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Cp Piston To Bore Clearance And Ring Gapping


TriniGT
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I am doing a fresh build and I wanna be sure on clearances. I know some people mostly shops will keep this to themselves but being that I am all the way over here and cannot send my engine to one of you'll competent builders maybe someone can give me an idea of what clearances to use. I have had the block decked and polished along with the crank and I am ready to check ring gapping and want to know what gapping to use on each ring, my power goal is somewhere around 500-600, not crossing 600. I want the build to be reliable and not losing oil due to big clearances. Can anyone advise me what is a safe gapping. This is a stock bored 86mm RB26 and I am using stock CP forgies with RB26 rods. Any help is greatly appreciated.

Jason.

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New engine block or is it just honed? If its new the 86 should actually measure to be a true 86mm bore. Measure it at 3 diagonals at 3 heights down each bore to be 100% sure of each cylinder. They can get it wrong at the factory so its best to be sure. Machine shops get it wrong too.

A honed old stock block will probably measure up a little over stock by no more than 0.001". The loosest you'd want to run the cylinder clearances is about 5 thou. I have heard of more being used on all out drag/nitrous engines but thats not what your building here. The downside of a loose bore is noise that can cause issues with knock sensors detecting piston slap, and also blow by is increased a little. So that can increase oil temps and mean you need to have a good catch can system to re-claim oil and return to the sump.

Bore clearances are generally built into the pistons. So an 86mm piston if measured with a micrometer will actually be 86mm - about 3 to 4 thou. Most pistons ive measured have never been perfectly round (including CP's). Measure at multiple diagonals under the base of the oil ring above where the skirt starts.

0.002" is minimum factory spec which is too tight for what your doing. Id be leaning towards the 2.5-3 thou though considering the engine is going to rev and when that happens the crank will bend and twist to a degree, as will the block.

Measure the crank and the inside diamater of all the assembled rods with the bearings installed at 3 diferent diagonals with a micrometer and not plastigauge. And once youve done this dont mix anything up.

I always measure everything 3 times and write down each measurment. If you get a big diference between the 3 measurements start again till you dont. If you do this right you wont be doing a post in a months time with pictures of your spun bearings on display.

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Thanks a lot measure three times and rebuild once. I will check all clearances and this block is a hones one, I will definitely check the bore at different positions to see if the bore is uniform and likewise for the rod bid end clearances and like you said write them down and number then to know everything is accounted for. Will let you know how it goes.

Also what ring gaping you recommend for the different rings, I have read to leave the oil ring as it is but can you give me some insight on what clearances one each of the ring so I can get it right the first time. Thanks.

Edited by TriniGT
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Sorry i went off on my own tangent last night. That happens when I post when its late sometimes.

The manufacturer spec sheet for the pistons and rings should recommend a ring gap for various applications. NA applications run the smallest gaps, then forced induction and finaly nitrous. Diferent ring materials also have diferent requirements.

The data in a CP booklet I have here says that for the standard chromoly rings supplied with CP's they recommend 0.007" top ring and 0.008 on the second ring. Oil rings should be 0.015 minimum.

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piston to bore: 3-4thou

top ring 15thou

second ring 17thou

cant remember oil rings..

the cp recomended ring gaps are to big!!

for a CP street engine mine are almost the same

piston-bore: 3 thou

top 16 thou

second 18 thou

oil rings more than 15 but no more than 20

i get a great seal with this combo if ran in correctly but leaves a little headroom if the engine see's more boost.

hey V-spec Tomei recommended piston to bore is 1.9-2.2thou!

Trini with that power level id use a little larger

Piston-bore 3.25thou

top 18 thou

second 20 thou

oil rings 16 thou

Edited by DiRTgarage
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the shop that honed the block, did they have the CP spec sheet b4 they started?

thats a great question...as any block honing should be measured from the exact pistons used.

i even went to the extreme of taking my measuring equipment to the machinist i use and got him to measure with his equipment and i did with mine so we could compare 'feel' so we knew we were measuring the same.

Edited by DiRTgarage
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bingo pauly :sick:

also i'm with the other 2, somewhere between 3-3.5thou piston to bore. also measure all the pistons, and all the bores. you shouldnt have problems with the CP's, but better to check.

1 thing i dont like with the CP rings is the oil control ring, not enough tention for my liking. great for a race engine wanting power, but for a non-smoky streeter grab a set of acl oil rings, cost about $70

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bingo pauly :sick:

also i'm with the other 2, somewhere between 3-3.5thou bore to piston, but make sure theyre all the same

1 thing i dont like with the CP rings is the oil control ring, not enough tention for my liking. great for a race engine wanting power, but for a non-smoky streeter grab a set of acl oil rings, cost about $70

Mate im a big fan of the ACL's but no bastard wants to use them. The thick upper rings seal really well...even in big clearanced drag race stuff and as you say the oil control is much better than the CP's as even at 16 thou the CP's tend to 'float' on the bore.

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I suppose it depends on the pistons you are using.

My specs are

Tomei 86.5 NON cooling channel pistons

2 thou clearence

GT block

Tomei stroker

Running anything up to 1.8 Bar on GT-RS's

Ring gaps im not 100% sure on so i wont post them up.

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I suppose it depends on the pistons you are using.

My specs are

Tomei 86.5 NON cooling channel pistons

2 thou clearence

GT block

Tomei stroker

Running anything up to 1.8 Bar on GT-RS's

Ring gaps im not 100% sure on so i wont post them up.

Yes different manufacturers use different materials that have differing expansion rates so clearances vary.

I just built a Tomei non cooling channel 2.8 stroker (87mm)...i was too scared to use the lower end of the clearance scale with them and as the engine will be seeing reasonable power and boost went 2.2thou.

Edited by DiRTgarage
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Mate im a big fan of the ACL's but no bastard wants to use them. The thick upper rings seal really well...even in big clearanced drag race stuff and as you say the oil control is much better than the CP's as even at 16 thou the CP's tend to 'float' on the bore.

yeh them ROSS pistons are pretty good huh

speaking of which, i wonder what acl are gonna do to mahle since the merger

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The machine shop said the CPs were one thou over the spec on the sheet unbelievably. He told me he gave clearance of 4 thou, wondering if I should be worried. I am going to be using the top 17, second 19 and oil as a recommended 16 just to be in between what was advised. Didn't think I would get help on this as I know people usually keep these things to themselves so thanks a lot. Will let you know how it goes and should I worry on the piston to bore clearance of four thou?

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I suppose it depends on the pistons you are using.

My specs are

Tomei 86.5 NON cooling channel pistons

2 thou clearence

GT block

Tomei stroker

Running anything up to 1.8 Bar on GT-RS's

Ring gaps im not 100% sure on so i wont post them up.

Would love to see how the GT-RS's go on a stroker :P

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Completely agree with this!

I ended up giving my CP's the flick in favour of a new set of Tomei's and never looked back

yeah i prefer not to use them either and if i do i supply my own rings, cp's are good but definatley not my first choice. Most of my comp engines running jap pistons (cosworth etc) are set around 2thou piston to bore, ring gaps vary depending on application, even the cp's and arias i set up tighter than they specify.. not quite 2thou though >_< .

No machining should be done prior to having pistons micced up and on hand. Machine to suit then hone to suit indiviual fitment... dont ever buy say 20thou ups and just bore the block 20thou..... it will end in a clunker that breaths its ass off..

Edited by URAS
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