Need help....i have a CDB i am puttin Mahle Forged Pistons in that has Oil Squitters(jets). One thing, if it is a closed deck block with oil jets, i will need to releive the piston skirts to clear them. it just means putting a small groove in the bottom to clear.

has anyone attempted this and can provide me with tips or a previous thread outlining how they've done it. does this affect the weight?

thanks in advance

Wayno

p.s. below is pics of what im supposed to do!

 

You need to do a "dry" (no sealant) build of the bottom end to check the clearance at BDC - no good assuming they need to be modified.
This also gives you the chance to measure the height of the pistons in relationship to the deck face so you can calculate the final Compression Ratio and head gasket thickness.


Mick

 

Hi Mick,

have you done something similiar yourself?

i understand it is best to do a dry fit without rings first to check clearence etc. There is no manual for this so i take it only common sense engineering.
The 'cut outs' im thinking of using are 6mm deep and 12mm wide but as I said before, a dry fit will indicate the position and point of contact between the piston skirt and oil jet. im thinking of using a ball nose rotary file for this.
Im concerned with the weight of the pistons after and of course possible spurs that could cause scoring or if i dont make the 'cut out' deep enuff or wide enuff it could cause the skirts to clip the oil jet and shatter.

so i wondering has anyone carried out this Modification successful? and if so any Do's & Dont's kinda advice.

I have the data for you regarding the cr.
one cylinder is 498.77cc
the head is 46cc
the 1.3mm gasket has 8.8cc
the piston has 12cc
The total clearence volume is 66.8cc
1 cylinder 498.77cc div by 66.8cc + 1 = 8.466:1cr
once again thanks again for any and all help

cheers

Wayno

 

You've not taken the "compression height" of the piston into account. This is 1.287"/32.7mm. This will put the piston circa .3mm down the bore @ TDC. This means your CR will be quite a bit lower.

The piston you're using is designed for the '01 > WRX, & STi, which is why you "may" have to modify the skirt, but I wouldn't cut anything unless you really have to.

As Merlin says, a simple dry build, and some Plasticine will tell you if it's required.


Mark.

 

I've had to do this on my recent build. I'm using 99.5mm (2.5L) RCMS/Omega pistons in a 2.0L CDB that's been sleeved and machined for 2.5 crank and pistons.

I did a dummy build and the engine would only rotate 180 degrees in either direction. It was the clearance on the squirters that was causing the problem.

The consenses of opinion and the prefered choice is to have the block machined so that the squirters sit lower in your block rather than taking material out of the pistons.

The other way (Which I have done) was to use a flapper wheel on a dremel and remove material that roughly equates to 5mm deep and 5mm wide. I spoke to Matt Clark from RCM about this and he advised me to do what I have done.

Of course you could take to an engineering shop who'll do the job far more precisely but will have the same outcome.

I would add that I then made sure all of the pistons weighed exactly the same. I got mine to within a 10th of a gram of each other.

I was talking to the guys from Cosworth about this at Silverstone and they are looking into changing the design of their piston because of this very problem.

The other option of course is to cap the oil squirter holes and run without the squirters.
I think those that have them though prefer to keep them.

Hope that helps

Daz

 

how much lower?

i was lookin for u Daz as i heard you did this but couldnt find the right Daz
u didnt by any chance video this or document it by pics?

will this cause any likely problems if i go 400 - 500 BHP?

thanks again lads


Wayno

 

Providing you're careful there's no reason why things shouldn't be ok. You're going the right way about it by cutting out a semi circular shape rather than a rectangular shape.
This means there are no right angles in your cut where cracks can start ( stress risers).

To be sure, using a bare piston (no rings) put it into the bore and gently place it at onto the oil squirter. Then have a look through the gudgeon pin access hole. If the pin hole in the piston sits higher than the hole in the block then you know its going to be a problem.

For a solid solution that you can forget about I would take the block to an engineering firm and get them to price up lowering the squirters into the block. That way you'll not be worrying about the integrity of the pistons.

I'm just going to have to wait and see if mine is OK. Several reliable sources have said it should be. Only time will tell.

 

does that intail them countersinking the original hole and retreading or extending the original thread or a bit of both?

 

I would imagine a bit of both.

 

rite then, let the games begin... gona do this later with a mate of mine...let ya know how we get on..

may aswell document it as doin a total rebuild from the crank up..
so might do a build project

thanks for the help

Wayno