Piston to cylinder wall clearance question

Hello, i have a set of 74mm stock bore CP pistons.

On the datasheet : cylinder to wall clearance: 0.0035(0.0889mm).

What does this number really mean? is it the minimum clearance? The maximum? An average?

The engine is a 4efte.

Thank you

This number means recommend piston to wall clearance which would ensure the pistion not getting stuck in the bore under the hight load when it is fully expended because of the heat. If you bore is 74mm the piston should be 0.0889mm smaller in diameter which is 73.9111 mm measured at proper point ( normally 0.5 inch above lowest piston skirt point) at 15-25 degrees C air temperature...

Hi, i understand that... but what if pistons sizes are not exactly 73.9911 ?

On the data sheet again piston size : 2,915 (73.990mm)

As long as the clearance is not less than 0.09 mm it is going to be OK. But if it is less some damage may occur...

The clearance specified is 0.0035" with the "0.0889 mm" being a lack of the normal rounding that would occur. For all intents and purposes, 0.09 mm is a perfectly acceptable working value*.

As Shota said, it's to allow for the differential expansion that occurs as an engine heats up and the aluminium piston expands in the bore. If you don't have at least the minimum you're at risk of seizing the engine, so you may add a tiny bit but NOT reduce it. If you're going to be using a LOT of boost, or other "power adders", you may need to use 0.0040" or even 0.0045" to allow for the greater heat expansion.

As a comparative figure, an average human hair is considered to be between 0.06 and 0.08mm in diameter, so a difference of 0.0011mm is very small, and not something even the best machine shops are unlikely to be able to measure without some very specialist tools (I've used some and they require very carefully controlled conditions!), as it's ~43.3 millionths of an inch...

Hi, ok thank you, i got it...So 0.0889mm is the minimum clearance and i could go to 0.1mm if a lot of boost.

A lot of boost or nitrous oxide injection or both - any high load on the piston will result in greater material expansion so bigger clearance is required.