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wall clearance and ring gap

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Hi Andre

I really want to know how can I calculate the piston to wall clearance because I plan to be running 800hp 4G63 evo9 engine .

I'd like hear some input to wall clearance and ring gap. I plan to running occasionally be hitting around 40psi boost pressure at

the track consistently high 26psi on the street with E85.

what material are your pistons.

generally the piston to wall clearance is about .08mm.

this is dependant on alloy and silicon content of pistons.

For most engines I have built the piston manufacturer provides specs like this for their pistons so the obvious question is what does the manufacturer of your pistons say. Same for piston rings but I dare say when running 40psi things may change a fair bit.

This is a tricky question and unfortunately I'm not going to be able to give you an absolute answer. The correct wall clearance depends on two parameters - The material the piston is manufactured from, and how much power (or more importantly heat) the piston is going to be subjected to.

For example most high performance forged pistons are manufactured from a 2618 alloy which has a higher rate of thermal expansion than the likes of a 4032 alloy or an OE cast piston and hence requires more bore clearance. What you can get away with for a drag application running on E85 will be different to what you would use on a circuit application running pump gas.

I can only recommend basing your piston to bore clearance on the manufacturer's recommendations for your specific application. The problem is that in my experience these recommendations tend to be on the larger side and while this is safe, it does tend to result in more blow by and it doesn't stabilise the ring pack as well due to the ability for the piston to rock in the bore. Basically the piston manufacturer never wants a piston to nip up in a bore due to their recommendations.

In my drag engines I always ended up a little tighter than recommended by about 0.005" but I had the advantage of building many engines and analysing the wear on the piston skirts before making decisions on what to do with the clearance.

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