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A rule of thumb is .002" to .003" of ring gap for each 1" of bore diameter. Thus, a ring for a 3" bore would have a .006" to .009" endgap. Use the piston, upside down, with no rings installed, to push the ring you are measuring quite a ways down in the bore, to the end of ring travel, and measure the ring gap there where the bore is not worn as much as it is near the top. Using the piston in this manner sets the ring square in the bore for an accurate measurement, and you want to measure it there because if you have an OK gap near the top of the bore, where there is more wear, the ring will bind lower down, where there is less wear. The reason the .010" OS rings you have overlap is because they are made for a cylinder that has been bored .010" in DIAMETER, and this makes the .010" OS rings considerably larger in CIRCUMFERANCE so they overlap! Hot-rodders buy rings that are slightly too big, and file the ends to get the close ring gap they desire. This can be carefully done by hand, or with a PISTON RING FILER. (I've included a link to a site with a photo of a piston ring filer). If this engine is for serious use, and needs to last, your bore is probably worn (and honed) too much for the original piston. It would be best to have it bored (or precision honed) to the appropriate OVERSIZE, and a matching piston and ring set installed.
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