TheMinel

iTrader: (2)

Hi guys,

this is aimed towards the engine builders who have Cosworth components experience.

My shop called me today as they were a bit baffled with the Cosworth pistons clearances. They say they've installed them before with a 0.05mm clearance as per spec, but now they say the piston specs seem to have changed to 0.1mm. So they checked with Cosworth and this is the reply from one of their Engineers.

"His block needs to be bored to 95.500mm as it says on the label the pistons are sent in. When up to working temperature the 4032 material pistons in a cast iron liner will be a perfect working fit in that bore.

These clearances may be twice as wide as some other manufacturers, who will almost definitely using a low expansion aluminium. This is common in automotive pistons, as tight clearances can be maintained, so reducing engine noise before it gets up to working temperature.

Boring the block out of spec to what Cosworth suggest may lead to piston seizure and failure when under load.

I have looked through are archive and checked the original piston calculations spreadsheet for this piston, and all information on the drawing is correct.

If your customer is used to different manufacturers pistons. they may find that the Cosworth piston is a little noisier on start up, but this is because of the larger clearances. However as mentioned, when this piston is up to working temperature, it will be a perfect working fit in that bore size and give excellent performance.

I hope this is of some help.

Regards,


Jonjo Hammond

Piston Design Engineer,
Cosworth Ltd”


So basically my shop was checking with me to see that I was happy to have a 0.1mm clearance as they say this could lead to noisier pistons and or excessive oil consumption.

I did provide the pistons in the first place (although I had a set of Arias ones before and the shop insisted to have Cosworth components...which I then provided), so I'm sort of stuck with them anyway, but what I wanted to ask to the engine builders on here (Sam, Sharif etc) is if 0.1mm has always been the clearance for the Cosworth pistons for a turbo application, or otherwise.


Thanks


Valerio

Zivman

iTrader: (19)

Link for help

TheMinel

iTrader: (2)

thanks for that, although it seems a bit weird that Cosworth pistons (believe them to be rebadged Carrillo) would fall into the category of "weaker alloy" but more "expandable". I seem to understand the Cosworth ones use the more expandable Alloy 4032, hence wouldn't they need MORE cylinder to wall clearance to allow for expansion?

So has any shop bored with tighter clearance than what was advised by the manufacturer for the Cosworth pistons?

Zivman

iTrader: (19)

Maybe I am getting confused by the units here.

you are talking .1mm of clearance. This would be .00393" or roughly .004". That is what I ran with my 4032 arias pistons. I think somewhere around .004" to .005" would be the optimal range (.1 to .13mm).

FWIW, On my new build, 2618 pistons, I am running .0045"

edit:
in short, .1mm would seem to be fine

TheMinel

iTrader: (2)

essentially my shop thought they could run the Cosworth pistons with tighter clearances than what Cosworth is saying in their spec sheet and confirmed by their engineer