp piaton tomorrow. Will they be able to supply the rings. Custom size i guess.??

[tridentt150v]

I guess what Cef33y is wondering is....Was the bore machined to the correct size for the piston you are using??? Was it accidentally overbored??? Wouldn't be the first time.

Specs would give you the piston dimensions, clearances, and the recommended bore size.

[Komdotkom]

The rings are a file fit for every CP piston I've ever used, so just get a new set for your pistons and adjust them to fit. Piston to bore is going to be a bigger concern if that's the way your rings are.

[MJTru]

Hmm. I thought it may have been bored wrong as well. I was just to busy today to do anything. Ok here is where i must admit im a bit weak. Most likely because im just being lazy. I at the time did to blocks in miami florida with guess who some Australian guys. They do work for the coast guards here in the states. Son does the heads and dad does the blocks. May be an honest mistake. Im going to buy a dial bore gauge tomorrow and check his work. Guess what cp pistons can out and cp pistons went back in. By no means i think its there fault i actually think its just a different batch of pistons by cp and im just doing a rebuild now. Noticing the difference. Damn why do i feel its a money thing now. Well. Best solution if i was.to guess woyld be to buy 1mm or 40thow over pistons and rebore it as 1mm is my personal max i would go. Damn. But before jumping the gun ill contact cp and measures the bore and see what they have to say. From my personal experience i think a new set of pistons is in order. 1mm or 40thow.over. damn i cant believe all this bs. Ill take my caliper to work tomorrow and measure the block. They may have mest up.

Anyone can teach me how to check bore to piston clearance. Using a v caliper. Damn and its for a friend not even mine. :(

[GTSBoy]

Youtube is your friend. There are some great instructional videos for all that shit.

[MJTru]

Thanks i have watched a couple of videos. Please look at the spec sheet that came with the piston is the piston to cylinder wall clearance what im pointing at. ??? If so thats small. I would opp to go with .004

[GTSBoy]

yup

[GTRPSI]

Bore can be slightly bigger than suggested, .004" is fine.

Most cheap bore gauges wont distinguish between .0035" and .004" accurately. Get your machine shop to measure them in front of you at various points down the bore in the X and Y plane and note them.

Ring end gap depends on the boost and purpose of the engine, i wouldn't run .008" end gaps unless it was a very low boost tight daily driver.

If you turn up the boost and lay into it a bit aim for 16 to 20 thou with the latter based on serious power numbers.....60 thou, somethings not quite right.......

[GTRNUR]

I'd put money on the bore size being too big for your pistons. I've never seen a set of CP rings that didn't need filing to get the gap right. You will probably find you have 12-16 thou at least, not 4.

[MJTru]

Aftet i removed one piston took the rings off and insert it upside down to measure the difference from piston to cylinder wall clearance it seems to be close to 20 thou smaller. That would put me right about right if i use a 87mm bore piston but will never know. If the clearance is .004 and my caliper isnt to accurate when i wonder the caliper around in the bore i get like 86.9 thats very close to 87mm i feel the accuracy of the caliper is the problem maybe its a 87mm bore piston i need. Damn if thats the case thats a big mistake. Like a 1300 dollar mistake. Guess its better than assembling the motor to have it fail. Im glad i spot this. Im going to order a set of new pistons and rods and go from there.

[tridentt150v]

Why do you need rods?

Pistons and rings I can understand.

This has the ring gaps you need for CP pistons and rings http://www.cp-carrillo.com/LinkClick.aspx?fileticket=LjC4jLmI4Ls%3D&tabid=74

They also recommend around 8 thou, but others may have better knowledge of it than manufacturers specs or me??

[MJTru]

It makes no sense to me to just buy pistons. As i rather save som cash and stock up on pistons by buying the rods and pistion combination. Thats all. I know that pistons are cheaper.