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John, i wouls say .005 to .010 should be enough. The minor dia. of the ring groove mainly lets gasses (from combustion or steam) in behind the rings to help force them out against the cylinder wall.Bill
John, The groove on my engine had been done by the origional owner, mine has one groove, 1/4" wide, and to a depth to take rings 1/16" thick, it was a bit scarey putting them on,but it prooved much easier than I thought it would be. If you don't want cast iron rings, you could use teflon tape. Ian S C