Piston Rings Part 2.
The Trimble articles describe the construction of a special purpose fixture required to support the rings during their heat treatment. Equations were provided for the dimensions of a 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, and this one was my fourth.
In the past, I've enclosed the fixture'd assembly in an argon-filled stainless steel foil bag order to protect the rings from scale deposits during heat treatment. After a number of builds I realized that scale build-up really wasn't a problem in the controlled temperature environment of an oven, but mysterious deposits continued to show up on the rings even when they were bagged. Typically, minor burnishing with a white Scotch Brite pad was enough to remove them while still supported on the mandrel.
Initially I suspected the deposits were lead coming out of the 12L14 that I typically used to machine the mandrel, but switching to 1020 didn't eliminate them. I didn't bag the Ford rings, but I machined the fixture from 303 stainless and thoroughly cleaned it and the rings with acetone before heating.
The fixture'd rings were heat soaked at 975F for three hours and allowed to cool overnight. Those pesky black deposits showed up again but only on the rings themselves and the spreader dowel which was a length of drill rod. The stainless steel mandrel, bolt, and nuts remained clean and shiny. This makes me wonder if those deposits are actually carbon coming out of solution from the cast iron and, this time, also out of the high carbon alloy of the drill rod.
Although they were easily burnished from the ring surfaces while still on the mandrel, this time the rings were stuck fast together. This wasn't at all unusual, but in the past the rings were easily separated with a sharp Xacto knife (starting from the ring gap). I didn't account for the rings' smaller areas when I tightened the mandrel bolt to clamp everything together, and this time probably got it too tight. Eventually, I got them apart, but I had to reheat the stack (sans mandrel) to 400F to do it.
After separation, their faces were lapped one last time on a glass plate using 1000 grit grease. After a thorough cleaning in solvent, their outside surfaces were individually burnished with a Q-tip in a lame attempt to polish them free of any remaining deposits.
The final step was to sort the rings by light testing them. A simple fixture was turned from black Delrin to adapt a 200 lumen flashlight to the bottom end of the spare liner inside which each ring was checked. Admittedly, my grading/acceptance criteria is pretty subjective, but it's kept particularly poor rings out of my engines.
The tests were done in a totally dark room where, ideally, light should only be seen escaping through the ring gap. Such rings received an "A" grade and will become the top compression ring in each cylinder. With all the machining and measuring uncertainties that can creep into the numerous process steps while trying to work within tenths, A's can be tough to get. Out of 23 rings, I wound up with only 6.
Rings that showed faint wisps of light leakage between the ring and cylinder wall were graded "B". My guesstimate is that the imperfections in these rings were due to circularity errors on the order of a few tenths and might even be a result of remnant deposits. These rings will likely bed into their cylinders during the first few several seconds of running (or maybe even during starting). I wound up with 8 of these, and they're destined for the pistons' second groove.
The "C" rings were saved as spares. Relative to the B rings, their leakages were noticeably brighter and were perhaps due to circularity errors that slipped through my process. More likely, however, they're a result of mishandling after being sliced from the blank. I wound up with 9 of these.
I should add that none of these C rings showed enough leakage through the ring'd test cylinder to be seen in daylight only. Whether or not they would have been 'good enough" for use is anyone's guess, but their grade will keep them out of the engine unless there's an installation catastrophe with the A's and B's.
Since I had more than enough A and B rings to cover my needs (and since I really don't enjoy making rings), the number one blank wasn't processed any further. The finished rings will be installed when the head is finally assembled to the block.
I have to say that even though the quality of the starting blanks was on par with what I've come to expect during my other builds, the quality of the finished rings was disappointing. Out of 23 completed rings, I expected to have some 15 A's and maybe 1 or 2 C's. My relatively poor yield may have been related to the tiny dimensions of these rather fragile rings. Compared with the hundreds of 1"+ rings I've made, these smaller more flexible rings were difficult to work with and seemingly more susceptible to handling damage especially while removing those #@% deposits. - Terry
