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To get an accurate reading of the gap, the ring must be inserted square in the bore. A ring-squaring tool or an inverted piston with a second ring installed as a stop can do the job. Before filing, the ring's end gap may butt solid; never force the ring in. The end gap is measured with a feeler gauge. Work up from a smaller feeler gauge a thousandth at a time until the next one won't go in. Closely examine the end gap by feel with the feeler gauge as well as by sight to judge whether the end gap is parallel.