I use George Trimble's method to make my piston rings. The only change I've made to his process is to use a normalization temperature of 975F rather than his originally published 1475F. In addition, I use a 200 lumen flashlight to check the fit of each finished ring inside a cylinder before it's installed on a piston. A ring that has any light leaking between it and its cylinder wall, other than through its running gap, is discarded.
My yields are typically limited by circularity issues that often show up during or after the blank's final machining. I use class 40 gray cast iron from a number of sources that's been laying around in my shop for years. The circularity errors have been as high as eight tenths over portions of the finished blanks. Typically, less than half of any blank passes my acceptance criteria of two tenths, and occasionally an entire blank is scrapped. Once a ring has been removed from a blank, it's nearly impossible to evaluate until it's fully finished and light tested. After slicing candidate rings from the well-behaved portions of the blanks, my yields are typically 80%-90%.
When starting a large batch of rings, I usually prepare several blanks from different sources. Even though only eight rings plus a few spares were needed for the Offy, I prepared three 3" long blanks. The Offy's rings require a finished diameter of 1.002", and so I started with one inch diameter raw material which actually measured 1.080".
All three blanks were turned down to 1.063" so they could be gripped in a standard collet during their final machining. The blanks were then drilled/bored to the rings' final i.d. for a depth of two inches leaving an inch spigot at one end for the collet during final machining. In order to relieve some internal casting stresses, the pre-finished blanks in my last few builds next have next received a 700F heat soak. This heat soak seems to have reduced issues with circularity errors which sometimes show up even days after the blanks' final machining. Rather than my usual five hour soak, an oven programming error soaked these three blanks for some 12 hours.
A few days later, the o.d. of the first blank was finished and polished to its final diameter with 600 grit paper. Quadrature measurements of the blank's diameter recorded continually along every half inch of its length during machining showed (surprisingly) negligible errors over the blank's two inch length. Using a .019" parting tool, I was able to get 25 candidate rings from that single blank, and so I didn't finish the remaining two blanks.
The inside corners of the parted rings were broken using a 1/4" diameter ceramic file. Using 600 grit grinding grease on a glass plate and a simple holding fixture, both flat sides were lapped to obtain a .001" clearance in the piston grooves. During combustion, the rings will seal against the lower walls of the pistons' groove to provide an important component of the combustion chamber's overall seal.
The Trimble article recommends a straight radial break in each ring for proper contact with the spreader dowel in the normalization fixture. Although 'good enough' results might be obtained by simply snapping the rings, I constructed a cleaver several years ago. After lapping and just before heating, each ring was cleaved and the running gap set to .004" with a diamond file. Each gap was verified inside a cylinder using a feeler gage.
The Trimble articles describe the construction of the fixture required to support the rings during their heat treatment. Equations were provided for the dimensions of the mandrel and spreader dowel which are its key components. This fixture isn't difficult to make, but its dimensions are specific to a particular ring diameter. I finally got to reuse one of my earlier made fixtures.
In the past I've sealed the fixture'd rings in an argon-filled stainless bag for protection during heat treatment, but the contents invariably wound up covered in a mysterious brown deposit. Although it wasn't difficult to remove, it was an annoying extra step that I began suspecting is related to the sulphur and lead that are alloyed into the free-machining steels used for the fixtures. This time I didn't cover the fixture, and only minor burnishing with a white Scotch Brite pad cleaned the o.d.'s up nicely.
My light tests yielded a dozen 'perfect' rings and only five that I labeled as rejects. The remainder were a bit off from being perfect, but in all likelihood would be 'good enough'. - Terry
