Rb25 Bottom End Build

[boostn0199]

Im thinking about starting my first bottom end build on the side to have it ready for when the time comes. It is used for a drift car with 300 to 320 kw on e85.

Im thinking of going 20 though cp pistons and going to aim for 2 thou main bearing clearance. Just have a few questions should i just go with a set of acl std race bearings and get the block align honed, crank polished and balanced. It is a 2 nd hand crank by the way. What is an accetable variance in clearances?

[GTScotT]

If you want the motor to last it would be best to get it tunnel bored and the bearings done to match individual clearances (blue printing).

Whacking in 'standard size' bearings has only ever been a get out option for me, never used in an application other than reviving a dead motor.

[boostn0199]

So getting a tunnel bore along with a set of oversized bearing which the machinist will indiually set for a specfic number would be the best.

[R31Nismoid]

You only get pistons as big as you need too. If you only need 86mm, get 86mm for instance. Don't just mince out to 87mm because

It will depend on the block, how much needs to be taken out etc.

[Gerg_R31]

Main clearance 0.0004"-0.0029"

Does the crank need a grind or a linish? has it been ground before?

Unless your going main studs a line hone isn't needed often

[Gerg_R31]

A good machinist can machine anything and be consistent with the sizes, blue printing is making every cylinder identical but with parts these days having better technology for better consistent quality, you find blue printing bearings and pistons don't need to be done these days

and a tunnel bore is machining the main cap parting faces then boring back to factory size. over size bearings are for crank grinds

[boostn0199]

I see, well whether or not the crank will need a grind or linish i wont know until i get it as im researching now at this stage.

So firstly ill have to check the crank to see if in spec or not and whether it will need grinding then get a set a set of acl std bearings if crank is fine and give the machinist the clearances i want so he can do what machning is required.

Ive read that just using a plsstigauge for checking clearances isnt enough and should use a mic aswell is this true?

[James_03]

I have found plastigauge to be fairly good in the past. Also don't go out and purchase bearings until you know the condition of the crank, no point purchasing standard bearings then finding out the crank needs to be ground.

If you want to run with standard shells, then find a low k's motor is probably what you want to be doing. The higher the k's, the more chance the tolerances will be out.

Cheers.

[boostn0199]

Thanks for the info everyone will consider it all. Can anyone recomend any good machining shops in melb?

And if anyone has any other advice or tips pleaae input.

[boostn0199]

In regards to the ring gap clearance for the cp pistons, having a look at there guide here http://www.cp-carrillo.com/LinkClick.aspx?fileticket=LjC4jLmI4Ls%3d&tabid=74

Ive worked out for a 86.5mm bore that they should be .019" for top ring and for the second ring .023" and .015" for the oil rings that was based on the Nitrous/turbo supercharged application. Are people following these guidelines when doing it or are they using there own different gaps? Also from what Ive read so far the piston to bore clearance for cp pistons is 3.5thou but I havent found a document on CP's website where it talks about the it any thoughts?