What do you do get a good ring seal?
Pray for a miracle? I got the two really good rings I ever had (out of dozens) entirely by having them supplied in the correct-era engine from the factory.
The issue appears to be the material used, and it seemed to vary drastically from batch to batch, both the factory and various aftermarket suppliers. Both of my good engines were "red box" engines, supposedly from the 60's or early 70's (according to Lew McFarland, who sold them to me) and had excellent compression even before break-in. Having wised up at that point, I never took these apart to try to check anything, I just ran them. The one I still have left still has great compression in both directions.
Most of the replacement rings were stock ST replacement parts. All the ones I ended up running fit properly to my ability to determine - they were round and fit the cylinder and had appropriate end gaps. I discarded a large number of them that did not fit properly. You push them into the liner with the bottom of the piston (so you know they are approximately square to the bore), then shine a bright flashlight around and see if you can see any light coming through between the ring and cylinder. MANY of them will pass light, toss those or grade them for later use when the "good" ones fail.
The older replacement rings were better than those sold in the late 70's to the end in about 1984. The good ones had the part number stamped on the package that was truncated "0075" as I recall. The later ones had the entire about 8-digit part number ("97120075" or something like that, Randy probably remembers or has one to look at).
The best replacement rings had end gaps around .002 with a feeler gauge, but I had one "0075" ring" that had an end gap of around 0.005 that was nonetheless one of the better examples and lasted much longer than the newer examples. You can try smaller or zero, but it will run funny for a while, start running OK and you can then see that the gap has worn itself to around .002 or so.
A visual test for a good ring, unfortunately only good when you have run it for a while, is that when you look in the exhaust, the ring will appear brownish compared to the aluminum piston. The bad ones will be bright silver. The feel test for good rings is whether it has the same (good) compression when flipping forwards and backwards. A bad one will have a big difference between forwards and backwards. When you first install it, that might be more forward than backward, or more backward than forward. But for whatever reason, when you run it, it tends to work itself to having less forward than backward, because, again, for whatever reason, the gap works itself to the intake side and that gives it more compression backwards than forwards. I am not sure which way you really want it, because what you really want is for it to be the same both ways.
The same test seemed to be true for aftermarket rings, the good ones were brown and the bad ones were silver at least for some of them. Ted told me the same thing about his Vic Garner Dykes ring, he had a great one, eventually he had Vic make a replacement, and it was silver after a bit of running and didn't run correctly.
There are a bunch of other crazy tricks. The most infamous was the brake cylinder hone to rough up the liner - you don't try to take off any metal, you do it just enough to remove any shiny spots. This was inspired by Scott Bair's observation of shiny spots and rings in his custom ST60 and how the ring was probably hydroplaning. You run the hone just enough at slow speed or by hand to get a cross-hatch everywhere, DON'T try to move any significant material. And for GOD'S SAKE, only do it to a ringed engine that shows odd running characteristics in-flight. NOT, repeat NOT, any modern engine with AAC/ABC/ABN systems. At best it accomplishes nothing (because the hone doesn't have much effect on chrome) and at worst it immediately ruins the entire thing and it must be replaced.
Note also that all of these tricks were rearranging deck chairs on the Titanic, you could get them to run smoothly and sort of reliably, but were down on power. It wasn't too difficult to get them to run reliably with mediocre or poor power for a while. It was difficult to the point of magic to get them to run reliably with strong power for any length of time, like more than an afternoon. A good factory ring had good power (in relative terms) and was smooth and reliable at the same time. As an example, when I was flying a lot, I would sometimes replace rings every few *weeks* after maybe 100 flights, starting out with mediocre power and ending up with less power than a McCoy 40 by the end.
Note also that even the best examples (like the one sitting on the shelf in my closet since 1988) are no match at all for even the lowly 46LA, when both are optimized.
The best solution to the "weak but smooth" problem, aside from getting a good ring, was to make the airplane smaller. When it started, the theory was (based on scaling with the prop diameter) was that an ST46 airplane should be 720 square inches. That might be swell for 1976 with a perfect factory ring, but for the "weak but smooth" versions, it will barely get to the top of the circle. People started working the size down, the best ST46 airplane was also one of the last, Ted's "Temptation" at 610 square inches with a relatively thick wing. And they needed to be very light. All of my ST46 airplanes were below 52 ounces and most of them were in the mid-upper 40's. This is where people's weight delusions started, because it really did matter. Of course, mostly we imagined it was wing loading so there were rules like "12 oz/square foot MAX!", which if you make the airplane smaller, does make it lighter and also solved the power loading issue that probably did matter.
A far better solution was evident as early as the late 70's - don't run ST46s. Various RC schnuerle engines were very obviously superior in terms of performance, when they ran correctly. Unfortunately, they didn't run correctly very often, so eventually some people went to the ST60, which had more power than a ST46, less power than a 40FSR, but ran a lot better. Later still, a few people tried 4-strokes (long before Windy declared them a "revolution" 10-15 years later - right after he sold his last ST60, presumably), some of which (like the Enya 46) also had more power than an ST46 but still were too unpredictable in some respects. The actual solution was discovered in 1986 or so, the tuned pipe.
Brett
Topic: OS Max LA .46 vs Super Tigre .46 (Read 5756 times)
