At a contest earlier this spring, I had my ST .46,  on my P51-B,  stop suddenly in flight during Profile.  Later it just would not start. Looking back I'm glad I turned down the use of an electric starter. Finally, last week, I removed the back plate, head, etc.   It was then I found several small pieces of aluminum all bent up in the engine.   I removed the back plate on another ST .46 and there it was.  That too has been removed. So, if you have an ST .46, or even a .60 the older G21 or V series, you may want to investigate and see it the ring is still in place. 

I have heard or the ring, but never have I seen one. I removed the back plate on another ST .46 and there it was.  That too has been removed. So, if you have an ST .46, or even a .60 the older G21 or V series, you may want to investigate and see it the ring is still in place.
Can anyone explain why it was ever on there?  I noticed the balance (counter weight) had grooves in it, to hold oil?  A search came up with nothing. 

   It was there to "fill up space" that would otherwise have been open around the crankpin side. I had never seen one of these fail, but Eric Viglione pointed out a few examples. There were are least three types of crank, the "porkchop" (that has cutaways on either side of the crankpin), the "full circle" (that is the porkchop, with added "gussets", maybe to make the crank stronger {why, I have no idea, the crank itself was plenty strong enough}), and then the "full circle with stuffer ring" that ou have. The purpose was to "stuff" the crankcase and probably reduce the oil drag - again, for no good reason I could see, even for R/C.

    As you found, the stuffer ring occasionally comes off. I now recommend cutting it off before it falls off on its own, one of the few engine modifications I would make  a priori, just as a reliability change.

     The "porkchop" crank was reputed to be the one to have back in the day, but any difference it might have made was entirely and utterly swamped by the quality of the ring you wound up with. People have taken either of the full circle versions and ground off the gussets to make them porkchop style. I never saw that make any difference, but the people who did stuff like that were usually so far down the rabbit hole already that you really couldn't separate out the changes.

     Brett