Soft, wear quickly, and have generally poor compression. I used them at the time, but they didn't last very long. I don't know if the same theory applies to ST51 or 60 rings.
The test for a good ring is to check the compression forward and backward. It should have a lot of compression in both directions. If it has a lot one way, and very little the other (usually has good compression backwards because the ring gap tends to move to the intake side for some unknown reasons) that means it is probably not good. The bad rings will run smoothly but use a lot of fuel and have minimal "power" which can go down noticeably in as little at 10-20 flights.
A good ring also turns brown after a few break-in runs. The bad ones tend to get gray/silver and shiny.
If it has poor compression in both directions, and it has been run on straight castor, or has been sitting for a long time, the problem is probably that the ring is glued into the groove by congealed castor oil or varnish. The solution is to switch to fuel with no more than 50% castor, put a 10-4 or even a 9-4 on it, and run it on the ground, at full RPM (peaked out lean) for a 10-15 minutes and that will loosen it up. If that doesn't work, the ring is probably just worn out, get a new one.
If I was still trying to run ST46s competitively, I would look for the Tony Cincotta ceramic-coated liner. Failing that, get out the brake cylinder hone, rough up the liner, and get an aftermarket ring from Frank Bowman.
Note that almost nothing new has been learned on this topic since about 1989 when David came in 6th at the TT with his ST46 "Central Gravity" running 15% Cool Power. That was pretty much the last gasp of the 4-2 break era. Then, as now, most of the "important tuning information" was a steaming load. What you want is a dead-stock engine, a selection of venturis from stock to about .181, and a good ring. And maybe some YS-20/20.
Everything I know about the topic (almost all gleaned from others who figured it out - I just absorbed it) is in some post here or SSW, just search. I may waver a bit on the actual part numbers, despite my generally good memory, the details are fading.
Brett
Brett That is your opinion, and your welcome to it, but from my finding, it is partly wrong, the engines ST 46 51 60 are MUCH better prepared properly as I did to many many 100s of them, first they all benefit from better bearing and sealed front ones, the ST 46 had some versions that had a aluminum band around the crank, this some times came loose in the engine and ruined things, also these particular cranks were not balanced well, and they were greatly improved by removing the band, and cutting them back to the older "T" shape, which was a better, smoother running crank, the true venturies I made for them was far better than the multi hole sprinkler, that had the really irritating way of filling with air bubbles at the end of the flight all around the "ditch" cut 360 degrees around the venturie to feed the fuel, the results was that it burped on and off for several laps before most of them would shutoff, Scott Bair used to run the stock venturies, that he "fixed" by inserting a 3/4 moon shaped blocking device and only allowing fuel to enter thru 1 or 2 holes, I just made new ones. Another was, with the limited range of glow plugs heat ranges, the heat dam cut around the cylinder heat, at the point the glow plugs installed, helped in keeping the motor running smoother . With respects to the rings, after setting up 100s of new ones and rebuilding many 100s of ST 46s, I found that most of the replacement rings, were machined to zero gap, many people would just stick these in and use them, they would heat up and grind the ring out of round, or flat on the side that the gaps were touching, this NEVER sealed right and always had inferior compression.. others called this "glazing" the ring. to set these up to work correctly, you absolutly had to set a proper ring end gap, and on the used ones, hone a crosshatch into the sleeve, then of course make sure the sleeve was still round and in good shape. The end gap also greatly depended on what sleeve you used also, there were thin ones, thick ones, chromed ones, standard ones, the chrome ones , normally used a larger end gap, whereas you could hone the steel ones, and setup the gap very tight, they would wear the ring in quickly, and open up to a proper end gap. The chrome ones were much much harder to hone, and many did not bother. Also the chrome ones ran hotter. Then you had many people who used older sleeves that were "mushroomed" out at the top, these never were ones that sealed well.
There are a lot of other things that will make the ST 46 51 60 better engines than stock, but I have covered enough to make the point.
Regards
Randy
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