Rusty Hopper's Modeler's Corner

O-ring Ramblings

| February/March 2004

Hello again everyone. The other day a friend of mine asked me about using an o-ring instead of piston rings in a scale engine. I love the idea, and I use o-rings in most of the models I build.

Further, sometimes I install a cast ring in the ring groove below the o-ring. After the engine is broken in I'll pull the o-ring and install another cast ring, which gives it great compression.

The o-ring method will give you some piston slap as you are running only one 'ring,' and it is at the top of the piston. In my experience this hasn't caused any problems.

If you want to run an o-ring, here is the way I do it. First, fit the piston close to your bore size -just so it passes smoothly through the bore. Next, cut the groove you normally would for a regular cast ring. I have found that each o-ring needs a custom fit as they come in many different sizes, both in diameter and thickness. With the o-ring installed, you want the piston to fit snug in the bore since the o-ring will expand some.

Now, I know some of you are thinking an o-ring won't last for long, and you're right. The engines I have built (and those of some friends') with o-rings will generally run for about 100 hours before the o-ring fails. If you think about it, that's some serious running time over the course of a season.

One benefit of using an o-ring is you don't need a great hone job to seat the rings, as a smooth bore works best with an o-ring. Have you ever looked at the inside of a hydraulic ram? They have almost a polished surface.