Possibility for ringless piston

[Kartagener]

Hi folks,

I am considering building my first model engine and would like to start slow. My current plan is to build a simple pneumatic model engine with a slide valve. Since I won't be running real steam, thermal expansion wouldn't be an issue. So is it feasible for a model to run without piston ring? (current plan is 1.2" or 3 cm diameter piston) I know sterling engines have fairly large gaps between piston and bore, but not sure if that could translate well here.

Any input appreciated! 

[Jasonb]

No problem running on air like that, I leave the ring off most of my engines as I mostly just run on air for display.

The power piston is a close fit on sterling engines, the displacer piston has a gap.

[Kartagener]

No problem running on air like that, I leave the ring off most of my engines as I mostly just run on air for display.

The power piston is a close fit on sterling engines, the displacer piston has a gap.

Thanks a lot! For my purpose would you say that 5 thou or 0.13 mm difference between bore & piston diameter would be a good enough fit?

[Jasonb]

No you really want a lot closer than that, even with rings that is a lot more than I would allow. something like 0.001" or 0.02mm on that sort of diameter if you want to put  a number on it but I just do it by feel. You also want a good finish on the bore (this one was not the best)

Have a look at this video of the fit for the engineI just completed. The end of the cylinder is just being held on a flat surface, with the steam/air port uncovered the piston drops straight to the bottom. If I put my finger over the port it takes a long while to drop. No Piston ring and no oil to seal anything.

<a href="https://www.youtube.com/watch?v=Vp2Vrq1Ziw4" target="_blank">http://www.youtube.com/watch?v=Vp2Vrq1Ziw4</a>

<a href="https://www.youtube.com/watch?v=jIhDdFKNSzA" target="_blank">http://www.youtube.com/watch?v=jIhDdFKNSzA</a>

[Kartagener]

No you really want a lot closer than that, even with rings that is a lot more than I would allow. something like 0.001" or 0.02mm on that sort of diameter if you want to put  a number on it but I just do it by feel. You also want a good finish on the bore (this one was not the best)

Have a look at this video of the fit for the engineI just completed. The end of the cylinder is just being held on a flat surface, with the steam/air port uncovered the piston drops straight to the bottom. If I put my finger over the port it takes a long while to drop. No Piston ring and no oil to seal anything.

<a href="https://www.youtube.com/watch?v=Vp2Vrq1Ziw4" target="_blank">http://www.youtube.com/watch?v=Vp2Vrq1Ziw4</a>

<a href="https://www.youtube.com/watch?v=jIhDdFKNSzA" target="_blank">http://www.youtube.com/watch?v=jIhDdFKNSzA</a>

Thanks! What a beautiful little engine!