Simple Hydraulics & Pressure trouble

[3trQN]

Im having a spot of trouble with some pretty elementary problem.

A mass sits inside a tube which is pointed vertically, the mass is the same shape as the pipe ( i.e. it behaves like a piston ), but the piston diameter is less than that of the pipe.

In order to lift the piston, what pressure must be applied to the fluid?

http://img235.imageshack.us/img235/4540/piston1lh2.th.jpg

The top of the chamber is eventually open to the atmosphere, and is filled with air at STP.

My problem seems to be the lack of a seal about the piston, i can see that it would be simple to calculate if it was simplified to a sealed piston, but with a piston that's smaller than the bore of the chamber its harder.

 

[Danger]

And that, essentially, is why engines have piston rings to seal that gap.

 

[Cyrus]

Yep, and even with piston rings you still get blow back.

 

[Danger]

Yeah. How irritating. That 650 hp 440 in my Roadrunner has a .008 ring gap (race-only TRW pistons), and it still wastes tons of potential through that.