quick question.....

Is it the E clip style?
and if so, are you locating the little tang vertically (which it should be) or horizontally (not really the way it is supposed to be)?

I had the same problem on my Picco 21's when I was running them until someone told me about which way to locate the little tangs on the E clips.

Carl

You might want to check the depth of the ring groove.  A typical setup might have the end of the wrist pin hit the retaining ring about 1/3-1/2 up the retaining ring.  If the wrist pin hits too low it can "dig" the ring out of the groove.  If it hits too high it can bind and lock in place.

I've always liked engines that did not have a through hole in the piston.  They had a ring of solid piston material left so only one wring is needed.