Q: I am working on a school project and need help picking the right size piston.

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I am working on a school project and have a black metal pipe with a 2.5" or 63.5 mm diameter. Now, the pipe will be standing up vertically will have water inside of it and I am trying to use a piston to push that water up from the bottom of the pipe. To do this, I have a bottle jack attached to the piston, which will essentially push it and hopefully the water all the way up. However, when i fitted the piston into the pipe and put water in from the top end, it all leaked out from the bottom. I feel like I may have ordered the wrong sized piston. I ordered a 63 mm piston with 0.25 bore dome rings. Do you have any suggestions on how I can fix this problem? Do I need a different piston?

The clearance you have between the piston and the cylinder pipe wall right now is about .020 inches (twenty thousandths of an inch). Typically, the clearance between a piston and the cylinder wall in an automotive engine is between .0015 inches and about .005 inches, depending on whether the piston is cast or forged and also depending on the quality of manufacture of the engine. To be "leakproof", the engine cylinder has to be perfectly round, typically within .0004 inches (four ten thousandths of an inch). Further complicating matters, for new rings, there really isn’t an optimal seal created until the engine has been broken in. At any rate, the interior surface of the pipe is probably not a perfect circle so even if you reduce the clearance to .0015 inches, the rings will not seal. What I would suggest doing is get a discarded lawnmower engine and use the piston and cylinder from that as your model. The piston to cylinder fit will probably be good enough to hold water. Of course, the depth of the cylinder is not very great but you could attach a pipe of ANY length to the top the cylinder and make THAT connection leakproof. If you have further questions or concerns, do not hesitate to re-contact YourMechanic as we are always here to help you.

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