The first oversize in a "Off the Shelf" forged piston will be 0.030". A "Custom" forged piston can be whatever size you want. The KB pistons are a good choice for a street motor, but need the top ring gap opened up due to the configuration of the piston, "Heat Dam", that isolates the top ring and not to mention the higher silicon content of the piston to reflect heat up towards the cylinder. On average, the top ring will need to be opened up to 0.026" to 0.028" gap.
Filing the rings is not difficult, but depending on what you have to use, care and patience counts. A simple flat file placed vertically in a vise can suffice. With the gap away from you, squeeze the ring onto the file at the portion of the file closest to you, and to carefully proceed to push the ring towards the vise. Once you finish with the stroke, release pressure on the ring to open the gap, and start over. NEVER stroke the ring back towards you as you will damage the moly coating on that type of ring. In order to maintain parallel ring ends at the gap, you squeeze the ring together and look up to the light to see how you are doing, and adjust stroking angles in order to keep the ends as parallel as possible. Before you put the ring into the bore to check the gap, use a small file to dress the 8 edges (top, bottom, in, and out) on both sides. Just a few strokes will suffice to break the sharp edge, and once again, only file into the Moly outside edge so as not to lift the coating.
In order to "square" the ring in the cylinder to check gap size, put the oil rings or second ring in a piston, and use it to push the ring being checked so it is square in the cylinder. The oil or second ring acts as a stop so the piston itself is square also. If you have (or get) a hand rotary filer which is inexpensive, the work will go faster, but you still need to adjust the angle of the ring end to maintain the parallelism of the ends. I would stay away from the cheap Chinese electric filers as they are JUNK!!
Steel rings are even more fun. It took an hour to file just one Cummins top steel ring by hand so I sprung for a professional electric filer...
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