In my defense, I should say that my experience and recommendations are for small, high performance two stroke racing engines, both spark and glow ignition. If you want to run an 8 hp., 26 cc engine up to 25,000 rpm, there is no substitute for an aluminum piston. If you are building low rpm, low output engines where service life isn't an issue, a large variety of materials will work. Cast iron is the old time choice both for ringed and ringless piston and liners.
Fox has used this system on some of their engines nearly as long as I have been alive. O-rings can't possibly work as a piston seal in any moderate to high performance internal combustion engine. All the engines I have worked with use commercial heat resistant, steel or stainless steel rings. A thin, single ring is standard. A friend has experimented with cast iron rings in these engines, but he hasn't found power gains as far as I know. I would seriously consider buying commercial rings.
OS makes a lot of different size engines and Tower Hobbies stocks the
27mm rings. You might even consider more stock parts.
When we started working with a commercial engine builder (
Quickdraw) their soft steel liners had a very short life. They switched to nitrided liners. They could add as much as 1/4 hp with the right hone pattern. A chromed liner would be even better, but that costs more. All the other engines we work with have either aluminum or brass liners with chrome or similar hard coatings. Even with their higher rate of expansion, it is common to increase the clearance .002" or more in the ring area of water cooled engines to prevent seizing.
Out of round cylinders also can seize pistons like the picture below. The builder, CMB, tried the same aluminum piston in a chromed aluminum liner they used in their glow ignition engines, but it never worked. We destroyed a lot of piston and liner sets like the one in the second picture below. More about our development of that 35 cc engine can be found
here.
I use a Sunnen bore gauge set against a Mitutoyo digital micrometer to check bores. Again, none of this is necessary for low performance engines, but they will still run better with round cylinders. The above posts are great descriptions of measuring and lapping methods amateur builders have used for a long time. You can get great results by hand fitting pistons to liners without knowing the exact dimensions of either.
Lohring Miller
