Weird - Oil ring pops out of groove while running on dyno

[davepl]

A friend of mine ran into a weird problem and I have no good ideas, so thought I’d run it by the board to see if anyone's ever heard of anything similar.

After a rebuild they were running it on the engine dyno and it started smoking badly; it turns out the oil ring had come out of the groove on one piston. Thinking he must have done something wrong during assembly, he ordered new rings to be sure, tore it down, reassembled, and guess what – the exact same thing happened again. In the same cylinder, but with the new rings.

The wrist pin is below the ring pack on this motor, so there’s no support rail and the pin couldn’t hit it from the back (that was my only guess).

The second time he assembled it he put the gap on the thrust side, which he doesn’t like to do, to make sure it would be “held” in place, but it still popped out. Fortunately didn’t damage the cylinder wall either time. I don't know much else about the engine (Big Block Chev or Chrysler RB).

Ever heard of anything like that, or have any random guesses? He's a machinist and a retired Winston Cup engine builder, so there's not a lot he hasn't seen before, but this was new to all of us!