Traditional piston ring sizing has been in fractional inch measurements. You&#;ll typically find top and 2nd rings in 5/64-inch, 1/16-inch, or .043-inch sizes, with oil rings typically in the 3/16-inch size. Modern engines moved to metric ring measurements of 1.5mm to 1.0mm for top and second rings with 3.0 to 2.0mm oil rings. These ring thicknesses have been the norm for decades, but moving to a thinner ring package has shown several advantages. With custom pistons, you&#;ll find types of piston rings as thin as .5mm (.020 inch). The thinner rings provide some great benefits, including increased horsepower and torque while reducing weight and compression height. Significant power gains can be had from utilizing thinner, modern rings and piston designs. While it has been more critical to use the proper piston ring installation pliers on thicker rings, we highly recommend that you use the same tool on thinner rings as well. The only types of piston rings that are OK to be &#;spiraled&#; onto the piston are the oil ring&#;s top and bottom rails. Never spiral the compression rings onto a piston.
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Teflon rings are probably the best choice for minimizing rust issues, they usually require a two or three peice piston design, since they will not expand and contract like an o-ring or metal ring for instalation. There are commecially made teflon rings made with the wiper shape, some with a spring metal part inside that make up of tolerances, creep and wear, these are wonderful seals for steam applications with. There some Teflon(TM) ring designs wich incorporate an o-ring also.
O-rings will work with the right bore and surface & gland tolerances and the correct surface finish, but they will have to be replaced perodically due to wear. Viton(TM) is proably the best choice for material. Silicone is not recomended for dynamic seals. If cylinders and or pistons are cast iron or steel then rust may be an issue, and could tear up o-rings very fast.
Bill Shields who contributes on here a lot has Teflon seals in Camel Back, I am sure he would be willing to share his piston & ring design.
All in all non-metalic rings work very well when tolerances and surface finishes are done right. They will not wear in like a cast iron ring will, all in all the old cast iron ring design is tolerant of larger surface finish and tolerance limits.
There is an Allen Mogal at our club with Teflon rings that has been running trouble free for several years. The owner just gives then a good squirt oil through the snifter valve ports running. I am not shure how the rings and pistons were configured.
Rob
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