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Advice Urgent Guys. Cp Clearances

rockabilly

By rockabilly,
in Engines & Forced Induction

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rockabilly Rising Star

rockabilly

Posted
Posted (edited)

hi guys, well my rb26/30 combo is well under way. block has been fully stripped core plugs the lot, and then chemically cleaned. crank has had proengines collar fitted and a nice polish, will be balanced next week. here is my problem. block is going of for boring and have done a search with not a definate answer. what piston to bore clearance do i need to tell my engine borer?. got the rods,pistons from spool but got not spec sheet, pm,ed him and he was finding out for me, but that was last wed. i need this info for monday morning uk time really.. below is my spec. any advice guys..bernie/uk

rb30 block

spool rods

cp 8.5 pistons.

king bearings

n1 oil water pimps

proengines collar

rb26 head polished/ported

jun cams 262/272 9.7mm lift

tomei camm buckets.

sard 1000cc inj

twin tomei in tank pumps fedding both ends of fuel rail

gt35r 1.06 rear on extreme manifold 44mm tail ex gate.

4" downpipe

1.0 tomei headgasket

trust deep sump extension..

post-13735-1186219418_thumb.jpg

Edited by rockabilly
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GTRsean Grand Master

GTRsean

Posted
Posted

Mate of mine built a 25/30 eng with a T78. CP pistons, argo rods, stock crank, clevite 77 bearings, total seal rings.

Built around year 2000.

Due to metallurgy of jap engines, compared to yank engs, you should run closer tolerances on the jap eng.

My mate used 001'-0015' tops and had no dramas.

It was quiet on startup, bugger all blowby and better boost response.

Don't run a big clearance as you don't need to with modern forgies these days, otherwise you get an eng that wears piston/rings/bore quicker and more blowby.

Stock nissan rings seem to be the go as well as they suit the block and bed in better, self explanitory(metallurgy again).

Hope that helps you make your decision.

GTRNUR Grand Master

GTRNUR

Posted
Posted (edited)

Sorry to disagree but that doesnt sound right. 0.001- 0.0015" is the clearance for aluminium piston in an aluminium cylinder, like on a porsche 911. The idea of close tolerances in those engines is that the cylinder grows at the same rates as the piston as things heat up. Porsche dont use a cast iron sleave, they have a machinable coating on the alloy called nickasel.

Enough porsche stuff... On to cast iron blocks.

Cast iron doesnt expand as fast as the alloy does. This is why the wall thickness needs to be larger, or the engine will get too tight as it heats up.

Another factor of consideration is that the RB30 block doesnt have piston squirters, which means the pistons run considerably higher crown temps than a normal forced induction RB20/25/26det(t) engine. This again means more piston growth.

CP uses 2618 alloy for their forged pistons, so when they heat up they grow more than hypereutectic cast as well as they get hotter because of lower silicon content.

If it were a NA engine 0.0015" would be ok, but tight. For forced induction and no piston squirters im 99% sure its 0.0025" because of the application being for forced induction and using a forged piston. You could even go as loose as 0.003" but unless your circuit racing the engine and are after all out response its probably too loose and will cause a fair amount of noise when its warming up, as well as wear out the pistons prematurely.

Edited by Vspec R33
T04GTR Grand Master

T04GTR

Posted
Posted

just do what the cp book says that came with your pistons. same for the fileback rings.

p-b is around 2.5-4' ring gaps are top:18' second:20' oil:18'

they should all need to filed. generaly out of the box they vary from 9-15'

GTRsean Grand Master

GTRsean

Posted
Posted
Sorry to disagree but that doesnt sound right. 0.001- 0.0015" is the clearance for aluminium piston in an aluminium cylinder, like on a porsche 911. The idea of close tolerances in those engines is that the cylinder grows at the same rates as the piston as things heat up. Porsche dont use a cast iron sleave, they have a machinable coating on the alloy called nickasel.

Enough porsche stuff... On to cast iron blocks.

Cast iron doesnt expand as fast as the alloy does. This is why the wall thickness needs to be larger, or the engine will get too tight as it heats up.

Another factor of consideration is that the RB30 block doesnt have piston squirters, which means the pistons run considerably higher crown temps than a normal forced induction RB20/25/26det(t) engine. This again means more piston growth.

CP uses 2618 alloy for their forged pistons, so when they heat up they grow more than hypereutectic cast as well as they get hotter because of lower silicon content.

If it were a NA engine 0.0015" would be ok, but tight. For forced induction and no piston squirters im 99% sure its 0.0025" because of the application being for forced induction and using a forged piston. You could even go as loose as 0.003" but unless your circuit racing the engine and are after all out response its probably too loose and will cause a fair amount of noise when its warming up, as well as wear out the pistons prematurely.

Mates rb30 bottom end had oil squirters installed into block. Basically it's up to you with what clearance you want to run. Consider the loose end of factory clearances, due to modern forged pistons having better metallurgy and manufacture, eg HKS, TOMEI etc. Consider using factory rings for best seal.

Not to point too much out but this engine had put out over 600HP for the past five years or so with no issues(mainly used on street). If I was you you should consultwith an engine builder who has the runs on the board so to speak instead of heresay like this.

Cheers

proengines Rising Star

proengines

Posted
Posted

Run them with the nominal clearance built into them. That is, if they are .5mm pistons finish the block at 86.5mm. CP knows a lot more about pistons than anyone here or their mate and thats the way they make them, with the correct clearance built in at the nominal size. You'll probably find it's .003"ish. There's a lot more to it than just the material they are made from.

  • 2 weeks later...
r33_racer Grand Master

r33_racer

Posted
Posted (edited)

we ran the rec 3 thou for our last few motors with no dramas. They were all cp pistons.

Also, obviously your ring end gaps will vary depending on the engines power and application. For circuit we run 20thou top and so it goes.

Edited by r33_racer

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