My STi was rebuilt with Cosworth pistons after the big end went last year.
it now sounds horrible when cold (very slappy) and seems to burn a lot of oil (although I haven't measured how much- will do so over the next month or two)
anyone know how much oil the standard car burns, or got any experience with the Cosworth pistons?
as far as I can see it's not smoking under power or on the overrun.

cheers all

Nick

 

Nick,

All the engines I've heard, using Cossie pistons are a bit slappy when cold. Seems to be inherent.

The level of "slap", depends on what bore clearances were used. This would also effect oil consumption. Also ring gaps, valve stem seals, etc'.

Mark

 

There was a revision on the Cosworth pistons, some time ago.

If they have used the competition spec ones, it will have more enormous amounts of piston slap.

... and will burn a lot of oil

 

MY UK94 had Cosworth pistons fitted last summer. It does sound a little "slappy" when cold, but can't hear anything abnormal when warmed up.

It uses a bit of oil, but nothing what I'd call excessive. Not enough to worry me anyway.

Doesn't use enough oil to smoke at any point.

Cheers

Mark

 

so why are they rebuilt with so much piston slap. surely if a standard turbo owner kicks off at a dealer because it's there when cold but not hot why doesnt the owner of a modded sti or something similar creating more power and torque do the same . i can't see no difference yet the latter owner lets it go because it's supposed to have it. if i had an engine rebuilt and it come back with piston slap i'd be furious.

 

Depends what the clearances are - mine are 3thou and I use around .35 litre per thousand miles - more when on track etc.

If you have proper forged pistons this is an unavoidable byproduct.

 

I partially agree with Craig...

If you are using forged pistons, you need to allow for quite a bit of expansion. If you are using big boost and very high cylinder temps, as a race/rally engine would, then you will need 3 thou.

A road going engine would be approx 2 thou and Cosworth do make closer tolerance pistons for less extreme applications.

 

The standard clearances for Subaru pistons is between 0.0004~0.0012". The limit is 0.0024", and if you had that, you'd be looking for a rebuild.

The Cossie pistons seem to have particuarly high expansion characteristics, unlike other brands of uprated forged pistons. Most rebuilders use the Cossie pistons, because they are "off the shelf", whilst other brands may take a few weeks. Most people aren't prepared to wait.

Cossie pistons are very good, just a bit noisy.

Pete's right, uprated pistons, always run bigger clearances, along with bigger ring gaps. These will inevitably lead to higher oil consumption.

Mark.

 

bit more info - when the car was first rebuilt it had the first release of Cosworth pistons - very noisy when cold.
RCMS then rebuilt it again with a replacement set (apparently the first were out of tolerance).

Incidentally I used to do analysis on pistons when I worked at Cosworth (few years ago).
I assume the forged material has a higher coefficient of thermal expansion.
Either that or it's just a design thing.

 

Interesting....

There were issue's with the "issue 1" pistons, but one of the problems was the clearance Cosworth were recommending (4thou).

Within reason, tollerance shouldn't come into it. You buy a set of pistons based on the condition of the bores (ie' standard, +25, or +50) then you number the pistons 1~4, and measure each one. The you either hone, or rebore, and hone each bore to match the piston.

Alternatively, you're unlucky, and the engine rebuilder is a muppet, and just figures everything will fit ok !!!!!!

It's a pretty easy job to put all the bits together, the time is in checking all the bits have the correct tollerances, and then correcting them !!! Like gapping 16 piston rings, finding the rods, crank, and barings are 1 thou' out of spec'......

There are very few "good" engine rebuilders, and the few that are, I couldn't afford, so in the end, I decided to do it myself Very glad I did now

Mark.

 

Mine is the same...need an answer also.

Just need some advise here. Cant get more than 180kw@tw and is using 500ml of oil a week.

Any ideas would help

Have rebuilt my RS Liberty engine with

Australian 1991 imported model RS 2.0L
Rebuilt engine with 19lb boost
Forged CP pistons with Eagle rods
WRX 97 heads
Bosh coil set
Lightened pulley and flywheel
VF34 IHI turbo..
Walbro 500 hp fuel pump
Front mount intercooler/ black
Reco gearbox
Custom Clutch 2800 lb clamp
3" Exhaust
GFB blow off valve
K&N with ram filter
Unichip

 

any oil in the throttle body.. any visible oil leaks??

 

No oil leaks, but if the car is parked for a few days then you start it you get a cloud of blue smoke...

 

"Like gapping 16 piston rings"

Thats rather a lot of rings... almost as many rings as there are valve followers to shim!