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I am planning the reassembly of my '69 350 engine. I would like to know the preferred piston ring gap as well as the preferred method of filing the rings.

From the Ducati Singles Manual, which is available online through a quick search, the stated (standard) in-position ring size for a 350 engine is a 76 mm diameter with a gap of 0.30-0.45 mm for the compression rings and a gap of 0.25-0.40 mm for the oil ring.